Beam Planning 2

Question 1

inverse square law states:

Answer 1

intensity of a radiation beam in a non-absorbing medium decreases or increases as the inverse of the distance

Question 2

fluence

Answer 2

number of photons per cross sectional area

Question 3

how does ISL relate to rt (2)

Answer 3

dose (rate) is directly related to the intensity of the beam

small changes in distance can have a large effect on the dose rate

Question 4

penumbra

Answer 4

area at the edge of the radiation beam at which the dose rate changes rapidly as a function of distance from the beam axis

Question 5

what are some causes to penumbra

Answer 5

geometric penumbra
transmission penumbra
scatter withing pt

Question 6

geometric penumbra

Answer 6

depends on size of source, distance from source, and source to diaphragm distance (SDD)

Question 7

what is the equation for penumbra

Answer 7

s(ssd+d-sdd) all over sdd

Question 8

transmission penumbra

Answer 8

region irradiated by photons that are transmitted through the edge of the collimators

Question 9

how can transmission penumbra be reduced physically

Answer 9

with divergent diaphragms (slanted collimators)

Question 10

scatter within pt

Answer 10

photons which have been deflected from their original direction by interacting with matter in the path of the beam

Question 11

physical penumbra

Answer 11

fuzziness at the edge of the beam that is actually measured on the pt
dosimetric concept defined as the lateral distance between 2 isodose lines at a specified depth

Question 12

what % do we wont most of the tumour to be treated at

Answer 12

95

Question 13

why is penumbra important in treatment planning

Answer 13

consideration in deciding field size
diff field sizes to cover same amount of tissue in cobalt vs linac
add an additional margin on the ptv to account for the penumbra

Question 14

in terms of mlcs why is penumbra important

Answer 14

we cannot put the MLC directly on ptv so we need a margin