Dose Course 2018_Flipping book

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Unresolved issues with electron pencil beam models

Beam modeling:

PB methods assume Gaussian characteristics of the incident beam. Hence, influence from “non Gaussian” features (collimator scattering, etc) yield profile errors and make output factors hard to calculate (has to be table lookup driven). Penumbra modeling through manipulation of incident pencil width may wash out effects of heterogeneities. Heterogeneities are well modeled at the first part of the depth range (since voxels are larger than the FE pencil width). At the end of the electron range, effects of localized heterogeneities (smaller than the FE pencil width using semi-infinite slab approximation) get washed out PB by models. “Redefinition” and “phase space evolution” models fix that, to the cost of CPU&memory… Requires homogeneous energy and directional distribution over the field.

Heterogeneities:

Dublin 2018