MV beams

Question 1

Explain the build-up region

Answer 1

Results from the relatively long range of secondary charged particles that are first release in the patient by photon interaction and then deposit their kinetic energy in the patient through coulomb interactions.

Question 2

Explain electronic equilibrium

Answer 2

The energy deposited outside the volume by ionisation created within equals the energy deposited in the volume by ionisation created outside.
Occurs only at DMAX

Question 3

Explain Skin sparing

Answer 3

Surface dose lower than dmx due to delay in photon interaction.

Question 4

6MV PDD

Answer 4

SD = 15%
DMAX= 1.5cm
@ 10cm depth 65%
@20cm depth 40%

Question 5

18MV PDD

Answer 5

SD = 10%
DMAX= 3.5cm
@ 10cm depth 80%
@ 20cm depth 55%