The effect of Gamma Knife radiosurgery on large posterior fossa metastases and the associated mass effect from peritumoral edema.

Gamma Knife radiosurgery (GKRS) as monotherapy is an option for the treatment of large (≥ 2 cm) posterior fossa brain metastases (LPFMs). However, there is concern regarding possible posttreatment increase in peritumoral edema (PTE) and associated compression of the fourth ventricle. This study evaluated the effects and safety of GKRS on tumor and PTE control in LPFM. The authors performed a single-center retrospective review of 49 patients with 51 LPFMs treated with GKRS. Patients with at least 1 clinical and radiological follow-up visit were included. Tumor, PTE, and fourth ventricle volumetric measurements were used to assess efficacy and safety. Overall survival was a secondary outcome. Fifty-one lesions in 49 consecutive patients were identified; 57.1% of patients were male. At the time of GKRS, the median age was 61.5 years, and the median Karnofsky Performance Status score was 90. The median number of LPFMs and overall brain metastases were 1 and 2, respectively. The median overall tumor, PTE, and fourth ventricle volumes at diagnosis were 4.96 cm3 (range 1.4-21.1 cm3), 14.98 cm3 (range 0.6-71.8 cm3), and 1.23 cm3 (range 0.3-3.2 cm3), respectively, and the median lesion diameter was 2.6 cm (range 2.0-5.07 cm). The median follow-up time was 7.3 months (range 1.6-57.2 months). At the first follow-up, 2 months posttreatment, the median tumor volume decreased by 58.66% (range -96.95% to +48.69%, p < 0.001), median PTE decreased by 78.10% (range -99.92% to +198.35%, p < 0.001), and the fourth ventricle increased by 24.97% (range -37.96% to +545.6%, p < 0.001). The local control rate at first follow-up was 98.1%. The median OS was 8.36 months. No patient required surgical intervention, external ventricular drainage, or shunting between treatment and first follow-up. However, 1 patient required a ventriculoperitoneal shunt at 23 months from treatment. Posttreatment, 65.30% received our general steroid taper, 6.12% received no steroids, and 28.58% required prolonged steroid treatment. In this retrospective analysis, patients with LPFMs treated with GKRS had a statistically significant posttreatment reduction in tumor size and PTE and marked opening of the fourth ventricle (all p < 0.001). This study demonstrates that GKRS is well tolerated and can be considered in the management of select cases of LPFMs, especially in patients who are poor surgical candidates.