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When can a plastic phantom be considered water-equivalent?
In MeV beams
Depths have to be scaled so that the spectra at the measuring point are similar, the same Sw,air is
applicable.
Measuring at a scaled depths, electron fluences differ between two materials, due to different
scattering powers.
→ Need of a fluence correction factor h
m
(may vary with depth)
Phantoms that are electrical insulators: Charge storage effects, which modify the electron fluence
incident on the ion chamber; cylindrical chambers more affected than plane parallel chambers
Recommended to use thin slabs (<2 cm thick) of plastic
McEwen, Med Phys 33(4), 2006, p876-