1
Q
What is SSD (Source to Skin Distance)?
A
The distance between the radiation source and the patient’s skin surface. Standard SSD: 50cm for Cs-137, 80-100cm for Co-60, 100cm for linear accelerator.
2
Q
What is SAD (Source to Axis Distance)?
A
The distance from the radiation source to the isocentre (axis of rotation). It is 80-100cm for cobalt units and 100cm for linear accelerators.
3
Q
Define isocentre in radiotherapy.
A
The point in space where the axes of rotation of the gantry, couch, collimator and the beam central axis meet, ensuring accurate beam direction when the tumor center is positioned at this point.
4
Q
What is the definition of field size in radiotherapy?
A
The width and length of the radiation beam at SSD or SAD, usually defined by the 50% width of the profile at that depth.
5
Q
Define penumbra in radiation therapy.
A
The unsharp edge of the radiation beam created mainly by finite source size. Radiological penumbra is defined as the 80%-20% width of the dose profile and includes geometric penumbra plus transmission and scatter.
6
Q
What factors affect geometric penumbra?
A
Geometric penumbra increases with: 1) Source size, 2) Source to surface distance (SSD), and decreases with source to diaphragm distance (SDD).
7
Q
State the inverse square law.
A
Intensity of radiation is inversely proportional to the square of the distance from the source. I₁/I₂ = (d₂)²/(d₁)²
8
Q
Define Percentage Depth Dose (PDD).
A
The ratio, expressed as a percentage, of the absorbed dose at any given depth to the absorbed dose at the depth of maximum dose (dmax) on the central axis, with SSD remaining constant.
9
Q
What factors increase PDD?
A
PDD increases with: 1) Beam energy/quality, 2) Field size, 3) Source-to-skin distance (SSD)
10
Q
Define Tissue Air Ratio (TAR).
A
The ratio of the absorbed dose at a given point in phantom to the dose in air at the same point with electronic equilibrium conditions. TAR is independent of SSD.
11
Q
What is the basic equation for treatment time or monitor units?
A
MU = Dose per beam per fraction / Dose rate at prescription point
12
Q
What two basic quantities must be determined for MU calculation?
A
1) Dose per beam per fraction, 2) Dose rate at the point for that beam (or dose per MU at that point for that beam)
13
Q
What is RDF (Relative Dose Factor)?
A
RDF is the output factor: RDF(S) = D(dref,S) / D(dref,Sref). It accounts for the change in dose due to field size variation relative to reference field size.
14
Q
What is TMR (Tissue Maximum Ratio)?
A
The ratio of absorbed dose at depth d to the absorbed dose at reference depth dmax in phantom with constant source-to-chamber distance. TMR(S,Q,d) = Dd / Dmax
15
Q
What is TPR (Tissue Phantom Ratio)?
A
The ratio of dose at depth d to dose at reference depth dref in phantom with constant source-to-chamber distance. TPR is a general form where TMR is a special case with dref = dmax.
16
Q
How do you correct for extended SSD treatment?
A
Apply inverse square law correction: multiply by (f/f₁)² where f is standard SSD and f₁ is extended SSD. Also correct field size and PDD for the extended distance.
17
Q
Define wedge factor.
A
Wedge factor (Wf) = Dose at reference point with wedge / Dose at reference point without wedge. It accounts for beam attenuation by the wedge filter.
18
Q
What is the purpose of wedges in radiotherapy?
A
Wedges are used to: 1) Compensate for missing tissue, 2) Reduce high dose areas (hot spots) in dose distribution, 3) Tilt isodose lines to match body contours.
19
Q
What is Scatter Air Ratio (SAR)?
A
SAR(s,Q,d) = TAR(s,Q,d) - TAR(0,Q,d). It represents the ratio of scattered dose at a given point in phantom to the dose in free space at the same point.
20
Q
What correction factors are needed when converting dose from one depth to another?
A
1) Field size correction (RDF), 2) Depth correction (TPR or TMR), 3) Beam modifier corrections (wedge factor, shielding tray factor), 4) Inverse square correction if SSD changes.
21
Q
What are the three levels of conformal radiotherapy classification?
A
Level 1: Basic CRT (2D planning), Level 2: 3D-CRT (CT-based planning with 3D dose calculation), Level 3: Advanced 3D-CRT (IMRT, image guidance, inverse planning)
22
Q
What are the advantages of SSD (fixed source-to-skin distance) planning?
A
1) More flexibility in patient positioning, 2) Greater range of beam entry positions, 3) Lower scatter dose from linac head, 4) Larger treatment fields possible
23
Q
What are the advantages of SAD (isocentric) planning?
A
1) Stable patient position, 2) Reduced treatment time, 3) More reliable field matching, 4) Option for rotational therapy
24
Q
What factors determine the choice of radiation energy?
A
The depth of the tumor. Deeper tumors require higher energy. Higher energy provides increased skin sparing and increased penetration.
25
What is the typical number of fields for radical intent treatments?
Head & Neck: 2-5 fields, Thorax: 2-5 fields, Pelvis: 3-5 fields, depending on the complexity and specific site
26
What is the purpose of beam shielding in radiotherapy?
To: 1) Shape the treatment field to match tumor volume, 2) Avoid critical structures, 3) Reduce dose to normal tissues
27
What is bolus and when is it used?
Tissue-equivalent material placed in contact with skin to: 1) Move the build-up region into the bolus, 2) Give therapeutic dose to the skin, 3) Fill cavities to prevent hot spots, 4) Compensate for missing tissue
28
What are compensating filters used for?
Made of metals to compensate for missing tissue while maintaining skin sparing effect, unlike bolus which increases surface dose.
29
What is beam weightage?
The contribution of each field to the prescribed dose per fraction. The sum of all field weightages equals one.
30
What are the two types of setup errors?
1) Random errors: Inconsistent deviations (patient movement, organ motion, inconsistent repositioning), 2) Systematic errors: Recurring errors (misinterpretation of setup, incorrectly prepared blocks, transcription errors)
31
Define GTV (Gross Tumor Volume) according to ICRU.
The palpable or visible/demonstrable extent and location of malignant growth. It represents the macroscopic tumor.
32
Define CTV (Clinical Target Volume) according to ICRU.
A volume that includes the GTV plus a margin that encompasses subclinical microscopic malignant disease. It is a pure anatomic-clinical concept.
33
Define PTV (Planning Target Volume) according to ICRU.
A geometric concept that accounts for variations in CTV (organ motion, setup inaccuracies) to ensure the CTV receives the prescribed dose. PTV = CTV + Internal Margin + Setup Margin
34
What is the Treated Volume according to ICRU?
The volume enclosed by an isodose surface (typically 95%) that is selected and specified by the treatment planner.
35
What is the Irradiated Volume according to ICRU?
The tissue volume that receives a dose considered significant in relation to normal tissue tolerance.
36
Define Internal Target Volume (ITV) from ICRU 62.
The volume that includes the CTV with an internal margin to compensate for internal physiological movements and variations in size, shape, and position of the CTV during therapy.
37
What is a Planning Organ at Risk Volume (PRV)?
A geometrical concept introduced in ICRU 62 to ensure adequate sparing of organs at risk by adding a margin to account for uncertainties in position and movement.
38
What dose homogeneity is recommended by ICRU 50?
The dose should be homogeneous within +7% to -5% of the prescribed dose. Values extending outside this range must be noted and justified.
39
What are the ICRU reference point criteria?
1) Within PTV representing most dose points, 2) Defined clearly and unambiguously, 3) Selected where it can be accurately determined, 4) Selected where there is no steep dose gradient
40
What is the Conformity Index and its ideal value?
CI = Treated Volume / PTV volume. The ideal value is 1, indicating perfect conformity of the treated volume to the PTV.
41
Define Exposure (X) in radiation dosimetry.
