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Treatment Planning Term Test 2 > Term Test 2 > Flashcards

Term Test 2 Flashcards

1
Q

what allows for an accurate prescribed dose to a target volume

A

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

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2
Q

What do we have to consider when calculating photon dose

A

1) non infinite patient
2) inhomogeneities

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3
Q

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

A
  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)
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4
Q

How do inhomogeneities change TERMA and dose

A
  • 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
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5
Q

What is charged particle equilibrium (CPE)

A
  • number of electrons entering and leaving small volume are equal, so ionizations due to all tracks are accounted for
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6
Q

What is needed for full CPE

A
  • must be along axis beam and laterality
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7
Q

Why is calculating dose easier when CPE is established

A
  • do not have to calculate all electron paths
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8
Q

When does CPE exist

A
  • volume is surrounded by material with same properties
  • minimum thickness equal to maximum range of electrons produced
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9
Q

When (or where) does CPE not exist

A
  • in the build up region
  • for very small fields and high photon energies
  • at the interface between tissues with different properties
    At beam edges
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10
Q

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

A

Fewer in photon interactions in lower densities

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11
Q

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

A

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

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12
Q

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

A

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

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13
Q

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

A

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

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14
Q

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

A
  • 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
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15
Q

What is the bone density

A

1.69

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16
Q

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

A
  • Compton interactions dominate
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17
Q

What is the atomic number of bone

A

13.5

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18
Q

In bone homogeneities for KV photons, which interactions will dominate

A

Photoelectric interactions

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19
Q

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

A
  • MV dose will be lower
  • KV dose will be higher
  • dose to bone will be the same
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20
Q

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

A
  • pair production
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21
Q

What are some correction based algorithms

A
  • 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
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22
Q

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

A
  • dose increases as energy increase
  • dose decreases as depth increases
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23
Q

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

A
  • dose increases and field size increases
  • dose increases as energy decreases
  • dose increases as depth increases
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24
Q

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

A

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

25
Q

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

A
  • the same horizontal plane
  • depth , field size, depth
26
Q

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

A

small
h , ISL
FS
PDD
ISL

27
Q

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

A

decreases as there is a tissue surplus

28
Q

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

A

increses as their is a tissue deficit

29
Q

what is the beam obliquity effect

A
  • 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
30
Q

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

A

95-105%

31
Q

what do missing tissue compensators do

A

achieve uniform dose in a plane orthogonal to beam axis

32
Q

what are examples of physical compensators

A

bolus
metal or other non tissue equivalent material
wedges

33
Q

What are examples of automated compensators

A
  • moving collimators (virtual wedging)
  • intensity modulated radiation delivery (field segmentation, MLCs)
34
Q

what are the advantages of bolus

A

inexpensive
malleable
quick to implement

35
Q

what are the disadvantages of bolus

A
  • can have air gaps
  • questionably reproducibility
  • increases skin dose
36
Q

why do we use wedges

A
  • contour corrections
  • fix dose gradient
37
Q

how do we use wedges

A
  • missing tissue compensator
  • dose gradient compensator
38
Q

what do wedges do to beam quality

A
  • physical wedges preferentially attenuate lower energy photons
  • therefore beam hardening occurs which creates a higher energy beam
39
Q

what are the 3 different types of wedges

A

manual :
universal / motorized
virtual / enhanced dynamic

40
Q

what does the presence of a wedge change

A
  • 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
41
Q

in wedges, theta is

A
  • angle by which an isodose curve is tilted
  • defined at the central axis
  • defined for a specific depth
42
Q

physical wedges may increase field size

A

true

43
Q

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

A

true

44
Q

what are the advantages of a non physical wedge

A
  • automation of treatment delivery
  • less peripheral dose eg contralteral breast
  • ergonomics and safety
  • cannot be stopped
45
Q

what are the disadvantages of a non physical wedge

A
  • greater dosimetric complexity in acquisition of commissioning data beam modeling for TPS and MU calcs
46
Q

what are the limitations of physical or virtual wedges

A
  • 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
47
Q

what is the hinge angle

A

angle between central axis of the two beams

48
Q

how do you calculate optimal angle

A

ca
theta = 90 - hinge angle / 2

49
Q

how does scatter dose occur

A
  • scatter off field tray
  • scatter off blocks
  • internal scatter (about 5% beyond 2 cm from field edge)
50
Q

what is the difference in MLCs between Elekta and varian

A

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

51
Q

what are the field shaped options and what do we use

A
  • infield placement
  • out of field plaement
  • cross boundary placement - midlead (use)
52
Q

what are the advantages of MLCs

A
  • flexibility in shaping fields
    -conformality to target
  • efficient
  • IMRT and modulation of beam intensity
    -IMRt and modulation of beam intensity
    dynamic wedges and compensation
53
Q

what are the disadvantages of MLCs

A
  • unable to create every shape
  • scalloped or stepped shape in distribution
  • interleaf leakage
    -1.0-1.5% transmission
54
Q

what are the rules for sementation and shaped ports

A
  • minimum leaf gap > 1.0 cm
  • minimum field opening >= 4x4 cm for 6MV
  • minimum MU >= 5MU
55
Q

what is the sc for varian, elekta, and siemens

A

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

56
Q

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

A

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

57
Q

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

A

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

58
Q
A

Treatment Planning Term Test 2 (1 decks)

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