Radiation Therapy Midterm Flashcards
What is the Off Axis Ratio (OAR)
OARd = Dose at POI / Dose at Center
Ratio of dose from on center of axis to somewhere off center for a given depth d. This is the basis of the isodose curves.
What is the 3 MeV region. Around what value does it exist?
Hint: it relates to lead and water
Region at which µ/p vs Ephoton is the same for water and lead
Region is around 1-4 MeV
What is the one major downside to using calorimeters to measure dose?
There is over or under estimation of heat due to enodthermic and exothermic reactions occuring.
What affect do the following have on PDD changes?
Higher Beam energy
Larger Depth
Larger Field Size
Larger SSD
Higher Beam energy - PDD drops slower due to less attenuations
Larger Depth - less incident photons, so lower PDD
Larger Field Size - More scattering occurence, so increases PDD
Larger SSD - 1/r2 changes become less pronounced, slower PDD decrease
What are some benefits to using FFF? (3 of them)
Less neutron contamination
Shorter treatment time
Reduces scatter dose out of field
What changes are made to the isodose curve when you get rid of the flattening filter? (4 answers)
Central peak is more pronounced with more E and FS
Profile shapes vary little with depth because photon energy spectrumv aries less off-axis
Reduces scatter dose out of field
PDD are lower due to the absence of a beam hardening
What is the difference between a physical and non-physical wedge filter?
Physical
- wedge-shaped absorber that causes progressive decrease in intensity across beam. Tilts the isodose curve
- literally physical
Non-Physical
- Electronic filter that generates a tilted dose distribution profile by moving collimating jaws around
What is the wedge angle defined as? Where is it defined?
Tilt caused by wedge relative to original flat (on isodose curve)
Defined at 10 cm depth. But it doesn’t change too much at other depths
On the isodose curve, what is physical penumbra defined as?
The lateral width from 90 - 20% PDD at Dmax
What is field size defined as in an isodose profile?
Lateral distance between the two occurences of 50% PDD
How is an isodose curve usually measured?
Probe in water tank with a small ion chamber is moved around and obtains data at different locations. Normalizes to the max value.
How do horns in isodose curves form?
Come from the flattening filter which are designed to overcompensate near the surface in order to obtain a flat isodose curve at greater depths
Pros and Cons to parallel opposed treatment
(1 con, 3 pros)
Con - Excess doses outside of the tumor
Pros - reproducible, homogenous and less likely to miss tumor
For multiple beam theraoy, what affect does having higher energy have on the hot spot?
What about having a thicker patient?
Higher Energy - lower hot spot
Thicker Patient - higher hot spot
What affect does a wedge have on the beam quality? (2-3 answers)
Small affect overall actually,
Hardens some of the beam
But also softens beam due to compton scatterings
When you’re moving a wedge during treatment, what affect will a lower wedge movement have on the wedge angle?
Makes it larger because the end of the path is attenuated much less than the start. Bigger difference between the two
Where is the wedge transmission factor typically taken?
Along the central axis
At a depth past dmax. 10 cm is good
Take dose measurements with and without the wedge
why do wedges need to be far from the patient?
So electron contamination doesn’t overdose the skin
Given separation of wedges (s), the wedge angle (theta) and the hinge angle (angle formed by intersection of bisecting lines through wedge centers) (Ø), what is the optimum wedge angle?
Why is that optimum?
theta = 90 - Ø/2
Want to have isodose curves parallel to bisector
In general why would you use more than one wedge in treatment? (2 reasons)
Gives very large falloff of dose in overlap regions
Always used premptively to avoid hotpots out of the target
What are the definitions of
Gross Tumor Volume
Clinical Target Volume
Planning Target Volume
Treated Volume
GTV: major part of volume containing mostly tymor. Extent and location of the primary tumor
CTV: consists of the tumor and any other tissue with presumed tumor. Much harder to see all the cells
PTV: volume you treat for which accounts for margin around CTV for movement or other uncertainties
TV: Volume covered by dose spreads. Contains ~ prescription dose. Holds additional margins allowing for limitations fo treatment technique
What is the one metric used to specify target dose. This is because nothing else is really necessary sicne everything else either requires difficult calculations/models or is just straight up inaccurate/impractical
Max Target Dose
What are the three modalities typically used for data acquisition pre treatment planning?
