S1267
Clinical - Urology
ESTRO 2026
cell carcinoma (RCC), achieving excellent local control with minimal decline in renal function.1,2 However, because FASTRACK II excluded patients with bowel abutting the target,2 NCCN guidelines advise against SBRT in such cases due to gastrointestinal toxicity risk. Adaptive radiotherapy (ART) allows on-table replanning to accommodate daily bowel motion while maintaining target coverage. This study evaluates the dosimetric benefits of ART compared with scheduled plans and whether a planning organ-at-risk volume (PRV) margin sufficiently accounts for anatomic variation. Material/Methods: We retrospectively reviewed patients with localized RCC treated with adaptive SBRT on the Ethos platform. OARs included small bowel, large bowel, stomach, and duodenum. Initial plans (PI) were created from simulation CTs, and adaptive plans (PA) were generated based on anatomy-of-the-day. For each fraction, PI was applied to the patient’s anatomy-of- the-day to obtain the scheduled plans (PS), and compared with the reoptimized PA using dose-volume histogram metrics, with selection of the superior plan for treatment. Plans with 5-mm bowel PRV margin (PPRV) were created from simulation CT. Results: Five patients (median age 77 years; range 59-88) were included. One patient received 26 Gy in 1 fraction, and four patients received 42 to 48 Gy in 3 fractions on alternate days. Median tumor size was 5.9 cm (3.2-8.5 cm). Coverage was comparable between PS and PA, with the dose covering 95% of the planning-target- volume (i.e., D95%) achieving 70.1% and 77.1%(p=0.16) of the prescription (Fig. 1), respectively. With a median follow-up of 5 months, the median glomerular filtrate rate decline was 3 mL/min from a baseline of 47 mL/min. Across all patients, bowel abutted the target in 12 of 13 delivered fractions. Applying PS to daily anatomy resulted in OAR constraint violations in all fractions – 7 stomach, 3 duodenum, 14 small bowel, and 6 large bowel – versus no violations with PA. Importantly, the abutting bowel segment fell outside the fixed PRV in treated fractions (mean volume 8.13 cm3), indicating that a fixed PRV would not have adequately captured inter-fraction bowel variation (Fig. 2).
Conclusion: Two-fraction and five-fraction SBRT showed similar acute-to-subacute hematologic AEs, while both regimens caused a significant drop in WBC count in the acute phase. The CD4+ count showed a numerically lower level 5F compared to 2F without statistical significance. Acceptable acute GU and GI AEs with effective PSA response were demonstrated. Keywords: White blood cell, SBRT, Prostate Expanding SBRT Eligibility in Renal Cell Carcinoma Through Adaptive Planning: Overcoming Bowel Proximity Limitations meiying xing 1 , qing li 1 , Kenneth Gregg 1 , Breanna Peyton 1 , Beatriz Guevara 1 , Nicholas Zaorsky 1 , Lauren Henke 1 , Lee Ponsky 2 , Daniel Spratt 1 , Rojano Kashani 1 , Alex Price 1 , Angela Jia 1 1 radiation oncology, university hospitals, cleveland, USA. 2 Department of Urology, university hospitals, cleveland, USA Purpose/Objective: Stereotactic body radiotherapy (SBRT) is an established non-invasive treatment option for renal Digital Poster 3497
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