Evaluation of Volumetric Response Assessment from Stereotactic Ablative Radiotherapy (SABR) for Renal Cell Carcinoma (RCC).

Abstract

SABR has recently been proven to be a safe, effective treatment for RCC. However, long term follow-up assessment can be challenging as traditional methods of assessment such as RECIST may not identify tumor response, and delayed responses to SABR are common. The purpose of this study was to quantify SABR response using volumetric changes from baseline throughout follow-up. We conducted a single-center retrospective study including all patients treated with SABR for RCC from 2013 to 2020. Patients were included if they had no metastatic disease, no resection after SABR, and had follow-up imaging available for at least 12 months following SABR. All available follow-up CT scans were collected and aligned, and tumors were contoured on all follow-up scans to measure volume and maximum linear dimension. Response to SABR was assessed by comparing tumor volume at follow-up to the initial tumor volume at the time of CT simulation. Twenty-four patients with 25 tumors were included. The dose was between 25Gy and 42Gy in 1-5 fractions; 64% of tumors received 35Gy in 5 fractions. Follow-up time was between 16 and 67 months; median follow-up was 32 months. A total of 164 follow-up scans were contoured. The relative volume and standard deviation (compared to baseline) at 3, 6, 9, 12, 24, 36, and 48 months was 0.76 (SD 0.22), 0.80 (SD 0.27), 0.76 (SD 0.32), 0.75 (SD 0.26), 0.61 (SD 0.33), 0.53 (SD 0.41), and 0.34 (SD 0.22) respectively. One tumor (4%) showed continued growth after SABR, while 24 of 25 tumors (96%) had durable volumetric response after SABR. Eighteen of 25 tumors (72%) decreased in volume on first follow-up scan and continued to shrink, while 6 of 25 tumors (24%) displayed growth after SABR (average 33% increase in volume) before shrinking below their initial volume. The median time to any volumetric response was 3.5 months (range 2-19 months). Nineteen tumors (76%) had 30% volumetric response. Median time to 30% volumetric response was 7 months (range 3-55 months). In contrast, 12 tumors (48%) had 30% decrease in maximum linear size, with median time to 30% linear response of 16 months (range 4-42 months). Thirteen tumors (52%) had 50% volumetric response; median time to 50% volumetric response was 11 months (range 4-37 months). Among tumors for which contrast-enhanced scans were available, 15 of 22 tumors (68%) displayed arterial enhancement at their most recent follow-up scan, at a median time of 24 months after SABR. SABR is a highly effective treatment for RCC, achieving durable local control in 96% of cases. Initial growth after SABR is common before volumetric response occurs. Volumetric response is more frequent and occurs earlier than decrease in maximum linear dimension, even with persistent arterial contrast enhancement following SABR.