1
Q
Definition of an element
A
Matter that cannot be decomposed
2
Q
Definition of a compound
A
Matter that can be decomposed, a combination of 2 or more elements
3
Q
Definition of an atom
A
Smallest particles of an element that can exist without losing chemical properties of the element
4
Q
Definition of a molecule
A
Two or more atoms bonded together.
Smallest particles of a compound that can exist without losing chemical properties of the compound e.g. a water molecule
5
Q
Explain atomic structure
A
Nucleus central made up of protons and neutrons
Electrons orbit the nucleus
Most atom is empty space
6
Q
Charge of a Proton
A
Positive
7
Q
Charge of a Neutron
A
Neutral
8
Q
Charge of an Electron
A
Negative
9
Q
What is the overall charge of a nucleus
A
Positive
10
Q
What is the atomic number
A
Bottom left (Z).
Number of protons in a nucleus
Denotes order of elements in periodic table.
11
Q
What is the mass number
A
Top right (A)
Total number of protons and neutrons
Can vary for the same element in isotopes
12
Q
Which number is higher mass or atomic number?
A
Mass number higher except in hydrogen when its the same
13
Q
What happens if you change the number of protons in an atom
A
The atom changes, atomic number changes
14
Q
What happens if you change the number of neutrons in an atom
A
Atom has different characteristics (isotype)
15
Q
Electrons reside in what around a nucleus
A
Shells
16
Q
Number of electrons in each shell
A
K-2, L-8, M-18, N-32
17
Q
What is binding energy
A
Energy required to overcome attraction of negatively charge electrons to positively charged protons in nucleus
18
Q
How do electrons escape an atom
A
Gaining enough energy to overthrow the binding energy
19
Q
What is ionisation
A
When an electron escapes an atom leaving it with net positive charge
20
Q
What increases binding energy of electrons
A
Closer to nucleus or more protons
21
Q
How does an electron move to a lower binding energy shell
A
Gaining energy
22
Q
How does an electron move to a higher binding energy shell
A
Losing energy
23
Q
Which shells have lower binding energy
A
Shells further from nucleus
24
Q
Which shells have higher binding energy
A
Shells closer to nucleus
25
What happens when electrons lose energy to move to a higher binding shell
Releases energy- produces Xray.
26
What is electromagnetism
The force of a magnetic field on a moving charged particle
27
What is radiation?
The release or transmission of energy
28
What are the methods of radiation?
Particulate radiation- streams of subatomic particles
Electromagnetic radiation- electromagnetic waves motion
29
Particulate radiation can be caused by what
Neutron
Proton
Electron
Ion
alpha particles
Beta particles
30
What is an ion
Atom where number of protons doesn't equal number of electrons
31
In decay what is made up of 2 protons and 2 neutrons
Alpha particle
32
An alpha particle is made up of what
2 protons and 2 neutrons
33
What is the charge of an alpha particle
Positive
34
What is a beta particle
High energy high speed electron
35
What is the charge of a beta particle
Negative
36
What is emitted during radioactive decay
Alpha and beta particles
37
What is electromagnetism
The force of a magnetic field on a moving charged particle
38
Electromagnetic radiation behaves as particles called
Photons
39
Moving magnetic fields and electrical fields travel in
Waves
40
Wavelength is
Distance between crests
41
The energy of a photon is proportional to
The frequency of the wave it is linked to
42
Wave frequency is
Number of crests that pass a given point in 1 second
43
How do you calculate the speed of a wave
Wavelength x frequency
44
Energy of each quantum can be calculated by
E (energy)= h (plancks constant) x v (frequency)
45
What is a photon
Packets of energy that behave like waves with no mass
46
High energy photons have what wavelength
Short
47
Low energy photons have what wavelength
Long wavelength
48
How does the production of xrays start
Always start with an incoming electron accelerated through an electric field
49
What does strong nuclear force do
Holds protons and neutrons together in nucleus
50
What is photoelectric effect
Xray or photon interacts with inner electron which is bound to shell, energy of photon transferred to electron which is ejected from shell, outer shell electron replaces which releases energy as radiation
51
Photoelectric effects produces what energy
Characteristic energies producing characteristic Xrays (fixed energy released in response to shell movement)
52
What is Bremsstrahlung radiation
Incoming electron attracted to the nucleus slows down and changes direction. Causes energy loss released as an Xray photon
53
Continuous spectra results from and is dependent on
Results from Bremsstrahlung
Dependent on incoming electron energy and atomic number of target
54
Discrete Spectra is linear because
Spikes of particular energy intensities to materials
55
In decay what is made up of a photon
Gamma Ray
56
Subatomic particles existing in unstable arrangements are knowns as what
Radioactive materials
57
What is beam energy
Maximum possible energy of a photon in a beam
58
What is beam quality
A beams penetrating power directly related to the energy range of photons in a beam
59
What is beam fluence
Intensity of beam
The number of photons passing through a unit area
60
How do you calculate beam flurence
number of photons divided by area
61
What does inverse square law describe
Beam intensity dropping with increased distance
62
Rule of energy change to distance in inverse square law
2 times the distance is a quarter of the energy
63
What is attenuation
Beam intensity reduced passing through a material
64
What is u (mew)
Linear attenuation coefficient Reduction in intensity of a photon beam per unit length of material
65
What is mass attenuation coefficient
u/p
mew/density
Reduction in intensity of a photon beam per unit mass of some material
66
Half Value Layer (HVL)
Thickness of a material required to cut the intensity of a beam by half
67
More penetrating beams change half value layer by
Increasing it
68
What is photoelectric effect
Incoming photon interacts with an orbiting electron.