The electric charge freed by ionization per unit mass of air. Units: Coulombs per kilogram (C/kg)
42
Define KERMA (K).
Kinetic Energy Released per unit Mass. The initial kinetic energy of all charged particles created by the ionization process. Units: Gray (Gy) or J/kg
43
Define Absorbed Dose (D).
The energy absorbed per unit mass of material. Units: Gray (Gy) where 1 Gy = 1 J/kg
44
What is the relationship between Exposure and Air KERMA?
Kair = X × (W/e), where W/e = 33.97 J/C is the mean energy required to create an ion pair in air per unit charge
45
What is Charged Particle Equilibrium (CPE)?
CPE exists when each charged particle of a given energy leaving a volume is replaced by an identical particle entering (mean energy in = mean energy out). When CPE exists: Absorbed dose = KERMA
46
How do you relate absorbed dose in different materials?
Dwater = Dair × (μen/ρ)water / (μen/ρ)air, using the ratio of mass energy absorption coefficients
47
What corrections are needed for ionization chamber readings?
1) Temperature and pressure (TPC), 2) Ion recombination, 3) Polarity effect, 4) Chamber calibration factor, 5) Corrections for non-reference conditions
48
What is the temperature and pressure correction formula?
TPC = [(273.15 + T°C) / 293.15] × [1013.25 / P(mbar)]
49
What causes ion recombination in chambers?
Positive and negative ions can recombine before reaching electrodes if they interact. Amount depends on: 1) Polarizing voltage, 2) Electrode separation, 3) Dose per pulse
50
What are the advantages of ionization chambers?
1) Ionization directly related to absorbed dose, 2) Various sizes/shapes available, 3) Can be used over wide energy range, 4) Accurate and reproducible
51
What are the three key principles of radiation protection?
1) TIME: Minimize exposure time, 2) DISTANCE: Increase distance from source (inverse square law), 3) SHIELDING: Use appropriate shielding for radiation type
52
Define deterministic radiation effects.
Effects where severity is proportional to dose received after a threshold is reached. Include acute damage like skin burns, sterility, loss of saliva production. Effects occur in short term.
53
Define stochastic radiation effects.
Effects where likelihood (not severity) is proportional to dose with no threshold. Include cancer and hereditary effects. Long-term damage occurring long after exposure.
54
What is the difference between absorbed dose, equivalent dose, and effective dose?
Absorbed Dose (D): Energy per unit mass (Gy). Equivalent Dose (HT): Absorbed dose × radiation weighting factor (Sv). Effective Dose (E): Sum of equivalent doses × tissue weighting factors (Sv).
55
What are the legal dose limits for radiation workers?
Whole body: 20 mSv/year, Extremities/Skin: 500 mSv/year, Lens of eye: 20 mSv/year (reduced from 150 mSv in IRR99)
56
What is ALARP principle?
As Low As Reasonably Practicable - aim to reduce doses to employees and public while balancing monetary cost if additional measures would only reduce doses insignificantly.
57
What are controlled areas in radiation facilities?
Areas where doses may exceed 6 mSv/year (whole body) or 15 mSv/year (lens) or 150 mSv/year (extremities). Must have signs, written entry procedures, designated RPS and local rules.
58
What are supervised areas?
Areas surrounding controlled areas where public dose limits (1 mSv/year whole body) could be exceeded. Must be kept under regular review.
59
Who is the Radiation Protection Advisor (RPA)?
A person meeting HSE competency criteria who advises on radiation safety. Must be consulted on specific aspects and appointed in writing (may be external or employee).
60
What is a Half Value Thickness (HVT)?
The thickness of shielding material that reduces the intensity of radiation by half. HVT = ln(2)/μ where μ is the linear attenuation coefficient.
61
What are the key characteristics of electron depth dose curves?
1) Small skin sparing effect, 2) Relatively homogeneous dose over specific depth range, 3) Rapid dose fall-off distally, 4) Bremsstrahlung tail
62
What are the rules of thumb for electron depth doses?
For depths in mm and energy in MeV: dmax = 2×Energy, R90 = 3×Energy, R50 = 4×Energy, Practical range Rp = 5×Energy
63
Define R50 for electron beams.
The depth in water where the dose is 50% of the maximum dose. Used to characterize electron beam energy.
64
What is the therapeutic range for electron beams?
Typically 85-90% of maximum dose, approximately 3 × Energy (in MeV) expressed in mm depth
65
How does electron beam PDD vary with applicator size?
For applicators >10×10cm: PDD independent of size (lateral scatter equilibrium). For <10×10cm: PDD strongly dependent on size and shape.
66
What happens to surface dose as electron energy increases?
Depth of dmax increases, relative surface dose increases, but absolute surface dose does not change significantly with energy.
67
What is the effect of extended FSD on electron beams?
1) Broader penumbra, 2) Treatment area doesn't increase significantly, 3) Absolute dose rate decreases (~inverse square), 4) Beam appears more penetrating (relative effect), 5) Absolute surface dose decreases, relative surface dose increases
68
What thickness of lead is required for electron shielding?
General rule: Lead thickness (mm) should be at least half the energy (MeV). E.g., for 10 MeV beam, at least 5mm lead required.
69
What is the effect of lead shielding on electron beams?
Electrons scatter from edge of lead into treatment area with lower energy, causing surface dose enhancement (hot spots) around the shield edge.
70
When should bolus be used with electron beams?
When including skin in treatment volume. Bolus moves build-up region into the bolus material, giving therapeutic dose to skin and filling cavities to prevent hot spots.
71
What is the half-life and what is the formula?
Time for half of radioactive sample to decay. T½ = ln(2)/λ ≈ 0.693/λ where λ is the decay constant. Activity = A₀e^(-λt)
72
What is effective half-life in nuclear medicine?
Combined physical and biological half-lives: 1/Teffective = 1/Tphysical + 1/Tbiological. Accounts for both radioactive decay and biological clearance.
73
What are the characteristics of ideal unsealed isotopes for therapy?
1) Pure beta emitter for local dose absorption, 2) Optional gamma ray (~200 keV) for imaging, 3) Moderately long effective half-life (days to weeks)
74
What are the characteristics of ideal sealed sources for brachytherapy?
1) Gamma ray emissions, 2) Energy high enough to penetrate tissue but not too high for shielding, 3) Long half-life for reusable sources or short for implanted sources
75
What is the half-life and energy of Iodine-125?
Half-life: 59.4 days. Emits gamma rays at 27-35 keV. Used for permanent prostate seed implants with low energy for easy shielding.
76
What is the half-life and energy of Iridium-192?
Half-life: 73.8 days. Emits gamma rays at ~397 keV. Most commonly used in HDR brachytherapy, needs replacement every 3 months.
77
State the inverse square law for point sources.
Radiation intensity ∝ 1/d². I₁/I₂ = d₂²/d₁². Doubling distance reduces intensity by factor of 4.
78
What is Reference Air KERMA Rate (RAKR)?
KERMA-rate to air from air at distance of 1m from sealed source. Units: μGy h⁻¹ @ 1m (or unit U where 1U = 1 μGy h⁻¹ at 1m). Used to characterize sealed source strength.
79
What are the dose rate classifications for brachytherapy?
Low Dose Rate (LDR): 0.4-2 Gy/h, Medium Dose Rate (MDR): 2-12 Gy/h, High Dose Rate (HDR): >12 Gy/h
80
What is the difference between pre-loading and afterloading?
Pre-loading: Manual insertion of radioactive source (high staff exposure). Afterloading: Catheters inserted first, then sources loaded remotely (reduced staff exposure).
81
What is the difference between Quality Assurance and Quality Control?
QA: Whole system ensuring quality (documented procedures, protocols, responsibilities, audits). QC: Specific tests to maintain standards (measurements, checks, verification).
