Term Test 2

Question 1

what allows for an accurate prescribed dose to a target volume

Answer 1

Accurate calibration at reference conditions in a uniform water phantom

Dose at any point in patient must be calculated and correlated to the calibration dose

Question 2

What do we have to consider when calculating photon dose

Answer 2

1) non infinite patient
2) inhomogeneities

Question 3

Deposition of energy from a photon beam has two stages. What are they

Answer 3

1. TERMA : total energy released per unit MAss (interaction of a photon with an atom: energy is transferred from photon to charged particles and scattered photons) \
2. Electrons set in motion (KERMA) then transfer energy to tissues via excitations and ionizations (DOSE step)

Question 4

How do inhomogeneities change TERMA and dose

Answer 4

• absorption of primary photon beam (changes number of photon interactions - changes in the probability of attenuation (changes in u and u/p)
• pattern and mean free path of scattered photons (mean free path is the average distance between photon interactions)
• change number in number of electrons produced
• change in range of electrons produced

Question 5

What is charged particle equilibrium (CPE)

Answer 5

• number of electrons entering and leaving small volume are equal, so ionizations due to all tracks are accounted for

Question 6

What is needed for full CPE

Answer 6

• must be along axis beam and laterality

Question 7

Why is calculating dose easier when CPE is established

Answer 7

• do not have to calculate all electron paths

Question 8

When does CPE exist

Answer 8

• volume is surrounded by material with same properties
• minimum thickness equal to maximum range of electrons produced

Question 9

When (or where) does CPE not exist

Answer 9

• in the build up region
• for very small fields and high photon energies
• at the interface between tissues with different properties
  At beam edges

Question 10

What is the relative PRIMARY photon interactions in a low density inhomogeneous tissue compared to homogenous tissue with higher density

Answer 10

Fewer in photon interactions in lower densities

Question 11

What is the relative scattered photon interactions in a low density inhomogeneous tissue

Answer 11

Number of scattered photons will be similar to a water density but the average energy of scattered electrons will be greater . Scattered photons have more space to move around and interact with other photons to set them in motion. Therefore more dose in less dense areas

Question 12

What is the relative electron interactions in a low density inhomogeneous tissue

Answer 12

Average energy of electrons is higher in lower density but dose will be equal or slightly greater

Question 13

Within a low density inhomogeneity (where CPE exists) describe the number of photons, energy of scattered photons, and secondary electrons relative to a homogeneous situation

Answer 13

Number of primary photons is much greater
Energy of scattered photons is slightly greater
Secondary electrons have a slightly higher energy

Overall: dose is greater in lower density

Question 14

When going from water, to air, to back to water, describe the dose at each interface

Answer 14

• interface 1: water into air - dose drops due to loss of photon/electron back scatter
  Interface 2: interface at air.- dose drops due to loss of CPE
  Interface 3: air to water - dose increase due to increase in electron back scatter
  Interface 4: water start - dose build up (drops locally) higher dose than if we had all water

Question 15

What is the bone density

Answer 15

1.69

Question 16

In bone inhomogeneties, for MV photons, which interactions dominate?

Answer 16

• Compton interactions dominate

Question 17

What is the atomic number of bone

Answer 17

13.5

Question 18

In bone homogeneities for KV photons, which interactions will dominate

Answer 18

Photoelectric interactions

Question 19

For KV photons and MV photons, what is their comparison to dose in water at the same point?

Answer 19

• MV dose will be lower
• KV dose will be higher
• dose to bone will be the same

Question 20

For very high energy photons, which interactions will dominate in bone inhomogeneties

Answer 20

• pair production

Question 21

What are some correction based algorithms

Answer 21

• flat contour
• homogenous
• dose measures at points along central axis
• symmetric field
• beam axis perpendicular to phantom surface
• infinite volume relative to range of scatter

Question 22

Due to primary photons, how does energy and depth effect dose ?

Answer 22

• dose increases as energy increase
• dose decreases as depth increases

Question 23

Due to scattered, how does field size, energy and depth effect dose ?

Answer 23

• dose increases and field size increases
• dose increases as energy decreases
• dose increases as depth increases

Question 24

With respect to PDD , what effects do SSD, beam energy, and field size have on it

Answer 24

PDD increases with
- increased SSD
- increased beam energy
- increasing field size increases

Question 25

TPR ratio fill in the blanks Compare doses in _______________________ Accounts for the _______ and the _________ at ________

Answer 25

- the same horizontal plane - depth , field size, depth

Question 26

Dose for low energy techniques - ________ nominal SSDs - effects of surplus / missing tissue substantial as __ is relatively closer to ____ - _____ change small so not considered - _____ change not relavent as typically prescribes at surface or very near - _____ factor VERY significant

Answer 26

small h , ISL FS PDD ISL

Question 27

how does a tissue surplus effect field size at surface, SSD, and dose at depth/point

