Principles of Radiotherapy 1

Question 1

what is absolute absorbed dose?

Answer 1

Amount of energy imparted by ionising particles to a unit mass of irradiated material at a place of interest.

Defined at build up depth

Measurement of energy absorobed by irradiated medium

relates to output of machine

gray

Question 2

what is relative dose

Answer 2

spatial distribution of dose (relative to point of determination of absolute dose)

%DD - distribution of dose related to the point of determination of absolute absorbed dose

Question 3

what is the central axis %DD?

Answer 3

dose distribution on central axis
central point of field
goes through isocentre

Question 4

what are the 2 regions

Answer 4

Build up region - increase of dose up to 100%, lack of electronic equilibrium
- skin sparing
- MV energy
-Rapid increase of dose, max at certain depth

attenuation region - loss of beam intensity, energy absorbed as beam gets through tissue equivalent
- scattering of photons

Question 5

where does electronic equilbrium occur

Answer 5

depth of maximum dose

Question 6

what is beam profile?

Answer 6

measurement of dose at different positions in a cross sectional plane

measured at defined depth

produces beam profile

Question 7

what is an isodose curve

Answer 7

a line that joins points of equal %DD

maps variation in dose as function of:
- depth
- trasnverse distance from central axis

curves related to reference point on the central axis

Question 8

what is an isodose chart?

Answer 8

set of isododse curves
show incremental %DD
maximum to 10%
plotted at 10% intervals
normalised to maximum dose
represents distribution of dose in one plane - map of dose distribution

Question 9

what is penumbra?

Answer 9

dose fall of rapidly in MV beam
lateral distance from central axis

width dependant on:
- distance from source
- scatter

Question 10

what is %DD used for

Answer 10

fixed FSD treatments

measured in water tanks
chamber moves hence measuring point moves

Question 11

what is Tissue maximum ratio used for

Answer 11

isocentric treatments

• constant measiring point
• measures absorption
• calculated from %DD values

Question 12

explain how beam type affects %DD

Answer 12

isodose curve characteristics are different:

• build up depth
• shape of curve
• penumbra
  -penetration of beam

Question 13

explain cobalt beam affecting %DD

Answer 13

DMAX at 5mm
profile less rounded
wide penumbra

Question 14

explain MV photons beam affecting %DD

Answer 14

Dmax at depth - 5mm from surface
flat - flattening filter
narrow penumbra - high energy so forward scatter

Question 15

explain electron beam affecting %DD

Answer 15

little skin sparing
rapid fall off
flat profiles
widening of lower valve isodoses at edges (bowing out)

Question 16

explain how increasing energy increases %DD

Answer 16

increasing primary photon penetration depth before interaction

• increasing %dd (isodose curves spread out)
• increased forward scatter
• 100% depth increases so greater skin sparing
• exit dose increases - bad skin effects

Question 17

what happens at MV beam

Answer 17

not mono energetic
beam hardening
lower energy photons absorbed at shallower depths - photons left are more energetic
greater fall of initally

Question 18

how is %dd affected by field size

Answer 18

scattered radiation occurs after initial interaction

as field size increases, scattered radiation at depth on central axis increases

not much change at build up depth

as energy increases, more forward scatter - effect on field size on %dd decreases

dose at dmax increases with field size so % change in dose decreases with field size increases

Question 19

How does field shape affect %dd

Answer 19

elongated field - less scatter to central axis than sqaure

rectangular field: backscatter similar to small area square

equivalent sqaure field:
same value of %dd as rectanglualr field of same radiation quality