82
What is the most important daily QC test for linacs?
Radiation output check using constancy check device or ion chamber to ensure machine is delivering correct dose before treatments begin.
83
How is photon beam energy characterized?
Using Tissue Phantom Ratio TPR20,10: ratio of dose at 20cm and 10cm depth with chamber at isocentre. Measured annually for calibration.
84
How is electron beam energy characterized?
Using R50,D: the depth where dose is 50% of maximum in water. Measured annually, with quicker constancy checks done 3-monthly.
85
Define beam flatness.
Flatness = (Dmax - Dmin) / (Dmax + Dmin) OR Flatness = [(Dmax/Dmin) - 1] × 100%. Measured across the flattened region of beam profile.
86
What are typical QC tolerances for linacs?
Radiation output: ±2%, Beam energy: ±1%, Flatness/Symmetry: ±2%, Mechanical (lasers, crosswires): 2mm, ODI: 2mm
87
What is patient-specific dosimetric verification?
Measuring plan dose before treatment to check deliverability. Methods: point measurement (ion chamber), film, array measurement, EPID measurement.
88
What are the advantages of CBCT for IGRT?
1) Good contrast and resolution, 2) 3D imaging, 3) Large field of view, 4) Daily adaptation possible, 5) Can acquire 4D images with respiratory trace
89
What is in vivo dosimetry?
Dose measurements performed during treatment using diodes or TLDs to detect errors from dose calculation, plan transfer, or patient setup. Usually done on first fraction.
90
What are reportable incidents under IR(ME)R SAUE criteria?
Therapy over-exposure: ≥10% whole course or ≥20% single fraction. Under-exposure: ≤90% whole course. Total geographic miss: all instances. Partial miss: >2.5× error margin AND guideline doses exceeded.
91
What are the different types of radioactive decay?
1) Alpha decay: Heavy nuclei emit alpha particles, 2) Beta(-) decay: Neutron-rich nuclei emit electrons, 3) Beta(+)/Positron emission: Proton-rich nuclei emit positrons, 4) Electron capture: Proton-rich nuclei capture orbital electron, 5) Gamma decay: Excited nuclei emit gamma rays
92
What is Cobalt-60 and its properties?
Produced by neutron bombardment of Co-59. Emits beta and gamma radiation. Average photon energy: 1.25 MeV (1.17 and 1.33 MeV). Half-life: 5.3 years. Used in teletherapy units with 10,000-12,000 Ci sources.
93
What is the advantage of linear accelerators over cobalt units?
1) Higher beam energy options (6-23 MV), 2) Higher beam output, 3) Multiple energies (photons 6-15 MV, electrons 5-18 MeV), 4) Sharper penumbra, 5) No radioactive source decay
94
What is the depth of maximum dose (dmax) and why does it occur?
The depth where absorbed dose is maximum. Occurs due to build-up of secondary electrons. For Co-60: 5mm, 6MV: 15mm, 10MV: 25mm, 15MV: 30mm. Increases with beam energy.
95
What is the Mayneord factor used for?
To convert PDD from one SSD to another: Fs = [(f₁+dm)/(f₂+dm)]² × [(f₂+d)/(f₁+d)]² where f is SSD, dm is dmax depth, d is depth of interest
96
What is beam quality index?
Usually defined as TPR20/10 for photon beams. E.g., 6MV: 0.67, 10MV: 0.736, 15MV: 0.77. Higher values indicate higher energy/more penetrating beams.
97
What is equivalent square and why is it needed?
Method to relate rectangular field to square field for which measured data exists. Used because dosimetric data is typically measured for square fields only.
98
What are the components of total scatter?
Total scatter = Collimator scatter (Sc) + Phantom scatter (Sp). RDF = Sc × Sp, accounting for scatter from collimator and within phantom.
99
What is Peak Scatter Factor (PSF)?
PSF = Dmax/Dair. It is the TAR value at depth dmax. Used to convert dose in air to dose in tissue. Also called Back Scatter Factor (BSF) for lower energies.
100
What is the sector integration or Clarkson's method?
Method to determine dose for irregular fields by dividing field into sectors (typically 36) and summing Scatter Air Ratios (SARs) for each sector to get total SAR and TAR.
101
What is a Digitally Reconstructed Radiograph (DRR)?
A synthetic X-ray image created from CT data set by the planning system. Used as reference image for setup verification by comparing with 2D planar images from treatment machine.
102
What are the types of geometric errors in radiotherapy?
Systematic errors: Same for every fraction (delineation, miscalibration, incorrect markers). Random errors: Different each fraction (organ motion, daily setup variation, respiration).
103
What immobilization devices are commonly used?
1) Masks and thermoplastic shells (head/neck), 2) Vacuum bags (various sites), 3) Wing boards (breast/thorax), 4) Stereotactic frames (brain), 5) Abdominal compression (lung)
104
What is the purpose of 4D-CT scanning?
To capture organ motion throughout respiratory cycle. Allows accounting for tumor movement with breathing for treatment planning and gating strategies.
105
What imaging modalities are used in radiotherapy planning?
CT (baseline for planning), MRI (soft tissue contrast), PET (metabolic/functional assessment), Conventional X-ray (2D planning), Ultrasound (some applications)
106
Compare kV and MV portal imaging.
kV: Better contrast (photoelectric dominates), better resolution, doesn't verify treatment beam, requires separate imaging system. MV: Poor contrast (Compton dominates), shows treatment beam position, no extra dose, uses treatment beam.
107
What is the advantage of MR-guided radiotherapy?
1) Superior soft tissue contrast, 2) No imaging dose, 3) 3D volumetric imaging, 4) Can image during treatment (intra-fraction monitoring), 5) Daily adaptation possible, 6) Functional imaging potential
108
What is DIBH technique?
Deep Inspiration Breath Hold - patient holds breath at deep inspiration during imaging and treatment. Reduces heart dose in left breast treatments by increasing distance between chest wall and heart.
109
What are offline imaging protocols?
Images acquired after treatment and reviewed later. Used to correct systematic setup errors by calculating average shift from first 3-5 fractions and applying to remaining treatments.
110
What is transit dosimetry?
Using EPID detector to measure exit dose from patient during treatment. Verifies correct delivery and patient position but difficult to implement due to lack of commercial software.
111
What is IMRT (Intensity Modulated Radiotherapy)?
Method of radiation delivery using beams of varying intensities across the field to produce concave dose distributions that sculpt dose around disease shape. Uses 5-9 non-opposing beams.
112
What are the IMRT delivery methods?
1) Step-and-shoot/Segmented: Beam off while MLCs move, 2) Dynamic: Beam on while MLCs move continuously, 3) VMAT/Arc: Rotating gantry with dynamic MLCs and dose rate
113
What is inverse planning?
Optimization process where beamlet weights/intensities are adjusted by computer to satisfy predefined dose distribution criteria. Used for IMRT. Opposite of forward planning where planner manually adjusts parameters.
114
What is field matching in radiotherapy?
Technique to join adjacent treatment fields without overlap or gap. Methods: asymmetric jaws, parallel central axes, tilted axes. Critical for treatments like breast (tangential fields matched to supraclavicular field).
115
What is tomotherapy?
Helical delivery of IMRT using 6MV linac mounted on slip-ring gantry with fan beam. Planning parameters: pitch (couch travel per rotation) and modulation factor (trade-off between efficiency and optimization freedom).
116
What corrections are needed for tissue inhomogeneities?
Methods: 1) TAR method: Use water-equivalent depth, 2) Power law (Batho): Raise TAR ratio to power of density difference, 3) Equivalent TAR (ETAR): Scale field size for scatter, 4) Differential scatter-air ratio
117
What is the effect of lung heterogeneity on dose?
Low density causes: 1) Increased dose beyond lung (less attenuation), 2) Reduced lateral scatter, 3) Dose perturbation at interfaces. Amount depends on beam energy (higher energy = less effect).
