ATP Barcelona 2017

2.5 Σ + 0.7 σ is a simplification • Dose gradients (‘penumbra’ = σ p lung  smaller margins for random errors ) very shallow in

σ σ 64.1 ) ( 64.1 5.2 2 2 − + +Σ = σ

M

p

p

• 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%:

σ σ 84.0 ) ( 84.0 5.2 2 2 − + +Σ = σ

M

p

p

( van Herk et al, IJROBP 47: 1121-1135, 2000)