Course book Advanced Treatment Planning 2018

Why switch to MC dose computation?

PB algorithm (especially in combination with range shifter) inaccurate for two reasons:

➢ Nuclear halo effect • Each pencil beam is modelled by 2 Gaussians (MCS, nuclear halo) • Lack of handling nuclear halo properly within the range shifter, then transporting the beam through vacuum (instead of air) and large heterogeneities (patient surface): causes lack of modelling accuracy especially for low energies where a greater angular spread of the protons is expected. ➢ Lateral heterogeneities • Each spot is split into 19 sub-pencil beams.

Courtesy N. Schreuder, Provision Knoxville, 2017

• In case of large spot sizes (combination of range shifter and larger gaps) the distance between subspots becomes larger than anatomic density variations within the patient.

Source: RS5 reference manual, RSL