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When can a plastic phantom be considered water-equivalent?
In kV photon beams
•Energy deposition is mainly due to
photo-electric effect and although
one would expect that solid plastics
with Z
eff
close to that of water would
be water equivalent, recent research
has shown otherwise.
8cm diameter
•Departure from water equivalence
worsens at low kV beams.
•It is thus important to check
equivalence with water also in kV
beams (comparison of relative dose
data, and back scatter factors )
Hill et al, Med Phys, 37 (8), 2010 p4355-