EFFECTS OF IRRADIATION GEOMETRY ON TREATMENT PLAN OPTIMIZATION WITH LINAC-BASED RADIOSURGERY

A comparison was made of different treatment plans to determine the ef fect on the three-dimensional dose distributions of varying the allowe d parameters in linac-based stereotactic radiosurgery with circular co llimators; these parameters are are position, length, and weighting, a nd collimator diameter. For the class of eccentrically shaped target v olumes that are not so irregular as to require several separate isocen ters, it was found that superior dose distributions could be achieved by varying are length, are position, are weighting, and collimator dia meter. An analysis of the results achieved with an automated planning program indicates that, in general, the variables of are position and are length are of greater importance than collimator size or beam weig hting. However, there are cases where varying these latter two paramet ers does result in markedly better dose distributions. A deeper invest igation into the effects of multiple collimators on the dose distribut ion in the area of steepest gradient demonstrated that multiple collim ator sizes do not significantly degrade the dose falloff, which is in fact mostly determined by the effects of intersecting arcs. (C) 1996 A merican Association of Physicists in Medicine.