S698
Clinical – Lower GI
ESTRO 2026
Complexity Surgical Oncology. Cancers (Basel) 2025;17:341. https://doi.org/10.3390/CANCERS17030341.[2] Fedorov A et al. 3D Slicer as an image computing platform for the Quantitative Imaging Network. Magn Reson Imaging 2012;30:1323–41. https://doi.org/10.1016/j.mri.2012.05.001.[3] 3D Slicer n.d. https://www.slicer.org/ (accessed November 4, 2025).[4] Groen HC et al. Use of Image-Guided Surgical Navigation during Resection of Locally Recurrent Rectal Cancer. Life 2022;12. https://doi.org/10.3390/life12050645. Keywords: Intraoperative Radiotherapy, Image-guided Surgery Neoadjuvant short-course radiotherapy followed by chemotherapy for rectal adenocarcinoma: real world analysis at a Brazilian public hospital Lucas Uglione Da Ros 1,2 , Lucas Fernando Fabra 3 , Valentina de Souza Stanham 1 , Laura Lessa Gaudie- Ley 1 , Arthur Bom Queiroz 1,3 , Rodrigo Perez Pereira 4 , Claudio Tarta 5 , Daniela Vargas Barletta 1,6 , Marta Nassif Pereira Lima 1,6 1 Radiotherapy, HCPA, Porto Alegre, Brazil. 2 PPGFT, UFRGS, Porto Alegre, Brazil. 3 PPGCM, UFRGS, Porto Alegre, Brazil. 4 Oncology, HCPA, Porto Alegre, Brazil. 5 Coloproctology, HCPA, Porto Alegre, Brazil. 6 Radiotherapy, Hospital Moinhos de Vento, Porto Alegre, Brazil Purpose/Objective: The multimodal approach to locally advanced rectal cancer aims to enhance oncological outcomes. Neoadjuvant chemotherapy and radiotherapy followed by surgery—a strategy known as Total Digital Poster 1567 Neoadjuvant Therapy (TNT)—has gained acceptance due to its improved tolerability and potential for achieving complete clinical response. Specifically, short-course radiotherapy (SCRT), as evaluated in trials like RAPIDO, has shown promise as an alternative for high-risk rectal cancer. This approach gained significant relevance during the COVID-19 pandemic, which necessitated adaptations in clinical practice to minimize patient exposure, optimize healthcare resource allocation, and ensure the continuity of essential cancer therapies. Furthermore, the reduction in the number of clinic visits offered by SCRT is particularly advantageous in low-income countries, such as Brazil. Material/Methods: This single-center retrospective cohort study analyzed patients treated with SCRT for rectal adenocarcinoma at a Brazilian public hospital. Inclusion comprised all patients with advanced rectal adenocarcinoma who
Analysis comprised qualitative visual inspection of the reconstructions and quantitative calculation of absolute and relative IOERT-PTV field overlap.
Results: IOERT fields were successfully reconstructed for all patients (Figure 2). In one case, reconstruction was challenging due to limited pointer accessibility in the deep and narrow lesser pelvis. Median overlap between reconstructed IOERT fields and neoadjuvant PTVs was 17.6 cm ³ (IQR 15.7-21.9 cm ³ ). This overlap was 88.2% (IQR 83.3-97.7%) relative to the IOERT field, meaning that IOERT was predominantly delivered within PTV. Subsequently, the median absolute IOERT field volume outside PTV comprised 3.0 cm ³ (IQR 0.4- 3.6 cm ³ ), representing 11.8% (IQR 2.3-16.7%) of the total IOERT field. These non-overlap regions involved anatomical structures not at risk, based on treating physicians’ expert opinion.
Conclusion: IOERT field reconstruction relative to PTV was feasible using an electromagnetic image-guided navigation system. The substantial overlap suggested spatial effectiveness, since IOERT was largely delivered within PTV and thus likely functioned as an effective boost. References: [1] Ewings S et al. The ELECTRA Trial: Approach to Contemporary Challenges in the Development and Implementation of Double-Blinded, Randomised, Controlled Clinical Trials in Low-Volume High-
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