Dose conformity of gamma knife radiosurgery and risk factors for complications

Abstract

Purpose: To quantitatively evaluate dose conformity achieved using Gamma Knife radiosurgery, compare results with those reported in the literature, and evaluate risk factors for complications.

Methods and Materials: All lesions treated at our institution with Gamma Knife radiosurgery from May 1993 (when volume criteria were routinely recorded) through December 1998 were reviewed. Lesions were excluded from analysis for reasons listed below. Conformity index (the ratio of prescription volume to target volume) was calculated for all evaluable lesions and for lesions comparable to those reported in the literature on conformity of linac radiosurgery. Univariate Cox regression models were used to test for associations between treatment parameters and toxicity.

Results: Of 1612 targets treated in 874 patients, 274 were excluded, most commonly for unavailability of individual prescription volume data because two or more lesions were included within the same dose matrix (176 lesions), intentional partial coverage for staged treatment of large arteriovenous malformations (AVMs) (33 lesions), and missing target volume data (26 lesions). 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 patients evaluable for complications, there were one Grade 5, eight Grade 4, and 27 Grade 3 complications. Increased risk of toxicity was associated with larger target volume, maximum lesion diameter, prescription volume, or volume of nontarget tissue within the prescription volume.

Conclusions: Gamma Knife radiosurgery achieves much more conformal dose distributions than those reported for conventional linac radiosurgery and somewhat more conformal dose distributions than sophisticated linac radiosurgery techniques. Larger target, nontarget, or prescription volumes are associated with increased risk of toxicity.

Introduction

Radiosurgery using the Gamma Knife (Elekta Corporation, Atlanta, GA) typically encompasses targets within a 50% isodose surface (normalized to a point dose maximum) using multiple isocenters created by 201 circular isocentric beams nominally 4, 8, 14, or 18 mm in diameter 1, 2. Linac-based radiosurgery offers differing dose distributions and dose conformity related to the number of arcs, isocenters, beam shaping, and beam modification used 3, 4, 5, 6, 7, 8, 9. If a large contribution to the risk of complications of radiosurgery is from the volume of normal tissue within or immediately adjacent to the prescribed isodose surface, as suggested by normal tissue complication probability modeling for radiosurgery (10), more conformal dose distributions should allow targets to be treated more safely. Conformity index (the ratio of prescription volume to target volume) has been reported to range from 2.7 for conventional linac radiosurgery (3) to 1.8 for micromultileaf collimator radiosurgery (5). Direct comparisons of linac, proton, and Gamma Knife radiosurgery dose distributions have only been performed for a small number of test cases 11, 12. The current study was performed to quantitatively describe the target coverage and dose conformity of routine Gamma Knife radiosurgery in a large number of lesions treated at the University of California San Francisco from 1993 to 1998 and compare the results with those reported in the literature for simple and complex linac radiosurgery techniques. In addition, complications of radiosurgery were graded and analyzed with respect to treatment and conformity parameters.