118
What is the effect of bone heterogeneity on dose?
High density causes: 1) Reduced dose beyond bone (more attenuation), 2) Dose perturbation at interfaces. Effect: -3.5%/cm for Co-60, -2%/cm for 10MV. Less effect at higher energies.
119
What is a Dose Volume Histogram (DVH)?
Graph showing volume of structure receiving each dose level. Two types: Differential (shows volume at each dose) and Cumulative (shows volume receiving at least that dose). Used for plan evaluation.
120
What parameters are reported from DVH for ICRU 83?
For PTV: D50% (median), D98% (near-minimum), D2% (near-maximum). For OARs: Mean dose and D2% (complication risk). Homogeneity Index: HI = (D2% - D98%) / D50%
121
What is a Record and Verify system?
System that sends treatment instructions to linac and records delivered dose. Manages entire pathway from scan to treatment. Prevents transcription errors through electronic transfer. Examples: MOSAIQ (Elekta), ARIA (Varian).
122
What is independent dose calculation?
Verification of TPS-calculated MUs using different method: hand calculation with data tables (simple plans) or independent computer calculation with different algorithm and beam data (complex plans).
123
What verification methods exist for IMRT?
1) Point measurement (ion chamber in phantom), 2) Film measurement (2D dose in phantom), 3) Array measurement (2D fluence map or 3D distribution), 4) EPID measurement (exit fluence)
124
What is gamma analysis for plan verification?
Combined dose-distance agreement metric. Typical criteria: 3% dose difference and 3mm distance-to-agreement (3%/3mm). Gamma <1 means measurement passes, >1 means fails. ICRU 83 recommends for IMRT verification.
125
What anthropomorphic phantoms are used for verification?
Phantoms mimicking human anatomy with varying complexity: simple blocks (solid water), semi-anthropomorphic (head/body shapes), fully anthropomorphic (ATOM family with tissue-equivalent materials for different organs).
126
What causes dose shadowing with in vivo diodes?
Diode contains build-up material that reduces delivered dose behind diode by several percent. Extent depends on thickness and energy. Negligible if only used for 1-2 fractions but significant for multiple uses.
127
What is the hierarchy of controls for radiation safety?
From most to least effective: 1) Elimination, 2) Substitution, 3) Engineering controls (bunkers, interlocks), 4) Administrative controls (local rules, procedures), 5) PPE (lead aprons)
128
What are local rules in radiation safety?
Key working instructions for controlled/supervised areas containing: management details, area descriptions, access arrangements, work instructions, dose investigation levels, contingency plans, training procedures. Must be brief, clear, site-specific.
129
What is the difference between IRR17 and IR(ME)R17?
IRR17 (Ionising Radiation Regulations): Protects staff and public from radiation, regulated by HSE. IR(ME)R17 (Ionising Radiation Medical Exposure Regulations): Protects patients undergoing medical radiation procedures, regulated by CQC.
130
What roles are defined under IRR17?
Employer: Overall legal responsibility. RPA (Radiation Protection Advisor): Advises on safety, must be consulted. RPS (Radiation Protection Supervisor): Ensures local rules compliance, checks staff working safely.
131
What is lateral scatter equilibrium for electron beams?
Occurs when distance from field edge to central axis exceeds electron lateral scatter range. Same number of electrons scattered toward CAX as away from CAX. Achieved with fields >10×10cm for clinical energies.
132
What causes the bremsstrahlung tail in electron depth dose curves?
High-energy electrons interacting with matter produce photons via bremsstrahlung. These photons penetrate deeper than electrons, creating a low-dose 'tail' beyond the practical range.
133
How does oblique incidence affect electron beam dosimetry?
1) Penumbra widens where surface moves away, 2) Isodose lines follow surface contour, 3) Dmax comes closer to surface, 4) Surface dose increases, 5) Effective depth increases by 1/cos(angle)
134
What is the rule for selecting wedge angle?
Rule of thumb: Wedge angle = 90° - (hinge angle)/2, where hinge angle is the angle between the two beams. For perpendicular beams (90°): wedge angle = 45°
135
What is dynamic wedge vs physical wedge?
Physical wedge: Metal filter in beam path, fixed angle, requires manual placement. Dynamic wedge: Created by moving jaw during treatment, programmable angles, no physical filter needed, variable transmission across field.
136
What is the Clarkson scatter integration formula?
SAR for irregular field = Σ(SARn)/36 where field divided into 36 sectors (10° each). TAR = TAR(0) + SAR. Used to calculate dose for irregularly shaped fields.
137
What is the purpose of guard ring in parallel plate chambers?
Helps define the collecting volume precisely and minimizes in-scattering of electrons from outside the chamber. Essential for accurate electron dosimetry.
138
What are typical tissue weighting factors (wT) from ICRP 103?
High (0.12): Red marrow, lung, breast, stomach, colon. Moderate (0.08): Gonads. Low (0.04): Thyroid, esophagus, liver, bladder. Very low (0.01): Brain, salivary glands, skin, bone surface.
139
What radiation weighting factors (wR) are used?
X-rays, gamma, beta, electrons: 1. Protons: 2. Alpha particles, heavy nuclei: 20. Neutrons: 2-20 (energy dependent). Higher wR means more biological damage per Gy.
140
What is Tenth Value Thickness (TVT)?
Thickness of material that reduces radiation intensity by factor of 10. TVT = ln(10)/μ ≈ 3.32 × HVT. Used in bunker design where large attenuation factors needed (typically 10⁶ reduction).
141
What are the typical HVTs for different energies?
Concrete: Co-60 ~60mm, 6MV ~80mm, 10MV ~110mm, 15MV ~130mm. Lead: Co-60 ~50mm, 6MV ~60mm, 10MV ~75mm. Higher energy requires thicker shielding.
142
What is off-axis ratio (OAR) method for isodose construction?
Older method combining central axis depth dose with beam profiles (off-axis ratios) at multiple depths to construct isodose curves. Largely replaced by direct measurement and beam modeling.
143
What is the advantage of carbon fiber couchtops?
Low atomic number (Z=6) means minimal attenuation of treatment beam, allowing posterior beams without significant dose reduction. Also radiolucent for imaging. Rigid for reproducible positioning.
144
What causes horns in photon beam profiles?
Flattening filter designed to over-flatten at surface to achieve flat isodose curves at depth. Due to beam hardening and scatter effects. More pronounced at higher energies.
145
What is the difference between coplanar and non-coplanar planning?
Coplanar: All beam central axes in same plane (typically transverse). Non-coplanar: Beams at different couch angles. Non-coplanar can improve dose distribution for H&N and brain but more complex to deliver.
146
What is the concept of remaining volume at risk (RVR)?
Difference between external patient contour and volumes of CTVs and OARs. Represents normal tissue not explicitly contoured. Used in ICRU 83 for comprehensive dose reporting.
147
What is fractionation sensitivity (α/β ratio)?
Radiobiological parameter describing tissue response to fraction size. Low α/β (2-3 Gy): Late-responding tissues, more sensitive to fraction size. High α/β (8-10 Gy): Tumors and early-responding tissues, less sensitive to fraction size.
148
What is the purpose of contrast in CT for treatment planning?
Enhances visualization of tumor and normal structures, particularly blood vessels and organs with similar density. IV contrast shows vascular structures. Oral contrast delineates bowel. Must document for dose calculation effects.
149
What is CT number and its relationship to electron density?
CT number (HU) = 1000 × (μ-μwater)/μwater where μ is linear attenuation coefficient. Related to electron density for dose calculation. Water = 0 HU, Air = -1000 HU, Bone = +1000 to +3000 HU.
150
What is the difference between LET and stopping power?