If the photon energy is greater than the binding energy of the electron it is ejected from the atom
69
Photoelectric effect changes with
1)mass density
2)electron density
3)x ray energy
4)atomic number
1)mass density- increase mass density- increased photoelectric effect
2)electron density - increased
3)x ray energy- higher when closer to electron binding energy
4)atomic number- higher atomic number increased photoelectric effect
70
What in imaging uses photoelectric effect
xray and CT
71
What is pair production
Photon passes by nucleus which creates electron and positron, energy of at least 1.022 MeV (high energies) required.
Electron and positron collide forming 2 photons
72
What is the Compton effect
X-ray photon hits loosely bound outer shell electron, absorbs photon energy (enough to eject electron) and is deflected, photon loses some of its energy and is scattered.
Can undergo multiple interactions
73
Compton scatter changes with
1)mass density
2)electron density
3)x ray energy
4)atomic number
1)density- increased with increased density
2)electron density- increased with increased electron density
3)x ray energy- increased with lower x-ray energy
4)atomic number-no change
74
At radiotherapy energy what interaction is dominant
Compton scattering
75
What interaction is dominant in low energies and high Z materials
Photoelectric effect
76
What interaction is dominant in higher energies and low z materials
Compton scattering
77
What interaction is dominant in high energies and high z materials
Pair Production
78
What is a photonuclear interaction
Very high energy photon ejects a nucleon (proton or neutron)
79
What is Auger effect
Photon gives energy to electron which is ejected and electron replaces is from looser shell releasing energy. Energy then hits another electron in the same atom also ejecting that
80
What is scattered radiation
Photon bounces off electron bound to parent atom
81
What is secondary electron effect
Produced after EM interacts with matter via PE or Compton and produces secondary electrons by scatter. These secondary electrons deposit the dose in tissue via electron matter interactions
82
What is linear energy transfer
Amount of energy transferred to a medium by ionising particles per unit distance traversed
83
What describes the energy deposition density of a type of radiation
Linear energy transfer
84
What particles have a high LET
alpha particles
protons
neutrons
85
What particles have a low LET
X-rays
gamma rays
Electrons
86
What is excitation
Electron gaining energy (by second electron) to move within a particle to a higher energy shell, this is unstable so drops back down releasing photon and scattered second electron
87
What is ionisation
Electron gaining enough energy to leave atom resulting in ion pair (free electron -ve and rest of atom +ve)
88
What is elastic interaction
When electron passes nucleus with no change in energy
89
What is inelastic interaction
When electron passes nucleus with energy release (bremstraahlung)
90
What is a soft collision
Particle passes atom at a distance larger than the atomic radius.
Most probable collision resulting in excitation or ionisation
91
What is a hard collision
Incoming electron closer to atom and therefore interact with individual electrons. These electrons have higher energy resulting in auger or characteristic radiation
92
What is collision loss
The process by which electrons lose energy when they interact with orbital electrons in a medium. This interaction can cause the atom to become excited or ionized
93
What is radiative loss
When radiation interacts with matter, electrons can be removed from atoms through a process called ionization
94
What is bremsstrahlung
Electron passes nucleus, attraction makes change of course and slow.
This releases energy as bremsstrahlung xray photon
95
What is stopping power
A materials ability to stop a particle.
