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Flashcards in Radiation Therapy Midterm Deck (44)
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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?


- wedge-shaped absorber that causes progressive decrease in intensity across beam. Tilts the isodose curve

- literally 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?



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