MRI, CT and Ultrasound
What are some considerations to take when determining patient contours? (5 answers)
Always take contour at position of treatment
Line representing tabletop must be indicated
Bony land marks and entry points must be indicated
Contours should be constantly checked if conditions change
Contours should be determined in more than one plane
What modalities are used to simulate treatments? What are their uses?
Radiographic - Positioning, setup fields. This is barely used
CT Simulator - All the same as radiographic, but done in 3-D and it’s all virtual
PET/CT - functional images on top of the CT simulator
What is CT # proportional to?
µ
and also
pe and Z
What three methods are used to verify the treatment? Briefly describe them.
Port Films: verify the treatment volume under actual treatment condiitons. literal film
Electronic Portal Imaging: transmitted beam through patient strikes a fluoroscreen/scintillator and light photons are reflected to a camera. Real time verification. Acts as a dosimeter. Used for IMRT QA
Cone-beam CT: x-ray tube mounted on rectractable arm. Flat panel of x-ray detector on outside. System can provide CBCT and 2-D radiography or fluroscopy images
How is effective SSD method used to account for countour irregularities?
Correction made with 1/r2 correction between distance of two difference dmax
Slides isodose chart up or down
How is TAr or TMR method used to account for countour irregularities?
CF = T(d,rA) / T(d+h, rA)
Pcorr = P’’ x CF
How is the isodose shift method used to account for countour irregularities?
Used to correct the entire isodose chart instead of point corrections like the other two methods
Correction factor decreases as energy increases
This is because shifting is less with higher energy
How is the corrected TAR method used to account for countour irregularities?
CF = T(d’,rd) / T(d,rd)
d’ = Σ di pei
How is the power law used to correct for tissue inhomogenities?
[T(d2+d3,rd) / T(d3,rd)] pe - 1
For tissue inhomogenities, what effects the dose within the inhomogenity?
What about the dose outside of the inhomogenity?
Inside: secondary electron fluence changes
Outside: photon fluence change
How is the equivalent TAR method used to account for tissue inhomogenities?
CF = T(d’,r’) / T(d,r)
Takes intoa ccount distance
Best suited for <= 6 MV
Batho ois best for >= 10 MV
For bone inside of tissue, what is the behavior of dose relative to had there not been any bone? (for low energy)
There is a spike in dose inside of the bone with a huge drop right outside
Huge spike because absorption attenuation ratio of bone is higher than tissue.
Huge drop afterwards because the bone attenuated the fluence more than the tissue would have
For bone inside of tissue, what is the behavior of dose relative to had there not been any bone? (for high energy)
Going into bone there is a drop in PDD because at high energies the ratio of attenuation coefficients is less than 1
At the end of the bone depth, there is a slight increase again, but overall the entire PDD curve gets dropped down
When you have soft tissue inside of the bone, what is the effect?
It is an effective BG cavity
Soft tissue inside gets more dose
When you have soft tissue surrounding bone, what happens to the dose to tissue adjacent to forward bone surface.
What happens to dose in the bone itself?
tissue adjacent to forward bone surface gets increase in dose due to electron backscatters
Dose in bone also increases due to a build up from electrons in the upstream
What is the major issue with dose planning for lung tissue? What is its effect?
CPE is lost in an air cavity.
Therefore there is less dose before and after the cavity. Dose to tissue beyond and infront of the cavity will be lower than expected
Not enough dose scatter ^
How does one need to treat the lung?
Make field size bigger to increase odds of CPE
Lower beam energy
Why can you use bolus on patient skin surface for low energy beams but not for high?
To preserve the skin sparing
What is the minimum distance a tissue compensator needs to be from the patient? Why?
20 cm
To avoid scattered electrons reaching surface and losing skin sparing effect
What does a tissue compensator do?
Provides missing beam attenuation from missing tissue
Some general things to remember about compensators
Can use multi-leaf collimator as well
Hold a collimator longer to simulate a thicker compensation portion
PDD decreases cause scatter is lsot
compensator shape depends on divergence and attenuation properties
