Radiation Therapy Midterm

Question 1

What is the Off Axis Ratio (OAR)

Answer 1

OAR_d = 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.

Question 2

What is the 3 MeV region. Around what value does it exist?

Hint: it relates to lead and water

Answer 2

Region at which µ/p vs E_photon is the same for water and lead

Region is around 1-4 MeV

Question 3

What is the one major downside to using calorimeters to measure dose?

Answer 3

There is over or under estimation of heat due to enodthermic and exothermic reactions occuring.

Question 4

What affect do the following have on PDD changes?

Higher Beam energy

Larger Depth

Larger Field Size

Larger SSD

Answer 4

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

Question 5

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

Answer 5

Less neutron contamination

Shorter treatment time

Reduces scatter dose out of field

Question 6

What changes are made to the isodose curve when you get rid of the flattening filter? (4 answers)

Answer 6

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

Question 7

What is the difference between a physical and non-physical wedge filter?

Answer 7

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

Question 8

What is the wedge angle defined as? Where is it defined?

Answer 8

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

Question 9

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

Answer 9

The lateral width from 90 - 20% PDD at Dmax

Question 10

What is field size defined as in an isodose profile?

Answer 10

Lateral distance between the two occurences of 50% PDD

Question 11

How is an isodose curve usually measured?

Answer 11

Probe in water tank with a small ion chamber is moved around and obtains data at different locations. Normalizes to the max value.

Question 12

How do horns in isodose curves form?

Answer 12

Come from the flattening filter which are designed to overcompensate near the surface in order to obtain a flat isodose curve at greater depths

Question 13

Pros and Cons to parallel opposed treatment

(1 con, 3 pros)

Answer 13

Con - Excess doses outside of the tumor

Pros - reproducible, homogenous and less likely to miss tumor

Question 14

For multiple beam theraoy, what affect does having higher energy have on the hot spot?

What about having a thicker patient?

Answer 14

Higher Energy - lower hot spot

Thicker Patient - higher hot spot

Question 15

What affect does a wedge have on the beam quality? (2-3 answers)

Answer 15

Small affect overall actually,

Hardens some of the beam

But also softens beam due to compton scatterings

Question 16

When you’re moving a wedge during treatment, what affect will a lower wedge movement have on the wedge angle?

Answer 16

Makes it larger because the end of the path is attenuated much less than the start. Bigger difference between the two

Question 17

Where is the wedge transmission factor typically taken?

Answer 17

Along the central axis

At a depth past dmax. 10 cm is good

Take dose measurements with and without the wedge

Question 18

why do wedges need to be far from the patient?

Answer 18

So electron contamination doesn’t overdose the skin

Question 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?

Answer 19

theta = 90 - Ø/2

Want to have isodose curves parallel to bisector

Question 20

In general why would you use more than one wedge in treatment? (2 reasons)

Answer 20

Gives very large falloff of dose in overlap regions

Always used premptively to avoid hotpots out of the target

Question 21

What are the definitions of

Gross Tumor Volume

Clinical Target Volume

Planning Target Volume

Treated Volume

Answer 21

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

Question 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

Answer 22

Max Target Dose

Question 23

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

Answer 23

MRI, CT and Ultrasound

Question 24

What are some considerations to take when determining patient contours? (5 answers)

Answer 24

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

Question 25

What modalities are used to simulate treatments? What are their uses?

Answer 25

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

Question 26

What is CT # proportional to?

Answer 26

µ

and also

p_e and Z

Question 27

What three methods are used to verify the treatment? Briefly describe them.

Answer 27

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

Question 28

How is effective SSD method used to account for countour irregularities?

Answer 28

Correction made with 1/r² correction between distance of two difference dmax

Slides isodose chart up or down

Question 29

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

Answer 29

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

P_corr = P^’’ x CF

Question 30

How is the isodose shift method used to account for countour irregularities?

Answer 30

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

Question 31

How is the corrected TAR method used to account for countour irregularities?

Answer 31

CF = T(d^’,r_d) / T(d,r_d)

d^’ = Σ d_i p_ei

Question 32

How is the power law used to correct for tissue inhomogenities?

Answer 32

[T(d₂+d₃,r_d) / T(d₃,r_d)] ^{p_e - 1}

Question 33

For tissue inhomogenities, what effects the dose within the inhomogenity?

What about the dose outside of the inhomogenity?

Answer 33

Inside: secondary electron fluence changes

Outside: photon fluence change

Question 34

How is the equivalent TAR method used to account for tissue inhomogenities?

Answer 34

CF = T(d’,r’) / T(d,r)

Takes intoa ccount distance

Best suited for <= 6 MV

Batho ois best for >= 10 MV

Question 35

For bone inside of tissue, what is the behavior of dose relative to had there not been any bone? (for low energy)

Answer 35

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

Question 36

For bone inside of tissue, what is the behavior of dose relative to had there not been any bone? (for high energy)

Answer 36

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

Question 37

When you have soft tissue inside of the bone, what is the effect?

Answer 37

It is an effective BG cavity

Soft tissue inside gets more dose

Question 38

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?

Answer 38

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

Question 39

What is the major issue with dose planning for lung tissue? What is its effect?

Answer 39

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 ^

Question 40

How does one need to treat the lung?

Answer 40

Make field size bigger to increase odds of CPE

Lower beam energy

Question 41

Why can you use bolus on patient skin surface for low energy beams but not for high?

Answer 41

To preserve the skin sparing

Question 42

What is the minimum distance a tissue compensator needs to be from the patient? Why?

Answer 42

20 cm

To avoid scattered electrons reaching surface and losing skin sparing effect

Question 43

What does a tissue compensator do?

Answer 43

Provides missing beam attenuation from missing tissue

Question 44

Some general things to remember about compensators

Answer 44

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