Improved linac dose distributions for radiosurgery with elliptically shaped fields.

Stereotactic radiosurgery techniques for a linear accelerator typically use circular radiation fields to produce an essentially spherical radiation distribution with a steep dose gradient. Target volumes are frequently irregular in shape, and circular distributions may irradiate normal tissues to high dose as well as the target volume. Improvements to the dose distribution have been made using multiple target points and optimizing the dose per arc to the target. A retrospective review of 20 radiosurgery patients has suggested that the use of elliptically shaped fields may further improve the match of the radiation distribution to the intended target volume. This hypothesis has been verified with film measurements of the radiation distribution obtained using elliptical radiation beam in a head phantom. Reductions of 40% of the high dose volume have been obtained with elliptical fields compared to circular fields without compromising the dose to the target volume.