EVALUATION OF VOLUMETRIC RESPONSE ASSESSMENT FROM STEREOTACTIC ABLATIVE RADIOTHERAPY (SABR) FOR RENAL CELL CARCINOMA (RCC)

Abstract

PurposeStereotactic ablative radiotherapy (SABR) is increasingly used to treat renal cell carcinoma (RCC). However, the optimal method to assess treatment response is unclear. We aimed to quantify changes in both volume and maximum linear size of tumors after SABR, and evaluate the utility of the two approaches in treatment response assessment. Methods and MaterialsWe retrospectively studied patients with RCC treated with SABR at our institution between 2013 and 2020. All available follow-up CT scans were aligned, and tumors were contoured on all scans. Volume and maximum linear size were measured at each follow-up, relative to these measurements at the time of CT simulation. ResultsTwenty-four patients with 25 tumors were included. Median follow-up was 32 months (range: 16-67). Nineteen tumors (76%) had 30% volumetric response at a median time of 7 months after SABR, while 12 tumors (48%) had 30% decrease in maximum linear size at a median time of 16 months. Eighteen tumors (72%) decreased in volume on first follow-up scan and continued to shrink, while 5 tumors (20%) displayed transient growth after SABR (average 24% increase in volume). Compared to T1a tumors, T1b or larger tumors were more likely to have transient growth (8% vs 33%, p=0.16), and had higher average relative volume 24 months after SABR (0.47 vs 0.8, p=0.022). ConclusionsVolume measurement results in more pronounced and earlier change compared to linear size measurement when assessing response to SABR. These findings may provide guidance when assessing treatment response for patients with RCC treated with SABR.