ESTRO 2025 - Abstract Book

S1226

Clinical – Lower GI

ESTRO 2025

Variability in 80 CBCTs and planning CTs: The mean of each average was as follows: anterior 2.7mm (±4.55), posterior 1.12mm (±2.42), right 4.99mm (±6.07), and left 1.6mm (±6.13). Based on the equations, the suggested CTV margin was anterior 1.3cm, posterior 0.7cm, right lateral 1.7cm, and left lateral 1.8cm. Conclusion: The lateral variability of the MR may be due to pelvic contraction. Keeping the bladder full is crucial. Expanding the CTV margin is feasible, considering the impact on nearby organs and toxicity. References: Nugent, K., O’Neill, B., Brennan, V., Lynch, J., Higgins, M., Dunne, M., & Skourou, C. (2022). Quantification of Organ Motion in Male and Female Patients Undergoing Long-Course Radiation Therapy for Rectal Cancer in the Supine Position. Advances In Radiation Oncology , 8 (1), 101109. https://doi.org/10.1016/j.adro.2022.101109 Boldrini, L., Chiloiro, G., Cusumano, D., Romano, A., Placidi, L., Turco, G., Antonelli, M. Alickikus, Z. A., Kuru, A., Aydin, B., Akcay, D., & Gorken, I. B. (2019). The importance of mesorectum motion in determining PTV margins in rectal cancer patients treated with neoadjuvant radiotherapy. Journal Of Radiation Research , 61 (2), 335-342. https://doi.org/10.1093/jrr/rrz092 Digital Poster Lower FDG PET/CТ SUVmax is associated with better response to neoadjuvant radiotherapy combined with chemotherapy in patients with rectal carcinoma Temenuzhka R Radev- Petkova 1 , Elitsa P Encheva 2 , Radina R Georgieva 1 , Teodora S Gugleva 3 , Aneliya D Klisarova 4 , Zhivka D Mezan 4 1 Radiation Oncology, University Hospital St. Marina, Medical University of Varna, Varna, Bulgaria. 2 Radiation Oncology Department, University Hospital St. Marina, Medical University of Varna, Varna, Bulgaria. 3 Radiation Oncology Department, University Hospital St. Marina, Varna, Bulgaria. 4 Nuclear medicine, University Hospital St. Marina, Medical University of Varna, Varna, Bulgaria Purpose/Objective: Positron emission tomography (PET) is used for distant staging of rectal carcinoma. Studies have shown that FDG uptake by the tumor and the maximum standardized uptake value (SUVmax) may have predictive role, as patients with high FDG uptake usually have less favorable outcomes. The aim of our study was to evaluate SUVmax value in patients with locally advanced rectal carcinoma (LARC) before the start of neoadjuvant radiotherapy combined with chemotherapy (nCCRT) as well to investigate its correlation with tumor`s response rate Material/Methods: Patients with LARC (n=65) treated between 2015 and 2021 were included. All patients received nCCRT (using 45– 50.4 Gy in 25–28 fractions) with Capecitabine. Radiotherapy was delivered with IMRT and VMAT techniques. The radiation dose was based on the dose covering 95% of the PTV. 18F-FDG PET/CT imaging was performed before nCCRT for distant metastasis evaluation. Overall response rate (ORR), is the proportion of patients who have a partial response (PR) or complete response (CR) to the treatment. We assessed tumor response using MRI, CT, PET/CT, colonoscopy. SUVmax values before nCCRT were analyzed in relation to the tumor response to the therapy. Keywords: Mesorrectal, Variability, Long-course 3080