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Spectral changes from off axis filtration
(
)
ln 2
direct beam
HVL
µ
≈
(for water)
Tailor, R.C.,
et al.
Med.Phys.,
1998. 25(5): p. 662-667
Lateral variation of the spectrum, such as off axis softening can be modelled by
varying coefficients of attenuation and energy release.
Off axis HVL values can therefore be modelled without explicit knowledge of
the spectrum change causing it!
30
δ
BEAM (EGS4) Elekta SL 25, 25 MV
BEAM (EGS4) Elekta SL 25, 6 MV
Monte Carlo data
Sheikh-Bagheri priv. com.