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Penumbra
Lateral scattering:
MCS: penumbra increases with
increasing penetration depth.
Exceeds penumbra of photons at
some point.
Presence of range shifter (combined
with low energies):
Substantial increase of spot size.
Dose calculation accuracy for PB
algorithm impaired.
Reduce air gap.
Courtesy Palmans 2006.
Water phantom
Low energy
High energy
Courtesy Grevillot 2014.