Linear Energy Transfer (LET): Energy deposited per unit path length. Stopping power: Energy lost by particle per unit path length (includes energy lost but not locally deposited). For electrons: Stopping power ≈ LET.
151
What is SSD (Source to Skin Distance)?
The distance between the radiation source and the patient's skin surface. Standard SSD: 50cm for Cs-137, 80-100cm for Co-60, 100cm for linear accelerator.
152
What is SAD (Source to Axis Distance)?
The distance from the radiation source to the isocentre (axis of rotation). It is 80-100cm for cobalt units and 100cm for linear accelerators.
153
Define isocentre in radiotherapy.
The point in space where the axes of rotation of the gantry, couch, collimator and the beam central axis meet, ensuring accurate beam direction when the tumor center is positioned at this point.
154
What is the definition of field size in radiotherapy?
The width and length of the radiation beam at SSD or SAD, usually defined by the 50% width of the profile at that depth.
155
Define penumbra in radiation therapy.
The unsharp edge of the radiation beam created mainly by finite source size. Radiological penumbra is defined as the 80%-20% width of the dose profile and includes geometric penumbra plus transmission and scatter.
156
What factors affect geometric penumbra?
Geometric penumbra increases with: 1) Source size, 2) Source to surface distance (SSD), and decreases with source to diaphragm distance (SDD).
157
State the inverse square law.
Intensity of radiation is inversely proportional to the square of the distance from the source. I₁/I₂ = (d₂)²/(d₁)²
158
Define Percentage Depth Dose (PDD).
The ratio, expressed as a percentage, of the absorbed dose at any given depth to the absorbed dose at the depth of maximum dose (dmax) on the central axis, with SSD remaining constant.
159
What factors increase PDD?
PDD increases with: 1) Beam energy/quality, 2) Field size, 3) Source-to-skin distance (SSD)
160
Define Tissue Air Ratio (TAR).
The ratio of the absorbed dose at a given point in phantom to the dose in air at the same point with electronic equilibrium conditions. TAR is independent of SSD.
161
What is the basic equation for treatment time or monitor units?
MU = Dose per beam per fraction / Dose rate at prescription point
162
What two basic quantities must be determined for MU calculation?
1) Dose per beam per fraction, 2) Dose rate at the point for that beam (or dose per MU at that point for that beam)
163
What is RDF (Relative Dose Factor)?
RDF is the output factor: RDF(S) = D(dref,S) / D(dref,Sref). It accounts for the change in dose due to field size variation relative to reference field size.
164
What is TMR (Tissue Maximum Ratio)?
The ratio of absorbed dose at depth d to the absorbed dose at reference depth dmax in phantom with constant source-to-chamber distance. TMR(S,Q,d) = Dd / Dmax
165
What is TPR (Tissue Phantom Ratio)?
The ratio of dose at depth d to dose at reference depth dref in phantom with constant source-to-chamber distance. TPR is a general form where TMR is a special case with dref = dmax.
166
How do you correct for extended SSD treatment?
Apply inverse square law correction: multiply by (f/f₁)² where f is standard SSD and f₁ is extended SSD. Also correct field size and PDD for the extended distance.
167
Define wedge factor.
Wedge factor (Wf) = Dose at reference point with wedge / Dose at reference point without wedge. It accounts for beam attenuation by the wedge filter.
168
What is the purpose of wedges in radiotherapy?
Wedges are used to: 1) Compensate for missing tissue, 2) Reduce high dose areas (hot spots) in dose distribution, 3) Tilt isodose lines to match body contours.
169
What is Scatter Air Ratio (SAR)?
SAR(s,Q,d) = TAR(s,Q,d) - TAR(0,Q,d). It represents the ratio of scattered dose at a given point in phantom to the dose in free space at the same point.
170
What correction factors are needed when converting dose from one depth to another?
1) Field size correction (RDF), 2) Depth correction (TPR or TMR), 3) Beam modifier corrections (wedge factor, shielding tray factor), 4) Inverse square correction if SSD changes.
171
What are the three levels of conformal radiotherapy classification?
Level 1: Basic CRT (2D planning), Level 2: 3D-CRT (CT-based planning with 3D dose calculation), Level 3: Advanced 3D-CRT (IMRT, image guidance, inverse planning)
172
What are the advantages of SSD (fixed source-to-skin distance) planning?
1) More flexibility in patient positioning, 2) Greater range of beam entry positions, 3) Lower scatter dose from linac head, 4) Larger treatment fields possible
173
What are the advantages of SAD (isocentric) planning?
1) Stable patient position, 2) Reduced treatment time, 3) More reliable field matching, 4) Option for rotational therapy
174
What factors determine the choice of radiation energy?
The depth of the tumor. Deeper tumors require higher energy. Higher energy provides increased skin sparing and increased penetration.
175
What is the typical number of fields for radical intent treatments?
Head & Neck: 2-5 fields, Thorax: 2-5 fields, Pelvis: 3-5 fields, depending on the complexity and specific site
176
What is the purpose of beam shielding in radiotherapy?
To: 1) Shape the treatment field to match tumor volume, 2) Avoid critical structures, 3) Reduce dose to normal tissues
177
What is bolus and when is it used?
Tissue-equivalent material placed in contact with skin to: 1) Move the build-up region into the bolus, 2) Give therapeutic dose to the skin, 3) Fill cavities to prevent hot spots, 4) Compensate for missing tissue
178
What are compensating filters used for?
Made of metals to compensate for missing tissue while maintaining skin sparing effect, unlike bolus which increases surface dose.
179
What is beam weightage?
The contribution of each field to the prescribed dose per fraction. The sum of all field weightages equals one.
180
What are the two types of setup errors?
1) Random errors: Inconsistent deviations (patient movement, organ motion, inconsistent repositioning), 2) Systematic errors: Recurring errors (misinterpretation of setup, incorrectly prepared blocks, transcription errors)
181
Define GTV (Gross Tumor Volume) according to ICRU.
The palpable or visible/demonstrable extent and location of malignant growth. It represents the macroscopic tumor.
182
Define CTV (Clinical Target Volume) according to ICRU.
A volume that includes the GTV plus a margin that encompasses subclinical microscopic malignant disease. It is a pure anatomic-clinical concept.
183
Define PTV (Planning Target Volume) according to ICRU.
A geometric concept that accounts for variations in CTV (organ motion, setup inaccuracies) to ensure the CTV receives the prescribed dose. PTV = CTV + Internal Margin + Setup Margin
184
What is the Treated Volume according to ICRU?
The volume enclosed by an isodose surface (typically 95%) that is selected and specified by the treatment planner.
185
What is the Irradiated Volume according to ICRU?
The tissue volume that receives a dose considered significant in relation to normal tissue tolerance.
186
Define Internal Target Volume (ITV) from ICRU 62.
The volume that includes the CTV with an internal margin to compensate for internal physiological movements and variations in size, shape, and position of the CTV during therapy.
187
What is a Planning Organ at Risk Volume (PRV)?
A geometrical concept introduced in ICRU 62 to ensure adequate sparing of organs at risk by adding a margin to account for uncertainties in position and movement.
188
What dose homogeneity is recommended by ICRU 50?
The dose should be homogeneous within +7% to -5% of the prescribed dose. Values extending outside this range must be noted and justified.
189
What are the ICRU reference point criteria?
1) Within PTV representing most dose points, 2) Defined clearly and unambiguously, 3) Selected where it can be accurately determined, 4) Selected where there is no steep dose gradient
190
What is the Conformity Index and its ideal value?
CI = Treated Volume / PTV volume. The ideal value is 1, indicating perfect conformity of the treated volume to the PTV.
191
Define Exposure (X) in radiation dosimetry.
The electric charge freed by ionization per unit mass of air. Units: Coulombs per kilogram (C/kg)
192
Define KERMA (K).
