IMRT

Question 1

conformal RT : we want short path length to PTV.
why?

Answer 1

push PTV further up depth dose curve in that direction allowing less absorption by other tissue.

Question 2

conformal RT : we want short trajectory thru patient.
why?

Answer 2

to reduce volume of tissue being irradicated

Question 3

conformal RT : we want small beam aperture.
why?

Answer 3

to cut beam down until it fits PTV

Question 4

types of MLC movements

Answer 4

dynamic / sliding window
step and shoot

Question 5

dynamic / sliding window

Answer 5

when beam is on , the MLC move and change shape
limitation= how fast the MLC moves

Question 6

step and shoot

Answer 6

MLC moves when beam is off

easier as it has fewer fields and each field is fixed

Question 7

loss of PTV dose homogenity caused by

Answer 7

reducing no. of beams

Question 8

Advantages of step and shoot compared to dynamic

Answer 8

-Easy to plan, deliver and verify
-less dependence on MLC speed
-interuptions easier to manage
-lower MLC
-less QA
-low leakage

Question 9

disadvantages of step and shoot compared to dynamic

Answer 9

-longer tx time
-hard on MLC software