Energy lost by unit length. (dE/dz) measure in MvV per cm.
Combination of collision loss and radiative loss. (collision with ionisation and excitation plus bremstrahlung)
96
What is particle range
Distance of a particle travelling through medium before stopping
97
What affects particle range
Medium property- Electron density, ration of atomic number to mass and density
Particle properties- Proportional to Zsquared inversely proportional to velocity squared
98
What causes an electron peak
Electron loses energy and therefore momentum as it travels through medium.
Low mass of electron means increased large angle scattering so no well defined peak
99
What effects particle ranges
More range in high energy beams and low Z materials
100
For electrons and protons of the same energy which has a higher range
Electrons
Protons heavier and therefore more likely to interact
101
Protons create what shape on a depth dose curve
Bragg peak
deep dose with sharp fall off
102
How to cover target with proton beams and a bragg peak
1. Increase Scatter or spread out the bragg peak
Use materials in beam to spread out energy
2. Spot scanning - multiple beams of different energy with different bragg peak
distances
103
How do you achieve depth in proton beams
Higher energies
104
What causes directly ionising radiation?
Electrons
105
What causes indirectly ionising radiation?
Photons
106
What is exposure?
Electric charge freed by ionisation per unit mass of air
107
What is the exposure equation
Exposure= Q/m
Q= charge produced when all electrons creasted have stopped
m=mass of air
108
What is KERMA
Kinetic Energy Released per unit Mass.
Kinetic energy of all the charged particles created by the ionisation process
109
What is the KERMA equation
KERMA= Etr/m
Etr= Mean energy released by ionisaing radiation
m=mass of material
110
What is KERMA measured in?
Units of joules per kilogram
Gy
111
What is absorbed dose?
Energy absorbed per unit mass of material
112
What is the absorbed dose calculation?
Absorbed dose= Eab/m
Eab= energy absorbed by material
m= mass of material
113
What is absorbed dose measured in?
Units of joules per kilogram
Gy
114
When are absorbed dose and KERMA equal?
Charged particle equilibrium
115
What is charged particle equilibrium?
When the energy (electrons) going into a medium are equal to the number going out
116
How to calculate absorbed dose
Absorbed dose= KERMA-Eout+Ein
117
CPE (Charged particle equilibrium) can only exist in what conditions
Medium is homogenous, Negligible attenuation of photon irradiation within volume, no inhomogenous electric/magnetic fields
118
When is the kerma proportional to the absorbed dose
After the Dmax
119
What is the difference between absorbed dose and KERMA
KERMA= where energy is release
Absorbed dose= where energy is eventually absorbed
120
How do we realted the dose measured in one material to the dose in another material?
Multiply the ratio of mass energy absorption coefficients
121
How does absorption of radiation vary in different tissues
Muscle, water air- Similar absorption with similar atomic number
In bone-Lower energy = higher absorbed dose (more photoelectric effect)
122
How do ionisation chambers work
Air chamber with eletrodes either side. Beam through air chamber, atoms and electrons measured either side therefore calculation dose by ionisation
123
What is a farmer chamber?
Central electrode made of aluminium
Outer electrode graphite
Graphite used as air equivalence
124
Electrometer role in measuring dose
Supplies electrode voltage and measures charge or current
125
What can effect volume of gas in a chamber?
Temperature and pressure.
126
What is the polarity effects?
Charge measured differs depending on polarity of collecting electrode
127
What is calorimetry?
Calculated deposited dose from temp rise in result of deposited energy and ioninsation
128
How does Radiographic film work?
Contains silver bromide, radiation exposure causes chemical change reduction to metallic silver which causes blackening
129
How does thermoluminescence measure radiation
Energy gain in atom from grounded state to conductive state, release of photons as light energy to go back to ground state, light output measured
130
What is a scintillatory counter
Generates light in response to radiation. Converts light to electric signal for dose measure
131
Whats re 2 common measurement devices used in vivo dosimetry
Diodes and thermolumiescnece detectors
132
How do you calculate an isodose plot
combination of beam profile and percentage depth dose (PDD)
133
How is PDD measured?
Chamber in a water tank moved up and down measured dose of beam at different points
134
How is tissue maximum ratio measured
Chamber in water tank at fixed isocentre but varying depth of water above it
135
What is a tissue phantom ratio?
