beam properties

Question 1

give 4 main beams used

Answer 1

kV
MV
electron
proton

Question 2

which beam is 100% at skin surface and gradual dose fall off

Answer 2

kV

Question 3

which beam has a Bragg peak

Answer 3

Proton

Question 4

which beam is 100% at skin surface with rapid dose fall off

Answer 4

e-

Question 5

which beam has build-up region

Answer 5

MV

Question 6

what causes MV beam dose fall off

Answer 6

Attenuation

Question 7

what is penumbra

Answer 7

normal tissue area outside of treatment area that receives moderate dose

Question 8

2 ways to reduce penumbra

Answer 8

increase distance of collimators from source (but will increase scatter)

reduce the size of the source

Question 9

what interaction is kV beam

Answer 9

photoelectric effect

Question 10

what is PE directly proportional to

Answer 10

atomic no. ^3 - meaning kV has preferential absorption in bone over soft tissue due to higher Z in bone

Question 11

is the penumbra in kV beam big/small and why?

Answer 11

very big - due to angled treatment causing a large source

Question 12

what is predominant interaction of MV

Answer 12

Compton scatter

Question 13

what causes DMAX

Answer 13

interaction occurs on skin surface, however scattered e- travels deeper into pt and delivers dose at 1.5cm for 6MV beam

Question 14

is the LAC high or low

Answer 14

LAC Is low, due o more penetrating beam, therefore less energy being absorbed

Question 15

is penumbra b ig/small in MV and why?

Answer 15

penumbra isn’t a thing due to point source (so small)

Question 16

is there high scatter in kV

Answer 16

yes, Compton scatter occurs causing lateral scatter and bowing of isodose

Question 17

is there high scatter in MV

Answer 17

no, as scattered e- has very minimal energy to scatter laterally

Question 18

give 3 advantages of kV

Answer 18

100% dose @ skin surface

possible to collimate at skin surface to reduce penumbra

preferential absorption in bone - good for tx of bones

Question 19

give 3 disadvantages of kV

Answer 19

high later scatter increasing dose out of field

tx of soft tissue above bone, will be preferentially absorbed by the bone below

DMAX @ skin, not good for deep tumour

Question 20

give 4 advantages of MV

Answer 20

no penumbra

no lateral scatter - so no dose out of field

more penetrating 0 good for deep tumour

DMAX @ depth - skin sparring

Question 21

give 2 disadvantages of MV

Answer 21

more penetrating - bad for superficial tumours - can use bolus

DMAX at depth - skin sparring

Question 22

main advantage of e- tx

Answer 22

rapid dose fall off - meaning if treating somewhere with vital behind/underneath, e- can avoid dose to OAR

Question 23

is scatter high or low for e-

Answer 23

very high lateral scatter, due to lots of scattered e- for delivery of dose - very bowing isodose

Question 24

where is DMAX for e- tx

Answer 24

can still occur at depth due to megavolt energy but b plus can be used to bring more superficial

Question 25

why do e- have rapid dose fall-off

Answer 25

due to e- having a finite stopping distance, meaning at that point e- cannot travel any further or deliver dose anywhere else

Question 26

name the other 2 treatment beams that aren't kV, MV or e-

Answer 26

Gamma (Brachy) Proton