Kinetic Energy Released per unit Mass. The initial kinetic energy of all charged particles created by the ionization process. Units: Gray (Gy) or J/kg
193
Define Absorbed Dose (D).
The energy absorbed per unit mass of material. Units: Gray (Gy) where 1 Gy = 1 J/kg
194
What is the relationship between Exposure and Air KERMA?
Kair = X × (W/e), where W/e = 33.97 J/C is the mean energy required to create an ion pair in air per unit charge
195
What is Charged Particle Equilibrium (CPE)?
CPE exists when each charged particle of a given energy leaving a volume is replaced by an identical particle entering (mean energy in = mean energy out). When CPE exists: Absorbed dose = KERMA
196
How do you relate absorbed dose in different materials?
Dwater = Dair × (μen/ρ)water / (μen/ρ)air, using the ratio of mass energy absorption coefficients
197
What corrections are needed for ionization chamber readings?
1) Temperature and pressure (TPC), 2) Ion recombination, 3) Polarity effect, 4) Chamber calibration factor, 5) Corrections for non-reference conditions
198
What is the temperature and pressure correction formula?
TPC = [(273.15 + T°C) / 293.15] × [1013.25 / P(mbar)]
199
What causes ion recombination in chambers?
Positive and negative ions can recombine before reaching electrodes if they interact. Amount depends on: 1) Polarizing voltage, 2) Electrode separation, 3) Dose per pulse
200
What are the advantages of ionization chambers?
1) Ionization directly related to absorbed dose, 2) Various sizes/shapes available, 3) Can be used over wide energy range, 4) Accurate and reproducible
201
What are the three key principles of radiation protection?
1) TIME: Minimize exposure time, 2) DISTANCE: Increase distance from source (inverse square law), 3) SHIELDING: Use appropriate shielding for radiation type
202
Define deterministic radiation effects.
Effects where severity is proportional to dose received after a threshold is reached. Include acute damage like skin burns, sterility, loss of saliva production. Effects occur in short term.
203
Define stochastic radiation effects.
Effects where likelihood (not severity) is proportional to dose with no threshold. Include cancer and hereditary effects. Long-term damage occurring long after exposure.
204
What is the difference between absorbed dose, equivalent dose, and effective dose?
Absorbed Dose (D): Energy per unit mass (Gy). Equivalent Dose (HT): Absorbed dose × radiation weighting factor (Sv). Effective Dose (E): Sum of equivalent doses × tissue weighting factors (Sv).
205
What are the legal dose limits for radiation workers?
Whole body: 20 mSv/year, Extremities/Skin: 500 mSv/year, Lens of eye: 20 mSv/year (reduced from 150 mSv in IRR99)
206
What is ALARP principle?
As Low As Reasonably Practicable - aim to reduce doses to employees and public while balancing monetary cost if additional measures would only reduce doses insignificantly.
207
What are controlled areas in radiation facilities?
Areas where doses may exceed 6 mSv/year (whole body) or 15 mSv/year (lens) or 150 mSv/year (extremities). Must have signs, written entry procedures, designated RPS and local rules.
208
What are supervised areas?
Areas surrounding controlled areas where public dose limits (1 mSv/year whole body) could be exceeded. Must be kept under regular review.
209
Who is the Radiation Protection Advisor (RPA)?
A person meeting HSE competency criteria who advises on radiation safety. Must be consulted on specific aspects and appointed in writing (may be external or employee).
210
What is a Half Value Thickness (HVT)?
The thickness of shielding material that reduces the intensity of radiation by half. HVT = ln(2)/μ where μ is the linear attenuation coefficient.
211
What are the key characteristics of electron depth dose curves?
1) Small skin sparing effect, 2) Relatively homogeneous dose over specific depth range, 3) Rapid dose fall-off distally, 4) Bremsstrahlung tail
212
What are the rules of thumb for electron depth doses?
For depths in mm and energy in MeV: dmax = 2×Energy, R90 = 3×Energy, R50 = 4×Energy, Practical range Rp = 5×Energy
213
Define R50 for electron beams.
The depth in water where the dose is 50% of the maximum dose. Used to characterize electron beam energy.
214
What is the therapeutic range for electron beams?
Typically 85-90% of maximum dose, approximately 3 × Energy (in MeV) expressed in mm depth
215
How does electron beam PDD vary with applicator size?
For applicators >10×10cm: PDD independent of size (lateral scatter equilibrium). For <10×10cm: PDD strongly dependent on size and shape.
216
What happens to surface dose as electron energy increases?
Depth of dmax increases, relative surface dose increases, but absolute surface dose does not change significantly with energy.
217
What is the effect of extended FSD on electron beams?
1) Broader penumbra, 2) Treatment area doesn't increase significantly, 3) Absolute dose rate decreases (~inverse square), 4) Beam appears more penetrating (relative effect), 5) Absolute surface dose decreases, relative surface dose increases
218
What thickness of lead is required for electron shielding?
General rule: Lead thickness (mm) should be at least half the energy (MeV). E.g., for 10 MeV beam, at least 5mm lead required.
219
What is the effect of lead shielding on electron beams?
Electrons scatter from edge of lead into treatment area with lower energy, causing surface dose enhancement (hot spots) around the shield edge.
220
When should bolus be used with electron beams?
When including skin in treatment volume. Bolus moves build-up region into the bolus material, giving therapeutic dose to skin and filling cavities to prevent hot spots.
221
What is the half-life and what is the formula?
Time for half of radioactive sample to decay. T½ = ln(2)/λ ≈ 0.693/λ where λ is the decay constant. Activity = A₀e^(-λt)
222
What is effective half-life in nuclear medicine?
Combined physical and biological half-lives: 1/Teffective = 1/Tphysical + 1/Tbiological. Accounts for both radioactive decay and biological clearance.
223
What are the characteristics of ideal unsealed isotopes for therapy?
1) Pure beta emitter for local dose absorption, 2) Optional gamma ray (~200 keV) for imaging, 3) Moderately long effective half-life (days to weeks)
224
What are the characteristics of ideal sealed sources for brachytherapy?
1) Gamma ray emissions, 2) Energy high enough to penetrate tissue but not too high for shielding, 3) Long half-life for reusable sources or short for implanted sources
225
What is the half-life and energy of Iodine-125?
Half-life: 59.4 days. Emits gamma rays at 27-35 keV. Used for permanent prostate seed implants with low energy for easy shielding.
226
What is the half-life and energy of Iridium-192?
Half-life: 73.8 days. Emits gamma rays at ~397 keV. Most commonly used in HDR brachytherapy, needs replacement every 3 months.
227
State the inverse square law for point sources.
Radiation intensity ∝ 1/d². I₁/I₂ = d₂²/d₁². Doubling distance reduces intensity by factor of 4.
228
What is Reference Air KERMA Rate (RAKR)?
KERMA-rate to air from air at distance of 1m from sealed source. Units: μGy h⁻¹ @ 1m (or unit U where 1U = 1 μGy h⁻¹ at 1m). Used to characterize sealed source strength.
229
What are the dose rate classifications for brachytherapy?
Low Dose Rate (LDR): 0.4-2 Gy/h, Medium Dose Rate (MDR): 2-12 Gy/h, High Dose Rate (HDR): >12 Gy/h
230
What is the difference between pre-loading and afterloading?
Pre-loading: Manual insertion of radioactive source (high staff exposure). Afterloading: Catheters inserted first, then sources loaded remotely (reduced staff exposure).
231
What is the difference between Quality Assurance and Quality Control?
QA: Whole system ensuring quality (documented procedures, protocols, responsibilities, audits). QC: Specific tests to maintain standards (measurements, checks, verification).
232
What is the most important daily QC test for linacs?
Radiation output check using constancy check device or ion chamber to ensure machine is delivering correct dose before treatments begin.
233
How is photon beam energy characterized?