Chamber in water at fixed isocentre but water depth above it varies. Difference measure between 2 depths ie. 10 and 20cm
136
What is relative dosimetry
looking at how dose chnages rather than actual measurements
137
What is absolute dosimetry
Direct dose measurement
138
What is Reference dosimetry
Dose derived from measurement using standard set up and calibrated against absolute dosimetry
139
What does code of practice in RT set out?
What primary standard is
What equipment you can use
Provision of scientific basis/reason for the code
140
What is primary standard for callibration
Nationally maintained (middlesex) measure in air KERMA
141
How is national primary standard callibrated
air KERMA EBRT
Graphite calorimetry electrons and protons
142
How is local callibraton done
Thimble ionisation chambers, farmer chambers and geiger counter, nationally calibrated every 3 months
143
What is beam quality?
Described how penetrating the beam is, higher energy, more penetrating
144
How do we measure beam quality in kV
Half value layer (HVL)
145
How do we measure beam quality in MV
Tissure phantom ratio (TPR) 20,10
146
How do we measure beam quality in MeV (electrons)
R50D
147
What is output in RT dosing
Dose per monitor unit
Can use to tell linac how many MU to deliver in order to acheive dose
148
What measures charge in dose delivery
Ionisation chamber in linac head which related to dose during calibration
149
What are the reference conditions for calibrating a linac
10x10cm field size, 100cm focus source distance (FSD), reference depth
150
What is the difference between the primary standard and secondary standard
Primary is national standard laboratory. Secondary is local and shared between multiple local locations calibrated against primary standard
151
What is R50D
The depth at which the dose has dropped to half the maximum value
152
Xray energy range for superficial
-Treatment potential
-Depth
-Use
Xray energy range for superficial
-Treatment potential 50-160kV
-Depth <5mm
-Use skin
153
Xray energy range for orthovoltage
-Treatment potential
-Depth
-Use
Xray energy range for orthovoltage
-Treatment potential 160-300 kV
-Depth <6cm
-Use bone, ribs, skin shallow
154
Xray energy range for megavoltage
-Treatment potential
-Depth
-Use
Xray energy range for megavoltage
-Treatment potential- >1MV
-Depth <30cm
-Use internal organs
155
What is an isocentre
a point in space about which everything rotates
156
What is FSD
Focus to skin distance
157
What is SSD
Source to surface distance
158
What is Dmax
Depth of maximum dose
159
What interaction is dominant in low energy beams
photoelectric
160
What interaction is dominant in mid energy beams
compton
161
what interaction is dominant in high energy beams
compton
162
What range of energy beam has the most back scatter factor
medium
163
Why do mid range energy beams have the highest back scatter
low energy dont penetrate enough to cause back scatter, high energy photons predominantly scatter forwards
164
PDD increases with
Increasing energy- more penetrating, less attnuated
Increased FSD
Increasing field size
Wedges, harder beam, low energy photons attenuated, high energy remain and more penetrating
165
Why does FSD effects PDD
More scatter with higher FSD
166
What do wedges do
Preferentially attenuate low energy photons
Beam hardening
Increase in average energy
Increased PDD
Adds asymmetry to beam
167
What is pemumbra
Region between 80-20% dose
168
What happends to penumbra with increased depth
gets larger
169
Whats is geometric field size
projection of front edge of collimator by lines drawn for centre of dose, correspons with 50% isodose line
170
What is dosimetric field size
Area enclosed by specific isodose lines
171
what causes penumbra
geometric cause- focakl spot and beams radiate out from it
Transmission- rays moving from source interacting with jaws (MLCs)
Dosimetric- scatter depends on energy
172
What does wedge do to a beam profile
add obliquity with beam hardening increasing dose
173
Effect on isodose curve- energy
Energy- Higher energy higher penetration
174
Effect on isodose curve- FSD
FSD- Increased FSD higher penetration, less skin dose
175
Effect on isodose curve- field size
Field size- Less steep fall off of curve with scatter and contamination. Higher skin dose higher field size.
176
Effect on isodose curve- surface obliquity
Surface obliquity- surface dose increases with oblique angle, more electron cross over
177
Effect on isodose curve- inhomogenous media
Inhomogenous media- Attenuation decreased when less dense
178
Effect on isodose curve- wedge
Modifys curve by increasing absoprtion and therefore protecting sensitive organs
179
What is a monitor unit
Measure of dose leaving linac head unit
180
How is monitor unit prescribed
Energy required to meet determined dose at isocentre.
