Treatment planning for gamma knife radiosurgery with multiple isocenters.

Many arteriovenous malformations and tumors suitable for radiosurgical treatment have non-spherical or irregular shapes. Forty-eight percent of the first 156 patients treated with the gamma unit at the University of Pittsburgh required treatment with two or more isocenters to optimize dose distributions. Dose distributions for combining gamma knife treatments to two or more isocenters were systematically investigated. High speed computerized dosimetry was performed using specially developed software and dose distributions were confirmed with film densitometry. We have developed guidelines for treatment to two or more isocenters which help reduce treatment planning time, and facilitate selection of treatment doses and optimum dose distributions. These guidelines include maintaining an account of the distances between all isocenters, using a catalogue of sample two-isocenter isodose plans, comparing dose volume histograms, and calculating complication probabilities using the integrated logistic formula.