Principles of Radiotherapy 1 Flashcards
what is absolute absorbed dose?
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
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what is relative dose
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
what is the central axis %DD?
dose distribution on central axis
central point of field
goes through isocentre
what are the 2 regions
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
where does electronic equilbrium occur
depth of maximum dose
what is beam profile?
measurement of dose at different positions in a cross sectional plane
measured at defined depth
produces beam profile
what is an isodose curve
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
what is an isodose chart?
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
what is penumbra?
dose fall of rapidly in MV beam
lateral distance from central axis
width dependant on:
- distance from source
- scatter
what is %DD used for
fixed FSD treatments
measured in water tanks
chamber moves hence measuring point moves
what is Tissue maximum ratio used for
isocentric treatments
- constant measiring point
- measures absorption
- calculated from %DD values
explain how beam type affects %DD
isodose curve characteristics are different:
- build up depth
- shape of curve
- penumbra
-penetration of beam
explain cobalt beam affecting %DD
DMAX at 5mm
profile less rounded
wide penumbra
explain MV photons beam affecting %DD
Dmax at depth - 5mm from surface
flat - flattening filter
narrow penumbra - high energy so forward scatter
explain electron beam affecting %DD
little skin sparing
rapid fall off
flat profiles
widening of lower valve isodoses at edges (bowing out)
explain how increasing energy increases %DD
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
what happens at MV beam
not mono energetic
beam hardening
lower energy photons absorbed at shallower depths - photons left are more energetic
greater fall of initally
how is %dd affected by field size
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
How does field shape affect %dd
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
