Dose-effect Relationship of Kidney Function After Stereotactic Ablative Body Radiotherapy for Primary Renal Cell Carcinoma: Trog 15.03 Fastrack Ii

Background Stereotactic ablative body radiotherapy (SABR) is a novel option to treat primary renal cell carcinoma (RCC). However, a high radiation dose may be received by the treated kidney, which may affect its function post-treatment. This study investigates the dose-effect relationship of kidney SABR with posttreatment renal function. Methods This was a prespecified secondary endpoint of the multicenter FASTRACK II clinical trial (NCT02613819). Patients received either 26 Gy in a single fraction (SF) for tumors with a maximal diameter of 4 cm or less, or 42 Gy in three fractions (multi-fraction (MF)) for larger tumors. To determine renal function change, 99mTc-DMSA SPECT/CT scans were acquired, and the glomerular filtration rate (GFR) was estimated at baseline, 12-, and 24-months post-treatment. Imaging datasets were rigidly registered to the planning CT where kidneys were segmented to calculate dose-response curves. Results From 71 enrolled patients, data from 36 (51%) and 26 (37%) patients were included in this study based on availability of post-treatment data at 12- and 24-months, respectively. The ipsilateral kidney GFR decreased from baseline by 42% and 39% in the SF cohort, and by 45% and 62% in the MF cohort, at 12- and 24-months respectively (p-values < 0.03). The loss in renal function was 3.6±0.8% and 4.5±1.0% in the SF cohort, and 1.7±0.1% and 1.7±0.2% in the MF cohort, at 12- and 24-months respectively. The major loss in renal function occurred in high-dose regions, where dose-response curves converged to a plateau. Conclusions For the first time in a multicenter study, the dose-effect relationship at 12- and 24-months post-SABR treatment for primary RCC was quantified. Kidney function reduces linearly with dose up to 100 Gy BED3.