Therefore increased density, depth, dose higher MU required
181
MU=
MU= dose per fraction (xfieldsize and z depth)
divided by
W (wedge factor) X T (Tray factor) X S (Scatter factors) X PDD (distance and depth) X machine calibration factor
182
Equivalent square theory
c= 2xaxb/a+b
c= 4xarea/perimeter
Turns into square shape equivalent
183
Fixed SSD formulas
dose/depth factorx output factor
X
wedge
X
tray
184
Dose distribution of electron beams effect by energy
Energy- range straggling means higher surface depth dose at higher energies with deeper dose
185
For electron beams at what depth is dmax?
2x energy
186
For electron beams at what depth is treatment depth R90
3xenergy
187
For electron beams at what depth is 50% dose R50
4x energy
188
For electron beams at what depth is practical range Rp
5x energy
189
Dose distribution of electron beams effect by field size
Minimal change too smallfield size smaller dose
190
Dose distribution of electron beams effect by build up dose
Build up as electron travel needs obliquity to deposit dose, upside down tree
191
Dose distribution of electron beams effect by obliquity and surface inhomogeneity
more obliquity more dose deposition
192
How do electrons scatter in relation to central axis
scatter away from central axis
193
Electron dose calculation rule?
2 3 4 5 rule
Dmax, R90, R50, Rp
194
Effect of applicator size on percentage depth dose of electrons
>10cm PDD independent of applicator size
<10cm strongly dependent on applicator size
195
As electron energy increases
Depth of dmax...
Relative surface dose...
Absolute surface dose....
Depth of dmax- increases
Relative surface dose- increases
Absolute surface dose- no significant change
196
Applicator size should be what in comparison to target size
1cm larger due to dose drop off at peripheries with scatter
197
What changes in elecrton beams when you increased the FSD
Penumbra becomes broader, does not chnage treatment area, causes absolute dose rate to decrease (inverse quare law)
Dose becomes more penetrating
198
Why does dose become more penetrating in electron beams with increase FSD
Inverse square law- relative dose increases
199
What causes dose enhancement effect with lead shielding?
Electrons scattered from hitting lead have lost energy and therefore absrobed very close to skin
200
How does bolus effects electron field dose
Gives artificial building up region therefore causing high dose at skin surface
201
What is the role of wax around lead shielding
Absorbs back scattered dose
202
What happend when electron beams hits surface obliquely?
isodose lines follow surface contour (oblique) bring dmax closer to surface
203
How does density effect electron dosing at depth
Less dense tissue allows for deeper dose deposition and more dose deposited
204
How does PET imaging use positrons to detect tumour location
Patient injected with F18-FDG cannot be distinguished from glucose.
F18 positron emitter
Positron annihalation gives 2 photons in opposite directions
Simultaneous detection =location of tumour when fused with CT
205
Considerations for patient immobilisation
Reproducibility
Patient comfort
Technique of set up
Aim rto reduce exposure to healthy tissue
Practical aspects- is equipment radioopaque
206
What is the role of verification imaging
accurate localisation and highlights modifications needeed to adjust to weight loss, bowel prep etc
207
Difference between CT and cone beam CT
Lower resolution images, useful for boney markers and some soft tissue info but not diagnostic
208
Techniques to counter patient breathing motion
Abdominal compression
Active breathing coordinator (ABC)
Varian RPM- box on chest wall
Vision RT- light feedback
4DCT
4DCBCT
209
4DCT planning is created how
Calculated max inhalation and exhalation and split into 'bins' which are matched across breathing pattern
composite images show regions over which tumour travels
210
What is PTV
planning target volume, movement or discrepancies in target
211
What is OAR in radiotherapy
Organs at risk defined by normal tissue complication probabilities
212
What is GTV
gross tumour volume, demonstratable extent of disease by imaging location
213
Whats is CTV
clinical target volume, volume treated to cover subclinical spread (cells)
214
What is ITV
internal treatment volume, variation of internal anatomy
215
PTV, OAR, CTV, GTV and ITV are outlined in which ICRU report
ICRU 50
216
ITV is amde up of what?
ITV- internal treatment volume
IM+ CTV
IM= expected motion of CTV within patient because of anatomy
217
PTV is made up of what?
