Dose conformity of Gamma Knife radiosurgery and risk factors for complications

Purpose: To quantitatively evaluate dose conformity achieved using Gamma Kn ife radiosurgery, compare results with those reported in the literature, an d evaluate risk factors for complications. Methods and Materials: All lesions treated at our institution with Gamma Kn ife radiosurgery from May 1993 (when volume criteria were routinely recorde d) through December 1998 were reviewed. Lesions were excluded from analysis for reasons listed below. Conformity index (the ratio of prescription volu me to target volume) was calculated for all evaluable lesions and for lesio ns comparable to those reported in the literature on conformity of linac ra diosurgery. Univariate Cox regression models were used to test for associat ions between treatment parameters and toxicity. Results: Of 1612 targets treated in 874 patients, 274 were excluded, most c ommonly for unavailability of individual prescription volume data because t wo or more lesions were included within the same dose matrix (176 lesions), intentional partial coverage for staged treatment of large arteriovenous m alformations (AVMs) (33 lesions), and missing target volume data (26 lesion s). The median conformity indices were 1.67 for all 1338 evaluable lesions and 1.40-1.43 for lesions comparable to two linac radiosurgery series that reported conformity indices of 1.8 and 2.7, respectively. Among all 651 pat ients evaluable for complications, there were one Grade 5, eight Grade 4, a nd 27 Grade 3 complications. Increased risk of toxicity was associated with larger target volume, maximum lesion diameter, prescription volume, or vol ume of nontarget tissue within the prescription volume. Conclusions: Gamma Knife radiosurgery achieves much more conformal dose dis tributions than those reported for conventional linac radiosurgery and some what more conformal dose distributions than sophisticated linac radiosurger y techniques. Larger target, nontarget, or prescription volumes are associa ted with increased risk of toxicity. (C) 2001 Elsevier Science Inc.