ESTRO 2025 - Abstract Book

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Late-breaking abstracts

ESTRO 2025

Conclusion In this very high-risk prostate cancer cohort, we validate previous findings of PSMA PET/CT and GC scores. PSMA PET/CT upstaged approximately one in three patients, and upstaging increased with the number of risk factors and GC score. Interestingly, over half of the patients with a very high GC score (>0.85) did not have extraprostatic disease on PSMA PET/CT. The THUNDER trial will assess PSMA- and GC-guided treatment adaptation strategies.

Keywords: THUNDER trial, PSMA PET, genomic classifier

References [1] Hofman MS, et al. Lancet. 2020;395(10231):1208-1216. doi:10.1016/S0140-6736(20)30314-7 [2] Nguyen PL, et al. Int J Radiat Oncol Biol Phys. 2023;116(3):521-529. doi:10.1016/j.ijrobp.2022.12.035 [3] Seifert R, et al. Eur Urol. 2023;83(5):405-412. doi:10.1016/j.eururo.2023.02.002 [4] Attard G, et al. Lancet. 2022;399(10323):447-460. doi:10.1016/S0140-6736(21)02437-5

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Proffered Paper Bone-sparing chemoradiotherapy for anal cancer – results of a phase II trial by the Danish Anal Cancer Group – The DACG II (NCT05385250) Karen-Lise G. Spindler 1,2 , Anne V. Jakobsen 1,2 , Erik M. Pedersen 3 , Christina M. Lutz 1 , Lars Nyvang 1 , Jolanta Hansen 1 , Anne Ramlov 1 , Christian A. Hvid 1 , Eva Serup-Hansen 4 , Eva E. Wilken 4 , Lars U. Fokdal 5 , Birgitte M. Havelund 5 , Sean P. McIlroy 5 , Camilla J.S. Kronborg 6 1 Dept. of Oncology, Aarhus University Hospital, Aarhus, Denmark. 2 Dept. of Clinical Medicine, Aarhus University, Aarhus, Denmark. 3 Dept. of Radiology, Aarhus University Hospital, Aarhus, Denmark. 4 Dept. of Oncology, Copenhagen University Hospital - Herlev and Gentofte, Herlev, Denmark. 5 Dept. of Oncology, Vejle Hospital, University Hospital of Southern Denmark, Vejle, Denmark. 6 Danish Center for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark Purpose/Objective Radiotherapy-induced bone damage and pelvic insufficiency fractures (PIF) are known complications from pelvic cancer treatment. The true incidence is unknown but reported up to 50 %. In two recent pilot studies, bone reactions detected with bone specific MRI were 35% in rectal and 50% in anal cancer, causing symptoms in ~50% of cases 1-2 . This phase II trial aimed to reduce PIF incidence to < 35% on bone-specific MRI findings and associated symptoms in squamous cell anal cancer (SCCA). Material/Methods Patients were prospectively included according to criteria; biopsy verified localized SCCA, eligible for standard (chemo)-radiotherapy with curative intent, written and oral consent, age >18 years. Exclusion criteria comprised previous pelvic radiotherapy or systemic therapy with severe bone marrow suppression, hematological diseases, hip-replacements and contraindications to MRI-scan. Treatment followed DACG guidelines 3 using VMAT with three or four arcs. Detailed bone sub-structures were delineated using a standardized atlas 4 and an AI-based delineation tool was developed, to reduce bone contouring time. Two treatment plans were compared—one standard and one optimized for bone-sparing— the optimal plan was selected for treatment. Toxicity data were collected at baseline, during chemoradiotherapy (CRT), and at 1- and 3-years post-therapy. The primary endpoint was PIF rate at one year on bone-specific MRI. N=85 patients would detect a reduction in PIF from 50% to 35 %. Results 100 patients were enrolled in three departments, most having T2 tumors and p16-positive disease. All patients except 3 completed the prescribed radiotherapy dose between 54–64 Gy, and 97% received concomitant chemotherapy. The primary endpoint was met in February 2025, and MRI available for 79 patients. At one-year follow-up, 95 patients were alive, 1 had died from SCCA, and 4 from other causes. Local failures occurred in 8 patients and 6 experienced distant failures. Bone-sparing treatment plans consistently reduced bone dose