ITV- internal tretament volume
+
SM- set up margin- inaccuracies in patient set up
218
What defines a serial organ at risk
a dose above tolerance to any portion of the organ will damage the whole organ function e.g. spinal cord- old christmas lights
219
What defines a parralel organ at risk
When part of organ receives over treatment dose, organ still functions- new christmas lights
220
Dose refernce point should be placed where
Area central axis of beam, with tissue homogeneity and representative of treatment volume
221
ICRU 62 recommends what for dose reporting
Dose volume histograms for PTV and OARs
222
What is PRV in radiotherapy planning
Planning organs at risk volume,OAR grown by margin to allow movement
223
What is IV in radiotherapy planning
Irradiated volume- volume of tissue receiving significant dose
224
What are isodose lines
link points receiving same dose like contour lines on a map
225
When reading a DVH V20Gy would want you to find what?
Volume exposed to 20Gy
226
When reading a DVH D50% would want you to find what?
Amount of dose 50% of the volume is exposed to
227
Prescription depth in single field RT
Prescriptoin depth, Dmax
228
Prescription dose in parallel opposed pair
Mid plane
229
Prescription dose in IMRT
Centre of target volume
230
Why adjust beam angle
Short distance form skin surface, avoid critical structure, shape teated volume around PTV
231
Characteristics of isodose curve in single field
-increased space with depth (flattened beam curve)
- widening penumbra depth
-increased rounding depth
232
Characterstics of isodose curve in parallel opposed beams
100% dose at isocentre but hot spots with exit/entrance cross over dose
233
When blocking is introduced what chnage occur in PTV
Primary radiation unaffected
Secondary radiation redcued
Central dose depends on scatter
234
What does sector intergration calcuate
Scatter contribution in irregualr blocking
235
ICE definition of a wedge angle
Angle of an isodose curve to a plane at right angles to the central axis at a depth of 10 cm
236
When might a wedge be used
minimise a dose gradiant across a PTV, compensate for missing tissue with patient shape ie. breast
237
Beam weighting is used for what
Improve dose uniformity across target volume
238
Pros and cons of coplanar and non coplanar planning
Coplanar- irradiated cvolume all in same slab of tissue, superior and inferior borders well defined
Non coplanar- PTV coverage improved, irradiated volume irregular shape, dose may fall off more gradually
239
Difference infixed SSD treatment and isocentric treatment
isocentric treatment- reference point in tumour always 100cm
fixed ssd- skin surface distance always the same
240
Pros and cons of fixed SSD treatment and isocentric treatment
SSD- simple data checking reducing error but also complex set up with requirements for patient to move
Isocentric- simple quick set up, different beam data
241
Shorter SSD causes what changes to dose, divergence and PDD
higher dose rates, greater divergence, greater PDD
242
Longer SSD causes what changes to dose, divergence and PDD
Lover dose rates, less divergence, smaller PDD
243
What is field matching
2 beams with parallel central axis, turn central axis (alter gantry angle) so 50% dose line parallel or block half beam therefore abutting central axis
244
What is a tangent pain in radiotherapy planning
Central axis of beam modified with gantry therefore beams at a tangent but central axis parallel
245
What is ARC therapy
Beam moves continuously around patient with a fixed field size
246
What is IMRT
Intensity modulated raiotherapy
247
How does IMRT create complexity in the beam
MLCs- step and shoot when beam off or sliding windows when beam is on
248
ICRU 83 updates what concepts with increasing IMRT use
Emphasis on DVH reports
Importance of OAR outlines
Reporting near maximum and near minumum point doses
ICRU reference point replaced by median dose
249
What is VMAT
IMRT but MLC and Gantry moves at same time. Dose rate and gantry speed can be modified through treatment
250
Benefits of VMAT
Improved treatment efficiency, smoother more conformal dose.
Can increase low dose 'bath'
251
Characteristic radiation is what
Energy given to electrons in shells, enough to reject.
Rejected electron leaves unstable atom and gap filled by outer electron.
Outer electron releases photon in predictable peaks
252
Bremstraahlung is what
Electron slows in field of nucleus, slowing electron releases energy as photon. Amount of energy varies- spectrum.
253
Higher electron energies have what direction of scatter
forwards
254
Lower electron energies have what direction of scatter
all directions, many colisions and varied pattern of attenuation
255
What is anode heel effect
Absorption of energy in beam target causing less intensity of beam proflie relative to target
256
How can you counter heel effect in beam profile
angling anode balanced with moving beam intensity
257
What is beam hardening
Filtering out low enrgy photons in order to increase penetration of photons and reduce surface dose
258
What is beam quality
A measure of penetration of the beam, depends on energy sprectrum. Hard beam has more energetic photons than a soft beam
259
What is quantity in xray emission
intensity of xrays
260
Name beam quality measures
HVL
D10- PDD at 10cm for a 10x10cm field with an SSD 100cm
R50D- depth that dose is half Dmax
PDD ratios
261
What measure do we use for kV beam quality
HVL
Measured using narrow beam geometry
262
Describe kV PDD
Dmax at surface
Steep drop off with depth
higher dose beyond treatment depth when compared with electrons
263
Talk through photon production in a Linac
Electron production in filament, focussing coils and steering coild, bending magntes to align beam, target
264
Power supply of linac deliver radiation in
pulses using pulse modulator.
