MONTE-CARLO AND ANALYTICAL CALCULATION OF PROTON PENCIL BEAMS FOR COMPUTERIZED TREATMENT PLAN OPTIMIZATION

Proton pencil beams in water, in a format suitable for treatment plann ing algorithms and covering the radiotherapy energy range (50-250 MeV) , have been calculated using a modified version of the Monte Carlo cod e PTRAN. A simple analytical model has also been developed for calcula ting proton broad-beam dose distributions which is in excellent agreem ent with the Monte Carlo calculations. Radial dose distributions are a lso calculated analytically and narrow proton pencil-beam dose distrib utions derived. The physical approximations in the Monte Carlo code an d in the analytical model together with their limitations are discusse d. Examples showing the use of the calculated set of proton pencil bea ms as input to an existing photon treatment planning algorithm based o n biological optimization are given for fully 3D scanned proton pencil beams; these include intensity modulated beams with range shift and s canning in the transversal plane.