Stereotactic body radiation therapy (SBRT) represents a novel, efficacious treatment for patients with kidney tumors who are medically inoperable or decline surgery. There is limited prospective data on the impact of kidney SBRT on renal function.

This was a prospective phase 2 single-arm clinical trial (clinicaltrials.gov NCT03747133) of kidney-directed SBRT in patients with primary or metastatic renal lesions who were medically inoperable or declined surgery. The primary outcome was the change in kidney function, assessed by the change in eGFR (estimated glomerular filtration rate) over 2 years. The a priori hypothesis was that eGFR (mL/min/1.73 m) does not decrease over time and was analyzed using a 1 sample t-test for noninferiority with a fixed margin of -6.974 based on published data at the time of trial design.

Thirty patients with 32 renal tumors enrolled, with a median (IQR) age of 76 (73-82), a Charlson comorbidity index of 8 (7-9), 93% with chronic kidney disease stage ≥2 (eGFR ≤60 mL/min/1.73 m), and the majority with cT1b disease with a median tumor size of 43 mm. Twenty-six patients (87%) had primary kidney cancer, and the remainder had metastatic lesions from non-kidney cancer histologies. Median radiation dose was 35 Gy in 5 fractions. Median follow-up was 24.5 months (IQR, 20-36.2). Median (IQR) eGFR levels (mL/min/1.73 m) were 47.5 (37.8-64.0) at baseline, 42.0 (35.2-54.2) at 1 year, and 39.5 (25.5-54.8) at 2 years. Eighteen of 30 patients were evaluable for eGFR at 2 years. Noninferiority was not established based on a mean reduction in eGFR between baseline and 2 years of -8.7 mL/min/1.73 m (95% 1-sided CI, -14.1, ∞; P = .71). Renal function decline was significantly associated with time, increasing age, baseline chronic kidney disease stages 3-4, and larger baseline tumor size on multivariable analysis. Local control was 96.7% at 2 years.

Kidney-directed SBRT results in modest clinical renal function loss up to 2 years following SBRT, based on evaluable patients in our study. Technical advances may further improve the therapeutic ratio.