581 / 1109
2.5
Σ
+ 0.7
σ
is a simplification
•
Dose gradients (‘penumbra’ =
σ
p
) very shallow in
lung
smaller margins for random errors
•
Number of fractions is small in hypofractionation
•
Residual mean of random error gives systematic error
•
Beam on time long
respiration causes dose blurring
•
If dose prescription is at 80% instead of 95%:
p
p
M
σ
σ
σ
64.1 )
( 64.1 5.2
2
2
− +
+Σ =
p
p
M
σ
σ
σ
84.0 )
( 84.0 5.2
2
2
− +
+Σ =
(
van Herk et al, IJROBP 47: 1121-1135, 2000)