Answer 27

decreases as there is a tissue surplus

Question 28

how does a tissue deficit effect field size at surface, SSD, and dose at depth/point

Answer 28

increses as their is a tissue deficit

Question 29

what is the beam obliquity effect

Answer 29

- increasing this results in the same pattern of spread for electrons but angled towards skin surface, reducing skin sparing - when beam is perpendicular, electron scatter projects away from skin

Question 30

what is the clinical requirement for a uniform dose percentage throughout a treatment plane

Answer 30

95-105%

Question 31

what do missing tissue compensators do

Answer 31

achieve uniform dose in a plane orthogonal to beam axis

Question 32

what are examples of physical compensators

Answer 32

bolus metal or other non tissue equivalent material wedges

Question 33

What are examples of automated compensators

Answer 33

- moving collimators (virtual wedging) - intensity modulated radiation delivery (field segmentation, MLCs)

Question 34

what are the advantages of bolus

Answer 34

inexpensive malleable quick to implement

Question 35

what are the disadvantages of bolus

Answer 35

- can have air gaps - questionably reproducibility - increases skin dose

Question 36

why do we use wedges

Answer 36

- contour corrections - fix dose gradient

Question 37

how do we use wedges

Answer 37

- missing tissue compensator - dose gradient compensator

Question 38

what do wedges do to beam quality

Answer 38

- physical wedges preferentially attenuate lower energy photons - therefore beam hardening occurs which creates a higher energy beam

Question 39

what are the 3 different types of wedges

Answer 39

manual : universal / motorized virtual / enhanced dynamic

Question 40

what does the presence of a wedge change

Answer 40

- the shape of the isodose lines based on the wedge angle - absolute dose at the reference depth which will be equal to the wedge factor

Question 41

in wedges, theta is

Answer 41

- angle by which an isodose curve is tilted - defined at the central axis - defined for a specific depth

Question 42

physical wedges may increase field size

Answer 42

true

Question 43

wedge factor will increase with increasing photon energy with the same wedge and field size

Answer 43

true

Question 44

what are the advantages of a non physical wedge

Answer 44

- automation of treatment delivery - less peripheral dose eg contralteral breast - ergonomics and safety - cannot be stopped

Question 45

what are the disadvantages of a non physical wedge

Answer 45

- greater dosimetric complexity in acquisition of commissioning data beam modeling for TPS and MU calcs

Question 46

what are the limitations of physical or virtual wedges

Answer 46

- dose compensation can only be achieved in a single plane - for non standard angles, combine open and wedged beams - as we have seen some wedges may have field size limitations

Question 47

what is the hinge angle

Answer 47

angle between central axis of the two beams

Question 48

how do you calculate optimal angle

Answer 48

ca theta = 90 - hinge angle / 2

Question 49

how does scatter dose occur

Answer 49

- scatter off field tray - scatter off blocks - internal scatter (about 5% beyond 2 cm from field edge)

Question 50

what is the difference in MLCs between Elekta and varian

Answer 50

elekta 40-80 pairs of leaves, varian has 40 or 60 elekta leaves replace one jaw set elekta jaw from other direction under the MLC leaves , varian has backup collimator above MLCs elekta calculations are done using effective field size varian has a black up collimator above MLC leaf Varian SC is jaw setting Elekta Sc is MLC setting

Question 51

what are the field shaped options and what do we use

Answer 51

- infield placement - out of field plaement - cross boundary placement - midlead (use)

Question 52

what are the advantages of MLCs

Answer 52

- flexibility in shaping fields -conformality to target - efficient - IMRT and modulation of beam intensity -IMRt and modulation of beam intensity dynamic wedges and compensation

Question 53

what are the disadvantages of MLCs

Answer 53

- unable to create every shape - scalloped or stepped shape in distribution - interleaf leakage -1.0-1.5% transmission

Question 54

what are the rules for sementation and shaped ports

Answer 54

- minimum leaf gap > 1.0 cm - minimum field opening >= 4x4 cm for 6MV - minimum MU >= 5MU

Question 55

what is the sc for varian, elekta, and siemens

Answer 55

varian: tertiary collimation system (collimator) elekta: upper jaw replacement (MLC) Siements: lower jaw replacement

Question 56

short answer: explain how the presence of a low density tissue heterogeneity will effect the distribution of dose within a patient

Answer 56

dose within a low density heterogeneity will increase due to a reduction in the attenuation of the beam through the lower density medium, compared to a uniform water equivalent density . this does not account for close to the field edge

Question 57

Short answer: what is the difference between TERMA and absorbed dose

Answer 57

terma: total energy released by primary photons at the point of primary photon interaction with the medium absorbed dose: the dose absorbed in the medium as a result of secondary interactions occurring at the points within the medium. includes also results from electrons causing ionizations and excitations