3215 - Adaptive Kidney Irradiation: The Impact of CT-Based Adaptive Radiotherapy on Kidney Cancer SBRT Delivery

Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) is an emerging treatment for localized kidney cancer. We report on our initial experience using computed tomography-based adaptive stereotactic body radiotherapy (CTA SBRT) to treat primary renal cell carcinoma (RCC) nonamenable to surgical management.

Materials/Methods: This single-institution retrospective analysis studied RCC patients treated definitively with CTA SBRT as their primary treatment from 2023-2025. The scheduled plan, (i.e, original plan recalculated on the updated contours), and adaptive plan, (i.e., reoptimized plan for updated contours) were compared daily. All patients were treated using SBRT techniques prescribing to either 42 Gy/3 fractions or 40 Gy/5 fractions. The distances from PTV to the nearest OAR were measured on the CBCT for each fraction in the axial plane. Standard descriptive statistics were used to summarize clinical and demographic characteristics. Generalized estimating equations (GEE) were used for the statistical analysis, with an identity link function, an exchangeable correlation structure, and robust standard errors. A two-sided significance level of 0.05 was applied.

Results: Forty-four fractions were included from the 10 eligible patients (mean age 75 years, 60% female). All patients treated had a history of chronic kidney disease with an average pretreatment eGFR of 50.6 and 4 (40%) patients had a solitary kidney. The average planned PTV volume was 138.4cc. Ninety-three percent (n=41) of the delivered plans were adaptive. The most common physician rationales for selecting adaptive treatment were improved OAR avoidance (76%) and/or improved target coverage (54%). In 7 (17%) treatment sessions, the scheduled plans were not available for evaluation due to variation between planned and updated daily anatomy, so these fractions were excluded from PTV and OAR outcome analyses. PTV V100% =95% goal was met in 84% (31) of adaptive plans and 46% (16) of scheduled plans. The adaptive PTV coverage was improved by an average of 2.24% (95% CI [0.43, 4.05]) when compared to scheduled (p=0.047). For fractions with PTV and OAR overlap, Dmax to nearest OAR was lower by an average of 82 cGy (95% CI [35.25,128.75]) with adapted plans (p=0.002). At a median follow up of 11 months, only one patient required initiation of renal replacement therapy, which was initiated approximately 7 months post-CTA-SBRT completion.

Conclusion: We demonstrate that adaptive plans improve target coverage to RCC lesions. Adaptive plans were also able to decrease the Dmax to the nearest OAR when there is evidence of PTV-OAR overlap. Future studies are necessary to determine the impact of CTA-SBRT on clinical outcomes, including eGFR and need for renal replacement therapy.