The effectiveness and factors related to treatment results of gamma knife radiosurgery for meningiomas.

A retrospective analysis was conducted on 80 patients with intracranial mengiomas treated with Gamma Knife radiosurgery between 1993 and 1996. The purpose was to analyze the efficacy of the treatment and to assess appropriate treatment parameters. The results were assessed by regular MR examinations, and tumor volume was measured at 6-month intervals. Mean follow-up duration was 21 months (range 6-45 months). 63 meningiomas were at the skull base and 17 were distal from the skull base. Tumor volumes <5 ml (n=38), 5-10 ml (n=21), 10-15 ml (n=14), 15-20 ml (n=7). The patients were divided into 3 groups according to the radiation dose. The groups were high-dose (peripheral dose 17-20 Gy, n=19), medium-dose (15-16 Gy, n=33) and low-dose (12-14 Gy, n=28) groups. A statistical method (Generalized Estimation Equation) was applied to compare treatment results in these groups with different doses and tumor volumes. The volume measurement at the latest follow-up showed 74% (59/80) meningiomas decreased in volume, 17% (14/80) had no tumor enlargement and 9% (7/80) had increased in volume. The increased volume was found more frequently in the patients with a short (6-12 months) follow-up period. In this series, the tumors had 32&percnt reduction in average tumor volume at 3 years after radiosurgery. At the range of 12-20 Gy peripheral dose (PD), radiosurgery was effective to reduce tumor volume 0.7% per month (p<0.05). However, higher doses had no significant difference on tumor volume reduction (p>0.05). On the other hand, high-dose (PD>17 Gy) treatment was associated with a higher risk of temporary tumor swelling and the development of adverse radiation effects (AREs). The AREs detected on MR images occurred in (25/80) 31% patients. Only 6/25 AREs were symptomatic and 2 had neurological sequelae. Peripheral doses, tumor volumes and their locations had significant impacts on the ARE (p<0. 05). In conclusion, a peripheral dose of 15-16 Gy may be adequate for meningiomas with small volumes (<5 ml). In larger tumors (>10 ml) a lower PD is preferred (12-14 Gy). To avoid initial tumor swelling and ARE, high-dose irradiation (PD>17 Gy) is not recommended for meningiomas larger than 5 ml.