ESTRO course Dose Modelling and Verification 2017

Flattening filter scatter the major extrafocal contribution Monte Carlo proof:

Mevatron & EGS4

Distribution of origin site. Note clouds at beam stopper, primary collimator and flattening filter

Lateral distribution of origin sites, projected to a common distance from the target.

All scatter sources merged to an effective source distribution.

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Chaney E L et al 1994 A Monte Carlo study of accelerator head scatter Med. Phys. 21 1383-9