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Why switch to MC dose computation?
PB algorithm (especially in combination with range
shifter) inaccurate for two reasons:
Source: RS5 reference manual, RSL
➢
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.
•
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.
Courtesy N. Schreuder, Provision Knoxville, 2017