Using Tissue Phantom Ratio TPR20,10: ratio of dose at 20cm and 10cm depth with chamber at isocentre. Measured annually for calibration.
234
How is electron beam energy characterized?
Using R50,D: the depth where dose is 50% of maximum in water. Measured annually, with quicker constancy checks done 3-monthly.
235
Define beam flatness.
Flatness = (Dmax - Dmin) / (Dmax + Dmin) OR Flatness = [(Dmax/Dmin) - 1] × 100%. Measured across the flattened region of beam profile.
236
What are typical QC tolerances for linacs?
Radiation output: ±2%, Beam energy: ±1%, Flatness/Symmetry: ±2%, Mechanical (lasers, crosswires): 2mm, ODI: 2mm
237
What is patient-specific dosimetric verification?
Measuring plan dose before treatment to check deliverability. Methods: point measurement (ion chamber), film, array measurement, EPID measurement.
238
What are the advantages of CBCT for IGRT?
1) Good contrast and resolution, 2) 3D imaging, 3) Large field of view, 4) Daily adaptation possible, 5) Can acquire 4D images with respiratory trace
239
What is in vivo dosimetry?
Dose measurements performed during treatment using diodes or TLDs to detect errors from dose calculation, plan transfer, or patient setup. Usually done on first fraction.
240
What are reportable incidents under IR(ME)R SAUE criteria?
Therapy over-exposure: ≥10% whole course or ≥20% single fraction. Under-exposure: ≤90% whole course. Total geographic miss: all instances. Partial miss: >2.5× error margin AND guideline doses exceeded.
241
What are the different types of radioactive decay?
1) Alpha decay: Heavy nuclei emit alpha particles, 2) Beta(-) decay: Neutron-rich nuclei emit electrons, 3) Beta(+)/Positron emission: Proton-rich nuclei emit positrons, 4) Electron capture: Proton-rich nuclei capture orbital electron, 5) Gamma decay: Excited nuclei emit gamma rays
242
What is Cobalt-60 and its properties?
Produced by neutron bombardment of Co-59. Emits beta and gamma radiation. Average photon energy: 1.25 MeV (1.17 and 1.33 MeV). Half-life: 5.3 years. Used in teletherapy units with 10,000-12,000 Ci sources.
243
What is the advantage of linear accelerators over cobalt units?
1) Higher beam energy options (6-23 MV), 2) Higher beam output, 3) Multiple energies (photons 6-15 MV, electrons 5-18 MeV), 4) Sharper penumbra, 5) No radioactive source decay
244
What is the depth of maximum dose (dmax) and why does it occur?
The depth where absorbed dose is maximum. Occurs due to build-up of secondary electrons. For Co-60: 5mm, 6MV: 15mm, 10MV: 25mm, 15MV: 30mm. Increases with beam energy.
245
What is the Mayneord factor used for?
To convert PDD from one SSD to another: Fs = [(f₁+dm)/(f₂+dm)]² × [(f₂+d)/(f₁+d)]² where f is SSD, dm is dmax depth, d is depth of interest
246
What is beam quality index?
Usually defined as TPR20/10 for photon beams. E.g., 6MV: 0.67, 10MV: 0.736, 15MV: 0.77. Higher values indicate higher energy/more penetrating beams.
247
What is equivalent square and why is it needed?
Method to relate rectangular field to square field for which measured data exists. Used because dosimetric data is typically measured for square fields only.
248
What are the components of total scatter?
Total scatter = Collimator scatter (Sc) + Phantom scatter (Sp). RDF = Sc × Sp, accounting for scatter from collimator and within phantom.
249
What is Peak Scatter Factor (PSF)?
PSF = Dmax/Dair. It is the TAR value at depth dmax. Used to convert dose in air to dose in tissue. Also called Back Scatter Factor (BSF) for lower energies.
250
What is the sector integration or Clarkson's method?
Method to determine dose for irregular fields by dividing field into sectors (typically 36) and summing Scatter Air Ratios (SARs) for each sector to get total SAR and TAR.
251
What is a Digitally Reconstructed Radiograph (DRR)?
A synthetic X-ray image created from CT data set by the planning system. Used as reference image for setup verification by comparing with 2D planar images from treatment machine.
252
What are the types of geometric errors in radiotherapy?
Systematic errors: Same for every fraction (delineation, miscalibration, incorrect markers). Random errors: Different each fraction (organ motion, daily setup variation, respiration).
253
What immobilization devices are commonly used?
1) Masks and thermoplastic shells (head/neck), 2) Vacuum bags (various sites), 3) Wing boards (breast/thorax), 4) Stereotactic frames (brain), 5) Abdominal compression (lung)
254
What is the purpose of 4D-CT scanning?
To capture organ motion throughout respiratory cycle. Allows accounting for tumor movement with breathing for treatment planning and gating strategies.
255
What imaging modalities are used in radiotherapy planning?
CT (baseline for planning), MRI (soft tissue contrast), PET (metabolic/functional assessment), Conventional X-ray (2D planning), Ultrasound (some applications)
256
Compare kV and MV portal imaging.
kV: Better contrast (photoelectric dominates), better resolution, doesn't verify treatment beam, requires separate imaging system. MV: Poor contrast (Compton dominates), shows treatment beam position, no extra dose, uses treatment beam.
257
What is the advantage of MR-guided radiotherapy?
1) Superior soft tissue contrast, 2) No imaging dose, 3) 3D volumetric imaging, 4) Can image during treatment (intra-fraction monitoring), 5) Daily adaptation possible, 6) Functional imaging potential
258
What is DIBH technique?
Deep Inspiration Breath Hold - patient holds breath at deep inspiration during imaging and treatment. Reduces heart dose in left breast treatments by increasing distance between chest wall and heart.
259
What are offline imaging protocols?
Images acquired after treatment and reviewed later. Used to correct systematic setup errors by calculating average shift from first 3-5 fractions and applying to remaining treatments.
260
What is transit dosimetry?
Using EPID detector to measure exit dose from patient during treatment. Verifies correct delivery and patient position but difficult to implement due to lack of commercial software.
261
What is IMRT (Intensity Modulated Radiotherapy)?
Method of radiation delivery using beams of varying intensities across the field to produce concave dose distributions that sculpt dose around disease shape. Uses 5-9 non-opposing beams.
262
What are the IMRT delivery methods?
1) Step-and-shoot/Segmented: Beam off while MLCs move, 2) Dynamic: Beam on while MLCs move continuously, 3) VMAT/Arc: Rotating gantry with dynamic MLCs and dose rate
263
What is inverse planning?
Optimization process where beamlet weights/intensities are adjusted by computer to satisfy predefined dose distribution criteria. Used for IMRT. Opposite of forward planning where planner manually adjusts parameters.
264
What is field matching in radiotherapy?
Technique to join adjacent treatment fields without overlap or gap. Methods: asymmetric jaws, parallel central axes, tilted axes. Critical for treatments like breast (tangential fields matched to supraclavicular field).
265
What is tomotherapy?
Helical delivery of IMRT using 6MV linac mounted on slip-ring gantry with fan beam. Planning parameters: pitch (couch travel per rotation) and modulation factor (trade-off between efficiency and optimization freedom).
266
What corrections are needed for tissue inhomogeneities?
Methods: 1) TAR method: Use water-equivalent depth, 2) Power law (Batho): Raise TAR ratio to power of density difference, 3) Equivalent TAR (ETAR): Scale field size for scatter, 4) Differential scatter-air ratio
267
What is the effect of lung heterogeneity on dose?
Low density causes: 1) Increased dose beyond lung (less attenuation), 2) Reduced lateral scatter, 3) Dose perturbation at interfaces. Amount depends on beam energy (higher energy = less effect).
268
What is the effect of bone heterogeneity on dose?
