Buildup region of high-energy x-ray beams in radiosurgery.

The depths of dose maxima of radiosurgical x-ray beams increase with the field diameter in the range from 10-30 mm. Furthermore, the degree of the increase is proportional to the photon beam energy. This behavior is in contrast to that of large radiotherapeutic fields, where the depths of dose maxima decrease with increasing field size. The surface doses, depths of dose maxima, and collimator scatter factors for radiosurgical beams with energies between 6 and 18 MV are presented. It is shown that the change in dmax for radiosurgical beams is a result of photon scatter in the phantom rather than in the collimator. Monte Carlo simulations of the radiosurgical beams are used to separate the total dose into two components: the primary dose and the scattered dose. Calculations indicate that buildup region characteristics are governed by the primary dose deposition in the medium. The scattered dose has a minimal effect on the total dose deposited in media by radiosurgical beams.