Single isocenter treatment planning for homogeneous dose delivery to nonspherical targets in multiarc linear...

Conventional radiosurgery refers to single isocenter focused beam irradiation of small intracranial targets with a single collimator. Conventional radiosurgery is characterized by spherical-shaped isodose surfaces. Nonspherical targets require a different approach to avoid exposing a large volume of nontarget brain tissue to high dose, particularly for lesions larger than 25 mm. Multiple isocenters are frequently used to treat nonspherical large targets, but multiple isocenter treatments are associated with a relative dose inhomogeneity of approximately 100% within the target volume, and may be correlated with an increased rate of complications. The feasibility of conformally treating elongated targets to an approximately homogeneous dose using a single isocenter methodology will be demonstrated.

A prolate ellipsoid of revolution, 25 mm in diameter, 35 mm in length, positioned at five representative locations in a head phantom, was used as a target model. The alignment of the target was taken to be parallel 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 arc weighting, selective limitation of the extent of individual arcs, and the use of different collimator apertures for the different arcs in accordance with the aperture size required to encompass the target for that arc. Treatment plans were selected based on considerations of dose-volume histograms and conformation of the 80% dose surface with the surface of the target. The goal was that the minimum target dose would not be less than 80% of the maximum dose.

Practical treatment plans for which the minimum target dose exceeded 80% were obtained for the three orthogonal orientations of the target for the five target locations. Plan parameters were essentially independent of the target position, dependent only on target orientation. The 80% isodose contour surface enclosed on average 2.8 cm3 larger volume (range: 2.1-3.9 cm3) than the prescribed 11.45 cm3 target. The minimum dose to the target 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 cm3 of nontarget volume (range: 12.7-15.9 cm3). Average deviation in the principal planes of the 80% isodose lines from the surface of the target volume was 0.95 mm for the 25 mm dimension (range 0.0 to +1.9 mm) and 0.86 mm (range 0.0 to + 2.4 mm) for the 35 mm dimension.

Standardized single isocenter treatment plans with the isocenter at the center of the target can achieve good conformation of the dose distribution to targets elongated along any of the principal axes, and located anywhere in the brain.