573 / 1161
2.5
S
+ 0.7
s
is a simplification
•
Dose gradients (‘penumbra’ =
s
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
s
s
s
64
.1
)
( 64.1 5.2
2
2
− +
+S =
p
p
M
s
s
s
84
.0
)
(
84.0 5.2
2
2
−
+
+S
=
(
van Herk et al, IJROBP 47: 1121-1135, 2000)