Mean Brain Dose Remains Uninfluenced by the Lesion Number for Gamma Knife Stereotactic Radiosurgery for 10+ Metastases

Objective: Gamma Knife (GK) stereotactic radiosurgery (SRS) is increasingly used as an initial treatment for patients with 10 or more brain metastases. However, the clinical and dosimetric consequences of this practice are not well established. Methods: We performed a single-institution, retrospective analysis of 30 patients who received Gamma Knife SRS for 10 or more brain metastases in 1 session. We utilized MIM Software to contour the whole brain and accumulated the doses from all treated lesions to determine the mean dose delivered to the whole brain. Patient outcomes were determined from chart review. Results: Our cohort had a median number of 13 treated lesions (range 10–26 lesions) for a total of 427 treated lesions. The mean dose to the whole brain was determined to be 1.8 ± 0.91 Gy (range 0.70–3.8 Gy). The mean dose to the whole brain did not correlate with the number of treated lesions (Pearson r = 0.23, P = 0.21), but was closely associated with tumor volume (Pearson r = 0.95, P < 0.0001). There were no significant correlations between overall survival and number of lesions or aggregate tumor volume. Fourteen patients (47%) underwent additional SRS sessions and 6 patients (20%) underwent whole-brain radiotherapy with a median of 6.6 months (range 3.0–50 months) after SRS. Two patients (6.6%) developed grade 2 radionecrosis following SRS beyond earlier whole-brain radiotherapy. Conclusion: The mean dose to the whole brain in patients treated with Gamma Knife SRS for 10 or more brain metastases remained low with an acceptable rate of radionecrosis. This strategy allowed the majority of patients to avoid subsequent whole-brain radiotherapy.