ATP Barcelona 2017

Penumbra

Lateral scattering: 

MCS: penumbra increases with

increasing penetration depth. 

Exceeds penumbra of photons

at some point.

Courtesy Palmans 2006

Presence of range shifter (combined with low energies):  Substantial increase of spot size.  Dose calculation accuracy for PB algorithm impaired.  Reduce air gap.

Water phantom

Low energy High energy

Courtesy Grevillot 2014