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Flashcards in Radiation Therapy Midterm Deck (44)
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1

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.

2

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

3

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.

4

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

5

What are some benefits to using FFF? (3 of them)

Less neutron contamination

Shorter treatment time

Reduces scatter dose out of field

6

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

7

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

8

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

9

On the isodose curve, what is physical penumbra defined as?

The lateral width from 90 - 20% PDD at Dmax

10

What is field size defined as in an isodose profile?

Lateral distance between the two occurences of 50% PDD

11

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.

12

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

13

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

14

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

15

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

16

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

17

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

18

why do wedges need to be far from the patient?

So electron contamination doesn't overdose the skin

19

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

20

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

21

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

22

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

23

What are the three modalities typically used for data acquisition pre treatment planning?

MRI, CT and Ultrasound

24

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

25

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

26

What is CT # proportional to?

µ

 

and also

 

pe and Z

27

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

28

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

29

How is TAr or TMR method used to account for countour irregularities?

CF = T(d,rA) / T(d+h, rA)

 

Pcorr = P'' x CF

30

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