16. GAMMA KNIFE CLINICAL DOSE PROFILE FOR EXTENSIVE BRAIN METASTASES

BACKGROUND: The use of Gamma Knife stereotactic radiosurgery (GKSRS) for the treatment of extensive intracranial metastases has been expanding due to its superior dosimetry and efficacy. However, there remains a dearth of data regarding the dose parameters in actual clinical scenarios. We endeavored to calculate the radiation dose to the brain when treating >15 brain metastases with GKSRS. METHODS: This retrospective analysis reviewed dosage characteristics for patients requiring single session GKSRS for the treatment of (3)15 brain metastases. Forty-two patients met the inclusion criteria between 2008 and 2017. The median number of tumors at the initial GKSRS procedure was 20 (15–39) which accounted for 865 tumors in this study. The median aggregate tumor volume was 3.1cm(3)(0.13–13.26) and the median marginal dose was 16Gy (14-19Gy). RESULTS: The median of the mean brain dose was 2.58Gy (range 0.95–3.67Gy) and 79% of patients had a dose <3Gy. The 12Gy dose volume was a median of 12.45cm(3), which was equivalent to 0.9% of the brain volume. The median percentage of brain receiving 5Gy and 3Gy was 6.7% and 20.4%, respectively. There was no correlation between the number of metastases and the mean dose to the brain (p=0.8). A higher tumor volume was significantly associated with an increased mean brain dose (p<0.001). The median of the mean dose to the bilateral hippocampi was 2.3Gy. Sixteen patients had supplementary GKSRS, resulting in an additional mean dose of 1.4Gy (0.2–3.8Gy) to the brain. CONCLUSION: GKSRS is a viable means of managing extensive brain metastases. This procedure provides a relatively low dose of radiation to the brain, especially when compared to traditional whole brain radiation protocols.