SINGLE ISOCENTER TREATMENT PLANNING FOR HOMOGENEOUS DOSE DELIVERY TO NONSPHERICAL TARGETS IN MULTIARC LINEAR-ACCELERATOR RADIOSURGERY

Purpose: Conventional radiosurgery refers to single isocenter focused beam irradiation of small intracranial targets with a single collimato r. Conventional radiosurgery is characterized by spherical-shaped isod ose surfaces. Nonspherical targets require a different approach to avo id exposing a large volume of nontarget brain tissue to high dose, par ticularly for lesions larger than 25 mm. Multiple isocenters are frequ ently used to treat nonspherical large targets, but multiple isocenter treatments are associated with a relative dose inhomogeneity of appro ximately 100% within the target volume, and may be correlated with an increased rate of complications. The feasibility of conformally treati ng elongated targets to an approximately homogeneous dose using a sing le isocenter methodology will be demonstrated. Methods and Materials: A prolate ellipsoid of revolution, 25 mm in diameter, 35 mm in length, positioned at five representative locations in a head phantom, was us ed as a target model. The alignment of the target was taken to be para llel in turn to each of the three principal axes of the head model (A/ P, R/L, and C/C). Dose conformation is achieved by nonuniform are weig hting, selective limitation of the extent of individual arcs, and the use of different collimator apertures for the different arcs in accord ance with the aperture size required to encompass the target for that are. Treatment plans mere selected based on considerations of dose-vol ume histograms and conformation of the 80% dose surface with the surfa ce of the target. The goal was that the minimum target dose would not be less than 80% of the maximum dose. Results: Practical treatment pla ns for which the minimum target dose exceeded 80% were obtained for th e three orthogonal orientations of the target for the five target loca tions. Plan parameters were essentially independent of the target posi tion, dependent only on target orientation. The 80% isodose contour su rface enclosed on average 2.8 cm(3) larger volume (range: 2.1-3.9 cm(3 )) than the prescribed 11.45 cm(3) target. The minimum dose to the tar get ranged from 80.1 to 84.5%, and the average dose to the target was 94.26%. The 80-to-50% dose volume enclosed an average of 14.8 cm(3) of nontarget volume (range: 12.7-15.9 cm(3)). Average deviation in the p rincipal planes of the 80% isodose lines from the surface of the targe t volume was 0.95 mm for the 25 mm dimension (range 0.0 to +1.9 mm) an d 0.86 mm (range 0.0 to +2.4 mm) for the 35 mm dimension. Conclusion: Standardized single isocenter treatment plans with the isocenter at th e center of the target can achieve good conformation of the dose distr ibution to targets elongated along any of the principal axes, and loca ted anywhere in the brain.