Gamma knife radiosurgery (GKRS) has recently been employed in patients with multiple brain metastases (METs), even those with five or more lesions. However, very little is known about whole-brain threshold doses, which may have adverse effects on the brain.

     We attempted to determine safe integral doses for the entire skull.

     

     We analyzed the treatment protocols of 1,246 GKRS procedures performed for 900 patients (40.3% of cohort) with five or more brain METs (maximum, 89) during the period between January 1999 and December 2009. The median lesion number was 10 (range: 5-89) and the median cumulative volume of all tumors was 7.3 (range: 0.1-115.3) cc. The median selected dose at the lesion periphery was 20 (range: 10-27) Gy.

     Integral skull doses were computed using the Leksell Gamma Plan (Elekta AB, Stockholm, Sweden). The median integral skull dose was 8.5 (0.2-28.7) Joules. The median brain volumes receiving >5 Gy and >12 Gy were 327 (range 5-1942) cc and 50 (range 0-313) cc. Among 578 patients (64%) who received less than 10 Joules, none experienced neurological death due to GKRS-related complications. Among 257 patients (29%) receiving doses between 10 and 15 Joules, only one case, who could not receive steroid therapy or undergo surgical removal due to poor systemic condition, experienced intractable seizures and severe perifocal edema after GKRS. Among 58 patients (6%) receiving doses between 15 and 20 Joules, one experienced gradual brain swelling due to necrotic change 8 months after GKRS. Among six patients (0.7%) who received over 20 Joules, one experienced acute brain swelling 2 days after GKRS.

     This was a retrospective study.

     We conclude that integral skull doses of 15 Joules or less are clearly safe, while those exceeding 20 Joules carry a relatively high risk of radiation-induced brain injury.

     The zone between lower and higher risk thus appears to be somewhere between 15 and 20 Joules.