Incidentally diagnosed renal masses are on the rise, in part due to increased use of cross-sectional imaging.¹ Treatment for renal masses and localized renal cell carcinoma (RCC) has evolved significantly and now prioritizes partial nephrectomy (PN), especially in cT1a renal masses. While PN remains the gold standard treatment in select patients, thermal ablation (TA) and active surveillance have arisen as excellent treatment options. 2021 AUA guidelines suggest considering ablation in lesions less than 3 cm but only recommend cryoablation and radiofrequency ablation (RFA).² Image-guided therapy has progressed, however, and newer techniques now exist to treat our patients with renal masses. Microwave ablation (MWA) has largely replaced RFA, and emerging data suggest that stereotactic ablative radiotherapy (SABR) may also play a role in renal mass management. We highlight these 2 modalities and how they may be best applied to provide optimal outcomes for our patients.

Despite the most recent AUA guideline recommendations, interventional radiologists do not limit TA of small renal masses (SRMs) to cryoablation or RFA, and Society of Interventional Radiology guidelines include MWA in the management of SRMs.³ MWA offers comparable treatment efficacy to cryoablation, with several studies and meta-analyses demonstrating local tumor control of 96%-100% and cancer-specific survival similar to that observed in PN.^4-6 Major complication rate after MWA is low, in the 1.8% (range 0.6%-3.3%), highlighting MWA as a safe, effective treatment for SRMs that offers comparable oncologic control to surgery if allowing for retreatment.⁵ Limitations to TA are primarily tumor size (typically <3 cm) and need for percutaneous access.⁷ Such limitations make alternative treatments such as SABR both attractive and medically necessary options.

Urologists may recall a time when radiation was deemed an ineffective treatment for RCC due to radioresistant behavior with conventional modalities such as external beam radiation.^8,9 Unlike traditional radiotherapy, SABR is administered either in single or hypofractionated fashion with ∼25 Gy and ∼40 Gy in 1-10 fractions delivered, respectively. This provides the ability to administer highly targeted radiation to renal tumors without the need for surgery or invasive percutaneous renal access.^7,10,11 Since its preclinical introduction in 2003 and the first clinical trials published on the topic in 2007, great strides have been made in examining the safety and efficacy of SABR in treating renal masses, with a recent publication by the International Radiosurgery Oncology Consortium for Kidney (IROCK) showing local failure of only 5.5% at 5 years.^12-15 SABR also eschews the limitations inherent to other noninvasive treatment options due to tumor location and size while offering comparable rates of cancer-specific survival to both PN and cryoablation or RFA at 90%-95%.^15-17 Complication rates with SABR remain analogous to other treatments, with the most common side effects being nausea, fatigue, dermatitis, and enteritis. Grade 3 toxicity rates are noted infrequently with SABR (between 0% and 15.8%).^11,18,19 Studies have also shown decrease in estimated glomerular filtration rate following SABR to be acceptably low, roughly −6 mL/min across several studies, which is akin to the outcomes of both PN and TA.^10,20,21

The management of incidentally diagnosed renal masses has evolved significantly with PN as the gold standard for select patients. TA and active surveillance have also emerged as viable treatment options, with MWA now largely replacing RFA and endorsed by the Society of Interventional Radiology. SABR has gained increasing attention as well, particularly for medically inoperable patients, as supported by the National Comprehensive Cancer Network guidelines, but needs further long-term evaluation to assess its role in the management of RCC. As the management evolves to include not only interventional radiologists but also radiation oncologists, it is imperative that urologists are actively involved in the appropriate evaluation and defining the appropriate integration of these and any other new technologies that involve the treatment of localized renal tumors.

In conclusion, both SABR and MWA are viable treatment options for appropriately selected patients in the ever-evolving landscape of RCC treatment. Additionally, we would like to stress the importance of active involvement by urologists in the appropriate integration of radiotherapy, ablation, and any new technologies the future may hold in our practices.