Term Test 2 Flashcards
what allows for an accurate prescribed dose to a target volume
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
What do we have to consider when calculating photon dose
1) non infinite patient
2) inhomogeneities
Deposition of energy from a photon beam has two stages. What are they
- 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) \
- Electrons set in motion (KERMA) then transfer energy to tissues via excitations and ionizations (DOSE step)
How do inhomogeneities change TERMA and dose
- 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
What is charged particle equilibrium (CPE)
- number of electrons entering and leaving small volume are equal, so ionizations due to all tracks are accounted for
What is needed for full CPE
- must be along axis beam and laterality
Why is calculating dose easier when CPE is established
- do not have to calculate all electron paths
When does CPE exist
- volume is surrounded by material with same properties
- minimum thickness equal to maximum range of electrons produced
When (or where) does CPE not exist
- in the build up region
- for very small fields and high photon energies
- at the interface between tissues with different properties
At beam edges
What is the relative PRIMARY photon interactions in a low density inhomogeneous tissue compared to homogenous tissue with higher density
Fewer in photon interactions in lower densities
What is the relative scattered photon interactions in a low density inhomogeneous tissue
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
What is the relative electron interactions in a low density inhomogeneous tissue
Average energy of electrons is higher in lower density but dose will be equal or slightly greater
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
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
When going from water, to air, to back to water, describe the dose at each interface
- 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
What is the bone density
1.69
In bone inhomogeneties, for MV photons, which interactions dominate?
- Compton interactions dominate
What is the atomic number of bone
13.5
In bone homogeneities for KV photons, which interactions will dominate
Photoelectric interactions
For KV photons and MV photons, what is their comparison to dose in water at the same point?
- MV dose will be lower
- KV dose will be higher
- dose to bone will be the same
For very high energy photons, which interactions will dominate in bone inhomogeneties
- pair production
What are some correction based algorithms
- 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
Due to primary photons, how does energy and depth effect dose ?
- dose increases as energy increase
- dose decreases as depth increases
Due to scattered, how does field size, energy and depth effect dose ?
- dose increases and field size increases
- dose increases as energy decreases
- dose increases as depth increases
With respect to PDD , what effects do SSD, beam energy, and field size have on it
PDD increases with
- increased SSD
- increased beam energy
- increasing field size increases
TPR ratio fill in the blanks
Compare doses in _______________________
Accounts for the _______ and the _________ at ________
- the same horizontal plane
- depth , field size, depth
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
small
h , ISL
FS
PDD
ISL
how does a tissue surplus effect field size at surface, SSD, and dose at depth/point
decreases as there is a tissue surplus
how does a tissue deficit effect field size at surface, SSD, and dose at depth/point
increses as their is a tissue deficit
what is the beam obliquity effect
- 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
what is the clinical requirement for a uniform dose percentage throughout a treatment plane
95-105%
what do missing tissue compensators do
achieve uniform dose in a plane orthogonal to beam axis
what are examples of physical compensators
bolus
metal or other non tissue equivalent material
wedges
What are examples of automated compensators
- moving collimators (virtual wedging)
- intensity modulated radiation delivery (field segmentation, MLCs)
what are the advantages of bolus
inexpensive
malleable
quick to implement
what are the disadvantages of bolus
- can have air gaps
- questionably reproducibility
- increases skin dose
why do we use wedges
- contour corrections
- fix dose gradient
how do we use wedges
- missing tissue compensator
- dose gradient compensator
what do wedges do to beam quality
- physical wedges preferentially attenuate lower energy photons
- therefore beam hardening occurs which creates a higher energy beam
what are the 3 different types of wedges
manual :
universal / motorized
virtual / enhanced dynamic
what does the presence of a wedge change
- 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
in wedges, theta is
- angle by which an isodose curve is tilted
- defined at the central axis
- defined for a specific depth
physical wedges may increase field size
true
wedge factor will increase with increasing photon energy with the same wedge and field size
true
what are the advantages of a non physical wedge
- automation of treatment delivery
- less peripheral dose eg contralteral breast
- ergonomics and safety
- cannot be stopped
what are the disadvantages of a non physical wedge
- greater dosimetric complexity in acquisition of commissioning data beam modeling for TPS and MU calcs
what are the limitations of physical or virtual wedges
- 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
what is the hinge angle
angle between central axis of the two beams
how do you calculate optimal angle
ca
theta = 90 - hinge angle / 2
how does scatter dose occur
- scatter off field tray
- scatter off blocks
- internal scatter (about 5% beyond 2 cm from field edge)
what is the difference in MLCs between Elekta and varian
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
what are the field shaped options and what do we use
- infield placement
- out of field plaement
- cross boundary placement - midlead (use)
what are the advantages of MLCs
- flexibility in shaping fields
-conformality to target - efficient
- IMRT and modulation of beam intensity
-IMRt and modulation of beam intensity
dynamic wedges and compensation
what are the disadvantages of MLCs
- unable to create every shape
- scalloped or stepped shape in distribution
- interleaf leakage
-1.0-1.5% transmission
what are the rules for sementation and shaped ports
- minimum leaf gap > 1.0 cm
- minimum field opening >= 4x4 cm for 6MV
- minimum MU >= 5MU
what is the sc for varian, elekta, and siemens
varian: tertiary collimation system (collimator)
elekta: upper jaw replacement (MLC)
Siements: lower jaw replacement
short answer: explain how the presence of a low density tissue heterogeneity will effect the distribution of dose within a patient
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
Short answer: what is the difference between TERMA and absorbed dose
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
