Brain Metastases Resection Cavity Radiosurgery Based Primarily on T2-weighted MRI: Technique Assessment

Abstract

Stereotactic radiosurgery (SRS) is commonly performed after surgical resection of brain metastases to reduce the chance of local tumor recurrence while maintaining cognitive function. Target delineation in these cases is typically based on T1-weighted post-gadolinium MRI (T1Gd). In this study, we report outcomes for patients having postoperative SRS in which the planning target volume (PTV) was based primarily on T2-weighted MRI (T2W). Sixty-two consecutive patients having single-fraction SRS after brain metastases resection were retrospectively reviewed. Patients with prior whole brain radiation therapy, multiple resection cavities, and small cell pathologies were excluded. Local control (LC), leptomeningeal disease (LMD), distant brain recurrence (DBR), and radiation necrosis (RN) were estimated by the Kaplan-Meier method. Cox proportional hazards model was performed on tumor and treatment factors (histology, number of metastases, preoperative tumor characteristics, time from surgery to SRS, extent of resection, PTV, and margin dose) against LC, LMD, and DBR. PTV based on T1Gd was independently contoured according to consensus guidelines and classified as decreased (<90% of the T2W PTV), unchanged (90%-110% of the T2W PTV), or increased (>110% of the T2W PTV). The median preoperative tumor size was 3.3 cm (range, 1.4-5.3); the median preoperative tumor volume was 12.9 cm3 (range, 1.4-62.6). The median PTV was 8.0 cm3; the median margin dose was 18 Gy. The median time from surgery to SRS was 11 days (range, 1-49); 26 patients (42%) had SRS ≤ 7 days. The median follow-up time after SRS was 15.1 months (range, 3-57). The 1-year LC, LMD, DBR, and RN rates were 88%, 25%, 36%, and 0%, respectively. No factor was associated with LC. Sub-total tumor resection was a risk factor for LMD (Hazard ratio [HR] 5.11, 95% Confidence interval [CI] 1.52-17.22, P = 0.003), whereas patients with multiple brain metastases had a greater risk of DBR (HR 2.88, 95% CI 1.24-6.67, = 0.01). Overall, the median PTV based on T2W was smaller compared to median PTV based on the consensus guidelines utilizing T1Gd (8.0 cm3 versus 9.1 cm3, P = 0.004). PTV based on T1Gd compared to T2W was decreased in 11 patients (18%; median volume decrease, -27%), unchanged in 14 patients (23%), and increased in 37 patients (60%; median volume increase, 29%). T2W MRI could provide accurate resection cavity delineation guidance even in the early postoperative period and was associated with decreased PTV compared to T1Gd MRI in the majority of cases.