265
In a linac, how does the electron gun filament work
heated releasing electrons from material surface. Attracted into linac by anode. Then accelerated by RF waves
266
In linac, how does waveguide work
metal tube with cavities along length. Buncher section tighter bunches electrons gained in pulses. Relativistic section, gain energy but not speed
267
In linac, how does a focussing coil work
magnetic fields wrapped around wave guides to focus beam
268
In linac, how does steering coil work
magnets at start and end of waveguide to straighten beam
269
In linac, how do bending magnets work
electron beam bent at end of waveguide slalom or 270 degree bend
270
What is beam quality
The penetration of a beam
271
How do we measure beam quality in an MV photon beam?
TPR20/10, ratio of readings taken at 10 and 20cm depth
272
How do we measure beam quality in electron beams
R50D, depth at which measured dose is half od Dmax
273
What is a target in a linac head
Electrons produced by bremstraahlung scatter via compton scattering through target with a forward momentum towards flattening filter
274
What is a flattening filter in a linac head
Filter with thicker area in centre, taregt produces highe beam intenisty centrally than peripherally, flattening filter attenuated central dose to produce flat beam profil
275
In electron beams what is used instead of a flattening filter
Scattering foils
276
Scattering foils acheive what in an electron beam
Scatters electron beam to be more clinically useful form very narrow beam
277
What do primary collimators do in a linac head
reduces a beam to its maximum size, reduces scatter outside of head
278
What do secondary colimators do
Jaws that set field size, adjustable and furtehr reduce scatter from linac head
279
What are the two sources of penumbra
Transmission and geometric
280
How does transmission penumbra occur
Some electrons travel through collimators with a degree of attenuation due to beam divergence
281
How does geometric penumbra occur
At beam edge looking up can only see part of source, this increases with source size, distance to patient and distance of source
282
What is monitor chamber in linac head
Ionisation chamber in linac head monitors linac output. Thin plate to minimise attenuation.
2 plates, one terminates once MU reached and second works as back up.
Can also check symmetry of beam
283
What is a monitor unit
Meausres output of a linac, output required to deliver 1cGy in reference conditions
284
How can we add a wedge to a beam
Physical wedge- placed infront of beam
Mechanical/universal wedge- 60 degree wedge in linac head
Enhanced dynamic/virtual beam- MLC/jaw moves across field in colimator
285
What is the role of a multileaf colimator
Attenuates non useful parts of beam to confrom to shape
286
What does tray factor account for?
Lead shielding placed in tray at linac head
287
Why does colimator need to be close to the patient in electron beams
Electrons ha a high scatter factor in air
288
Stereotactic treatment requires what MLCs
Small and fine MLC to increase precision
289
What localisation meathods are used in gamma knife or stereotactic treatement
Rigid frames most reporoducible
XVI imaging and thermoplastic head shells less accurate
290
What is an isocentre
Point in space 1m from radiation source where machine and couch rotate around
291
What is a sealed source
Radioactive material which in encapsulated in a container
292
What is an unsealed sourse
Radioactive material administered in liquid form directly into patients body- IV or PO
293
What is an isotope
Same number of protons but different number of neutrons, same chemical properties but different atomic mass
294
What happens to an atom in radioactive decay
Unstable nuclei split into smaller particle and releases energy
295
In alpha decay what is produced
2 protons and 2 neutrons
296
In beta decay what is produced
An electron ot positron
297
In gamma decay what is produced
a gamma ray (high energy photon)
298
What is physical half life
The time point where half your atoms have decayed
299
What is inverse square law
From a point source, beams diverge out and therefore density of beam spread out
new dose rate=old dose rate x (old distance/newdistance)squared
300
What is an A point in brachytherapy
2cm anterior and lateral to dose point to cover malignant tissue
301
What is the manchester pear in brachytherapy
3D demonstration of the A point in brachytherapy
302
What is hot loading in brachytherapy
Preloaded applicator with sources prior to placement
303
What is afterloading in brachytherapy
Applicator in place before source loaded
304
Rules with radiotherapy exposure
Time distance container
305
Sealed RT is measure in
Curie (Ci)
306
Unsealed RT is measured in
Becquerel (Bq)
307
What is biological half life
