BREMSSTRAHLUNG PRODUCTION AT 50 MEV IN DIFFERENT TARGET MATERIALS ANDCONFIGURATIONS

A combination of Monte Carlo, convolution, and experimental techniques have been used to investigate bremsstrahlung production at 50 MeV in full-range targets to produce narrow elementary photon beams for scann ing. Calculations using the ITS 3.0 Monte Carlo system for various tar get designs, including particle transport through the treatment head o f an MM50 racetrack microtron and a water phantom, have been compared to experimental dose profiles from narrow photon beams at 10-cm depth in water. A reduction in the ITS 3.0 default substep size has been fou nd necessary even for incomplete agreement, in consistency with the fi ndings of Faddegon and Rogers [Nucl. Instrum. Meth. A 327, 556-565 (19 93)] for a different experimental setup and energy using the previous version of ITS. Results show that the calculated shape of the tail of dose distributions from narrow photon beams agrees well with measureme nts, but CYLTRAN/ITS 3.0 fails to reproduce the central part of the di stribution. The discrepancy at small angles, reported previously for E GS4 and ITS 2.1 simulations, possess a limitation to Monte Carlo simul ations of narrow photon beams used in scanned systems of clinical acce lerators. Radial dose profiles have been calculated by convolution of the energy fluence at the exit of the target with one polyenergetic Mo nte Carlo calculated dose kernel and also a database consisting of ten different dose kernels corresponding to different monoenergetic photo n pencil beams for comparison. The agreement with the much slower full y detailed Monte Carlo calculations was better when using the database kernels than the polyenergetic kernel. Results for the mean energy, m ean polar angle, and energy fluence at different depths within various targets have been obtained. These are discussed in the context of the design characteristics of bremsstrahlung targets with emphasis on the ir utilization for scanning photon beam techniques. (C) 1996 American Association of Physicists in Medicine.