MONTE-CARLO BASED PHASE-SPACE EVOLUTION FOR ELECTRON DOSE CALCULATION

A system of computer codes based on phase-space evolution is developed and applied to low energy therapeutic electron beams. Monte Carlo (EG S4) is used to pre-calculate the electron transport and dose depositio n in a 0.5 cm width cubic voxel. Dose calculations at larger scales ar e computed from the pre-calculated data using phase-space evolution. T his approach has the theoretical accuracy of Monte Carlo with potentia lly significant speed gains resulting from the pre-calculation. This s tudy demonstrates the accuracy of this technique while providing a pre liminary assessment of the calculation time. For a 4.3 MeV electron be am in water with a 0.5 cm thick slab of either water (homogeneous), ai r, or aluminum at 1 cm depth, we observe differences relative to Monte Carlo of less than 3% along the central axis for a pencil-beam. For a 3.5 cmx3.5 cm field we observe a maximum difference on the central ax is of 4% in the build-up region and less than 0.1 cm in the fall-off r egion for all three phantoms. Calculation times are disappointing; how ever, there is high potential for their reduction to values comparable to or better than condensed history Monte Carlo while retaining clini cally acceptable accuracy. (C) 1997 American Association of Physicists in Medicine.