Is RFA or SBRT superior for patients with small renal masses?

Presenting author:
Anand Swaminath, MD

By Amar U. Kishan, MD, University of California, Los Angeles
Presenting author - Anand Swaminath, MD, Juravinski Cancer Centre

The incidence of renal cell carcinoma (RCC) is rising, with the peak incidence occurring between the seventh and eighth decades of life. Because additional risk factors include smoking and hypertension, many patients with localized RCC may harbor a significant risk from anesthesia and surgery. This highlights the need for non-invasive but effective treatments. External beam radiotherapy would theoretically represent the ideal, completely non-invasive approach; however, RCC was thought to be radioresistant, at least in the context of conventional fraction sizes. Thus, radiofrequency ablation (RFA) has been a standard minimally invasive ablative option in this population. Over time, however, mounting in vitro and clinical data suggested that RCC may respond well to hypofractionated radiotherapy and particularly to ultrahypofractionated radiotherapy. Simultaneous improvements in radiation planning and delivery techniques led to the development of stereotactic body radiotherapy (SBRT) or stereotactic ablative radiotherapy (SABR), which allows the precise delivery of extremely high doses of radiation per fraction. SBRT for RCC is now supported by large multi-institutional series. As of 2023, the National Comprehensive Cancer Network notes that “Stereotactic body radiation therapy (SBRT) may be considered for medically inoperable patients with stage I kidney cancer (category 2B) or with stage II/III kidney cancer (both category 3).”

The natural question is whether RFA or SBRT would be superior for patients with small masses, defined as those measuring four centimeters or less. As an early test of feasibility of randomization, Swaminath and colleagues performed the Randomized Pilot Trial of Stereotactic Body Radiation Therapy vs. Radiofrequency Ablation for the Management of Small Renal Masses (RADSTER) trial. In this trial, patients who were deemed inoperable or declined surgery or surveillance were randomized to receive SBRT or RFA at a tertiary center. Crossover was allowed if the assigned treatment could not be delivered. SBRT was delivered as a single fraction of 25 Gy, while RFA was conducted with two cycles of eight minutes each. Imaging by CT or MRI was conducted every three months, with diagnostic and one-year biopsies required.

Twenty-four patients were enrolled from January 2020-June 2021, the majority of whom had clear cell RCC. Ultimately, 14 patients received SBRT and seven had RFA (with patients crossing over due to technical inability to perform RFA); three declined treatment. No late toxicity through one year was found in either group. At one year, no radiographic local failure (RECIST) was observed. Mean reduction of estimated glomerular filtration rate was similar at one year (RFA -3 mL/min, SBRT -5.3 mL/min, p=0.7). Biopsies were performed in 20/24 patients at one year. On per-protocol analysis seven out of seven (100%) of RFA patients had no evidence of residual RCC, whereas with SBRT four out of 13 (31%) patients had no evidence of residual RCC, two out of 13 (15%) had scant/minimal residual disease, and seven out of 13 (54%) had evidence of RCC.

Overall, the RADSTER trial results suggest that randomization between SBRT and RFA is feasible, supporting the execution of a larger randomized trial to definitely compare efficacy. The results also may be important with respect to clinical trial design. The clinical data from non-randomized series, such as the IROCK series, suggest five-year local failure rates after SBRT of only 5.5%, while in RADSTER, the one-year biopsy rate suggested RCC persistence in 54%. This may suggest that pathologic response to SBRT, as opposed to RFA, may be a more prolonged process, and a better understanding is necessary for interpreting and designing future clinical trials.

Abstract 260 - Final Results from a Prospective Randomized Pilot Trial of Stereotactic Body Radiation Therapy vs. Radiofrequency Ablation for the Management of Small Renal Masses (RADSTER) was presented on Tuesday, October 3 during the SS 28: GU 4: Radiotherapy for Kidney Cancer and Post-Prostectomy session at the 2023 ASTRO Annual Meeting.

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