High density causes: 1) Reduced dose beyond bone (more attenuation), 2) Dose perturbation at interfaces. Effect: -3.5%/cm for Co-60, -2%/cm for 10MV. Less effect at higher energies.
269
What is a Dose Volume Histogram (DVH)?
Graph showing volume of structure receiving each dose level. Two types: Differential (shows volume at each dose) and Cumulative (shows volume receiving at least that dose). Used for plan evaluation.
270
What parameters are reported from DVH for ICRU 83?
For PTV: D50% (median), D98% (near-minimum), D2% (near-maximum). For OARs: Mean dose and D2% (complication risk). Homogeneity Index: HI = (D2% - D98%) / D50%
271
What is a Record and Verify system?
System that sends treatment instructions to linac and records delivered dose. Manages entire pathway from scan to treatment. Prevents transcription errors through electronic transfer. Examples: MOSAIQ (Elekta), ARIA (Varian).
272
What is independent dose calculation?
Verification of TPS-calculated MUs using different method: hand calculation with data tables (simple plans) or independent computer calculation with different algorithm and beam data (complex plans).
273
What verification methods exist for IMRT?
1) Point measurement (ion chamber in phantom), 2) Film measurement (2D dose in phantom), 3) Array measurement (2D fluence map or 3D distribution), 4) EPID measurement (exit fluence)
274
What is gamma analysis for plan verification?
Combined dose-distance agreement metric. Typical criteria: 3% dose difference and 3mm distance-to-agreement (3%/3mm). Gamma <1 means measurement passes, >1 means fails. ICRU 83 recommends for IMRT verification.
275
What anthropomorphic phantoms are used for verification?
Phantoms mimicking human anatomy with varying complexity: simple blocks (solid water), semi-anthropomorphic (head/body shapes), fully anthropomorphic (ATOM family with tissue-equivalent materials for different organs).
276
What causes dose shadowing with in vivo diodes?
Diode contains build-up material that reduces delivered dose behind diode by several percent. Extent depends on thickness and energy. Negligible if only used for 1-2 fractions but significant for multiple uses.
277
What is the hierarchy of controls for radiation safety?
From most to least effective: 1) Elimination, 2) Substitution, 3) Engineering controls (bunkers, interlocks), 4) Administrative controls (local rules, procedures), 5) PPE (lead aprons)
278
What are local rules in radiation safety?
Key working instructions for controlled/supervised areas containing: management details, area descriptions, access arrangements, work instructions, dose investigation levels, contingency plans, training procedures. Must be brief, clear, site-specific.
279
What is the difference between IRR17 and IR(ME)R17?
IRR17 (Ionising Radiation Regulations): Protects staff and public from radiation, regulated by HSE. IR(ME)R17 (Ionising Radiation Medical Exposure Regulations): Protects patients undergoing medical radiation procedures, regulated by CQC.
280
What roles are defined under IRR17?
Employer: Overall legal responsibility. RPA (Radiation Protection Advisor): Advises on safety, must be consulted. RPS (Radiation Protection Supervisor): Ensures local rules compliance, checks staff working safely.
281
What is lateral scatter equilibrium for electron beams?
Occurs when distance from field edge to central axis exceeds electron lateral scatter range. Same number of electrons scattered toward CAX as away from CAX. Achieved with fields >10×10cm for clinical energies.
282
What causes the bremsstrahlung tail in electron depth dose curves?
High-energy electrons interacting with matter produce photons via bremsstrahlung. These photons penetrate deeper than electrons, creating a low-dose 'tail' beyond the practical range.
283
How does oblique incidence affect electron beam dosimetry?
1) Penumbra widens where surface moves away, 2) Isodose lines follow surface contour, 3) Dmax comes closer to surface, 4) Surface dose increases, 5) Effective depth increases by 1/cos(angle)
284
What is the rule for selecting wedge angle?
Rule of thumb: Wedge angle = 90° - (hinge angle)/2, where hinge angle is the angle between the two beams. For perpendicular beams (90°): wedge angle = 45°
285
What is dynamic wedge vs physical wedge?
Physical wedge: Metal filter in beam path, fixed angle, requires manual placement. Dynamic wedge: Created by moving jaw during treatment, programmable angles, no physical filter needed, variable transmission across field.
286
What is the Clarkson scatter integration formula?
SAR for irregular field = Σ(SARn)/36 where field divided into 36 sectors (10° each). TAR = TAR(0) + SAR. Used to calculate dose for irregularly shaped fields.
287
What is the purpose of guard ring in parallel plate chambers?
Helps define the collecting volume precisely and minimizes in-scattering of electrons from outside the chamber. Essential for accurate electron dosimetry.
288
What are typical tissue weighting factors (wT) from ICRP 103?
High (0.12): Red marrow, lung, breast, stomach, colon. Moderate (0.08): Gonads. Low (0.04): Thyroid, esophagus, liver, bladder. Very low (0.01): Brain, salivary glands, skin, bone surface.
289
What radiation weighting factors (wR) are used?
X-rays, gamma, beta, electrons: 1. Protons: 2. Alpha particles, heavy nuclei: 20. Neutrons: 2-20 (energy dependent). Higher wR means more biological damage per Gy.
290
What is Tenth Value Thickness (TVT)?
Thickness of material that reduces radiation intensity by factor of 10. TVT = ln(10)/μ ≈ 3.32 × HVT. Used in bunker design where large attenuation factors needed (typically 10⁶ reduction).
291
What are the typical HVTs for different energies?
Concrete: Co-60 ~60mm, 6MV ~80mm, 10MV ~110mm, 15MV ~130mm. Lead: Co-60 ~50mm, 6MV ~60mm, 10MV ~75mm. Higher energy requires thicker shielding.
292
What is off-axis ratio (OAR) method for isodose construction?
Older method combining central axis depth dose with beam profiles (off-axis ratios) at multiple depths to construct isodose curves. Largely replaced by direct measurement and beam modeling.
293
What is the advantage of carbon fiber couchtops?
Low atomic number (Z=6) means minimal attenuation of treatment beam, allowing posterior beams without significant dose reduction. Also radiolucent for imaging. Rigid for reproducible positioning.
294
What causes horns in photon beam profiles?
Flattening filter designed to over-flatten at surface to achieve flat isodose curves at depth. Due to beam hardening and scatter effects. More pronounced at higher energies.
295
What is the difference between coplanar and non-coplanar planning?
Coplanar: All beam central axes in same plane (typically transverse). Non-coplanar: Beams at different couch angles. Non-coplanar can improve dose distribution for H&N and brain but more complex to deliver.
296
What is the concept of remaining volume at risk (RVR)?
Difference between external patient contour and volumes of CTVs and OARs. Represents normal tissue not explicitly contoured. Used in ICRU 83 for comprehensive dose reporting.
297
What is fractionation sensitivity (α/β ratio)?
Radiobiological parameter describing tissue response to fraction size. Low α/β (2-3 Gy): Late-responding tissues, more sensitive to fraction size. High α/β (8-10 Gy): Tumors and early-responding tissues, less sensitive to fraction size.
298
What is the purpose of contrast in CT for treatment planning?
Enhances visualization of tumor and normal structures, particularly blood vessels and organs with similar density. IV contrast shows vascular structures. Oral contrast delineates bowel. Must document for dose calculation effects.
299
What is CT number and its relationship to electron density?
CT number (HU) = 1000 × (μ-μwater)/μwater where μ is linear attenuation coefficient. Related to electron density for dose calculation. Water = 0 HU, Air = -1000 HU, Bone = +1000 to +3000 HU.
300
What is the difference between LET and stopping power?
Linear Energy Transfer (LET): Energy deposited per unit path length. Stopping power: Energy lost by particle per unit path length (includes energy lost but not locally deposited). For electrons: Stopping power ≈ LET.
FRCR ONCOLOGY
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