time taken for the body to eliminate half the amount of substance
308
what is physcial half life
Time taken for half of radioactive atoms to decay by half
309
What is effective half life
Time taken for administered radioisotope to reduce by half from physical and biological process
310
Radium 223 is what radiation
Alpha particles
311
Lutetium-177 is what radiation
Gamma and beta
312
Risk of radiation exposure with unsealed RT
Close contact pregnant women & young children
Bathroom hygeine
Seperate toilet where possible
No sexual activity
Travel- not directly adjacent to other passengers
313
What is deterministic effect in radiation
Dose threshold that causes loss of function or cell death
314
What is stochastic effect in radiation
Mutated cells that dont die but can lead to complications like cancers
315
What is ALARP
As low as reasonably practicable - radiation exposure
316
What dose is used to measure stochastoc effects in radiation
Equivalent dose= absorbed dosex radiation weighting factor
Effective dose= equivalent dose x organ weighted factor
317
What dose is used to measure deterministic effects
Absorbed dose =Gy
318
Hazards of radiation eposure include
External radiation, contamination, internal radiation
319
Alpha radiation requires what minimal shielding
Piece of paper
320
Beta radiation requires what minimal shileding
Aluminium, perspex
321
Gamma radiaition requires what minimal shielding
lead, concrete
322
Increased distance affects change to radiation how
Inverse square law, doubling distance reduces radiation to a quarter
323
Dose of radiation exposure is effected how my time
Proportional chnage ie. helf time exposed half radiation exposed
324
How to reduce radiation exposure when drawing up an unsealed source
Using a vacuum cupboard
325
What UK legislation is used in UK radiation exposure guidance
ICRP
326
What is the ICRP
International commission on radiological protection
327
What is the IAEA
International atomic energy agency- ntergovernental organisation whihc uses ICRP to publish safety standard as global reference
328
EU legislation for radiation
Provides guidance via EURATOM for EU members- UK now associate member
329
UK legislation for radiation
IRR- Protects public
IRMER- Protects patients
330
Radiation protection legislation is based on what 3 principles
Justification- benefit outweighs risk
Optimisation- exposure to as few and as low as possible
Dose limitations- on an individual level
331
What is the IRR 17
Ionising radiations regulations 2017- staff and public enforced by HSE in order to keep ALARP
332
Dose limits of employees over 18
Body
Lens of eye
Skin
Extermities
Body- 20Sv
Lens- 20Sv
Skin- 500Sv
Extremities- 500Sv
333
What are TLDS
Thermoluminescent dosinmeter, worn for 3 months and dose exposure chekced, body badges and finger badge for handling unsealed raidation
334
IRMER is applicable to who
patients, carers and comforters
335
For adjacent public areas to radiation what dose constraint is worked to
0.3Sv
336
For staff who dont work with radiation and memeber of public what is the target dose constraint
<1Sv
337
What materials are used for Linac bunkers
Concrete, steel, lead
338
What do primary barriers protect against in radiation room design
Where primary beam strikes wall
339
What do secondary barriers protect against in radiation room design
Scattered radiation and leakage radiation (through equipment shielding)
340
What is the design of a linac maze/door
Ensures no direct path for scatter to leave the room, dose must be calculated at maze door
341
What is an interlocked entry to a linac
Last man out button on closure of door or gate, beam turns off when someone walks in
342
What is monte carlo modelling?
considers photon interaction probability for every interaction and then every interaction of resulting electrons and photons to calculate dose. Too large number but gold standard.
343
What is pencil beam modelling
Uses kernels based on monte carlo
344
What are kernels in modelling
Map of dose from energy of beam, added into required shape lateral scatter not considered so dose can be under/overestimated e.g breast gets less scatter from lung
345
Whats is superposition algorithm?
dose scaled in kernels in proportion to density of materials travelled through.
346
What is field matching
Aligning the edges of adjacent radiation beams to ensure a smooth transition in dose distribution between them, preventing gaps or overlaps.
Overlap=hotspot
347
What is quality assurance
standard of practice to maintain quality. Ie. procedures, records, documentation and definition of responsibility
348
what is quality control
test or monitoring to ensure criteria are met
349
According to PM77 what is reported as over dose in a course and one treatment
10% over a course
20% in one dose
FRCR
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