ON COMPENSATOR DESIGN FOR PHOTON-BEAM INTENSITY-MODULATED CONFORMAL THERAPY

Recently the compensator has been shown to be an inexpensive and relia ble dose delivery device for photon beam intensity-modulated radiation therapy (IMRT). The goal of IMRT compensator design is to produce an optimized primary fluence profile at the patient's surface obtained fr om the optimization procedure. In this paper some of the problems asso ciated with IMRT compensator design, specifically the beam perturbatio ns caused by the compensator, are discussed. A simple formula is deriv ed to calculate the optimal compensator thickness profile from an opti mized primary fluence profile. The change of characteristics of a 6 MV beam caused by the introduction of cerrobend compensators in the beam is investigated using OMEGA Monte Carlo codes. It is found that the c ompensator significantly changes the energy spectrum and the mean ener gy of the primary photons at the patient's surface. However, beam hard ening does not have as significant an effect on the percent depth dose as it does on the energy spectrum. We conclude that in most situation s the beam hardening effect can be ignored during compensator design a nd dose calculation. The influence of the compensator on the contamina nt electron buildup dose is found to be small and independent of the c ompensator thickness of interest. Therefore, it can be ignored in the compensator design and included as a correction into the final dose di stribution. The scattered photons from the compensator are found to ha ve no effect on the surface dose. These photons produce a uniform low fluence distribution at the patient's surface, which is independent of compensator shape. This is also true for very large fields and extrem ely asymmetric and nonuniform compensator thickness profiles. Compared to the primary photons, the scattered photons have much larger angula r spread and similar energy spectrum at the patient's surface. These c haracteristics allow the compensator thickness profile and the dose di stribution to be calculated from the optimized fluence profile of prim ary photons, without considering the scattered photons. (C) 1998 Ameri can Association of Physicists in Medicine.