First Ever Use of Proton Stereotactic Body Radiation Therapy Delivered with Curative Intent to Bilateral Synchronous Primary Renal Cell Carcinomas

: Limited therapeutic options exist for inoperable bilateral kidney tumors. We report the first ever use of proton therapy to treat primary renal cell carcinoma (RCC) and the first report of proton stereotactic body radiation therapy (SBRT) for RCC in an inoperable patient with synchronous RCCs treated with proton SBRT. The patient is a 47-year-old 450-pound female with multiple medical comorbidities, including Stage 3 chronic kidney disease (CKD), who was found to have bilateral renal masses during work-up for cellulitis and sepsis. Following resolution of her sepsis, subsequent cross-sectional imaging demonstrated interval growth of the left renal mass to 4.4 x 4.8 cm and the right renal mass to 2.0 x 2.6 cm. Bilateral biopsies were performed, with pathology revealing Furhman Grade 1-2 clear cell RCC on both sides. A customized SBRT plan delivered a total dose of 3,000 cGy in five fractions to the bilateral kidneys every other day using proton beam therapy. The patient experienced no grade > 1 acute adverse toxicities from proton therapy, and now at one year after treatment, she has had no clinical symptoms of late radiation-induced toxicities. Although a marginal decline in post-treatment glomerular filtration rate (GFR) was observed (baseline 34 mL/min/1.73m2, one-year post-SBRT 29 mL/min/1.73m2), the patient remains asymptomatic and without a requirement for intervention. In presenting a case in which proton SBRT was performed safely and effectively for a medically complex patient with inoperable synchronous bilateral RCC, we suggest that proton therapy is a promising therapeutic approach for even unilateral primary RCC where preservation of renal function is of importance and should be considered for medically inoperable patients. Further experience is needed to determine the optimal dose and fractionation regimen for renal SBRT with proton therapy.