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Margins for random errors
Dose level
PTV margin
PTV margin*
(
ψ
= 3.2 mm)
80%
0.84
√ (σ
²
+ ψ
²
) − 0.84 ψ
0.4
σ
85%
1.03
√ (σ
²
+ ψ
²
) − 1.03 ψ
0.5
σ
90%
1.28
√ (σ
²
+ ψ
²
) − 1.28 ψ
0.6
σ
95%
1.64
√ (σ
²
+ ψ
²
) − 1.64 ψ
0.7
σ
*linear approximation
σ
= SD of random errors,
ψ
=
σ
of Gaussian penumbra