I'm trying to find some good information on pencil beam calculations for photon beams.

The question is from assignment 09, C3:

Various computer algorithms are used by radiation therapy planning systems to model the dose distribution of photon beams. Briefly describe the underlying principles (algorithm equations are not required) of the following photon dose calculation algorithms –

a) Superposition/convolution

b) Monte Carlo

c) Pencil Beam methods

Evaluate their comparative advantages and limitations.

Superposition/convolution and Monte Carlo are covered well in textbooks, but pencil beam methods are not. I would presume that pencil beams are the 'old' way of calculating dose distribution, using TAR/Batho/ETAR methods to compensate for tissue inhomogeneity?

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