Intraoperative radiotherapy (IORT) for surgically resected brain metastases: Local control and dosimetric analysis.

114 Background: The optimal use of adjuvant radiation following surgical resection of large brain metastases (BM) remains undetermined. Time to initiation following surgery and target delineation both impact local control (LC). Intraoperative radiotherapy (IORT) allows for elimination of lag time between surgery and radiation, direct cavity targeting, and safe dose escalation beyond traditional stereotactic radiosurgery (SRS). The current study provides an analysis of local disease control and dosimetric parameters related to intracranial IORT. Methods: Retrospective data was collected on patients treated with IORT immediately following surgical resection of BMs at three institutions according to the approval of individual IRBs. All patients were treated with the Zeiss Intrabeam device (Carl Zeiss Meditech, Germany) using spherical applicators ranging from 1.5 to 4.0cm with 50kV output. Statistical analyses were performed using SPSS (IBM) with endpoints of LC and incidence of RN, with p < 0.05 considered significant. Dosimetric comparisons between IORT and SRS were made based on V10, V12, and dose homogeneity based on percent of GTV receiving greater than 20Gy or 30Gy. Results: 54 patients were treated with IORT with a median age of 64 years. The most common primary diagnosis was non-small cell lung cancer (40%) with the most common location in the frontal lobe (38%). Median follow-up was 7.2 months and 1-year LC rate was 88% with radiation necrosis (RN) present in 4 patients (7%). The dosimetric comparison of a single IORT case revealed non-target V10 and V12 volumes as 24.75cm3 and 14.76cm3, respectively, for the SRS treatment plan of 16Gy to the margin. The V10 and V12 for the IORT treatment plan were 20.83cm3 and 9.93cm3 with a surface dose of 30Gy. The volumes exceeding 20Gy and 30Gy in the SRS plan were 14.73cm3 and 0.328cm3, respectively, while the corresponding volumes in the IORT plan were 9.8cm3 and 0cm3. Conclusions: IORT is a safe and effective means of delivering adjuvant radiation to the BM resection cavities with a high rate of LC, low incidence of RN, increased homogeneity of target dose and ability to escalate dose beyond traditional SRS plans.