Response evaluation and tumor shrinkage pattern post-SBRT for renal cell carcinoma

Purpose

Despite the promising application of stereotactic body radiotherapy (SBRT) in renal cell carcinoma (RCC), the optimal time and method for assessing tumor responses to SBRT remain unclear. We aimed to compare the utility of Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 and modified RECIST (mRECIST) in RCC response assessment and clarify the tumor shrinkage pattern post-SBRT.

Materials and methods

We retrospectively studied patients with RCC treated with SBRT at our institution between November 2016 and December 2021. Baseline and follow-up images were evaluated using RECIST 1.1 and mRECIST. Landmark analyses were conducted to assess the association between local control and tumor response status evaluated using the above two criteria.

Results

Ninety-one RCC patients with 103 primary or metastatic lesions were included. Objective response rates (ORRs) were higher and pseudoprogression was less commonly assessed by mRECIST than by RECIST 1.1 within each time period. The median shrinkage rate of bone lesions was much lower using RECIST 1.1 than that using mRECIST (−0.8 vs −1.7 cm/year). According to both criteria, the ORR tended to be stable on the 9–12 months post-SBRT. Patients with a tumor response at 9–12 months had a lower probability of infield recurrence using mRECIST (P = 0.047), this was not observed using RECIST 1.1 (P = 0.061).

Conclusion

mRECIST results in an earlier and more pronounced response evaluation compared to RECIST 1.1 in patients with RCC treated with SBRT. Nine to twelve months post-SBRT may serve as an early landmark for tumor response evaluation by mRECIST.