S697
Clinical – Lower GI
ESTRO 2026
Digital Poster 1332
Spatial effectiveness of intraoperative electron radiotherapy in locally recurrent rectal cancer using electromagnetic image-guided navigation Marit Brouwer 1,2 , An-Sofie Verrijssen 3 , Fleur Vande Kerckhove 2,4 , Heike Peulen 3 , Joost Nederend 5 , Cathryn Huibregtse Bimmel-Nagel 3 , Rutger Stijns 2 , Grard Nieuwenhuijzen 2 , Jip Tolenaar 2 , Pim Burger 2,4 , Chiron Morsink 2 1 Technical Medicine, University of Twente, Enschede, Netherlands. 2 Department of Surgery, Catharina Hospital, Eindhoven, Netherlands. 3 Department of Radiation Oncology, Catharina Hospital, Eindhoven, Netherlands. 4 GROW Research Institute for Oncology and Reproduction, Maastricht University, Maastricht, Netherlands. 5 Department of Radiology, Catharina Hospital, Eindhoven, Netherlands Purpose/Objective: In locally recurrent rectal cancer (LRRC), intraoperative electron radiotherapy (IOERT) can be given as a radiotherapy boost following neoadjuvant chemo- reirradiation when achieving a radical resection is deemed at risk. IOERT is effective only when delivered within tissue that received neoadjuvant radiotherapy, i.e., within neoadjuvant planning target volumes (PTV). However, evaluating this spatial effectiveness has long been limited due to the lack of spatial IOERT field documentation [1]. At the Catharina Hospital (The Netherlands), an electromagnetic navigation system enables intraoperative reconstruction of the IOERT field. This exploratory study aimed to assess the spatial overlap between reconstructed IOERT fields and PTVs in five LRRC patients. Material/Methods: Data originated from a single-centre prospective interventional study (QUART, NL86388.100.24). LRRC patients treated between May 2024 and October 2025 were included, who underwent neoadjuvant chemo- reirradiation followed by surgery with IOERT, guided by electromagnetic navigation. Patient-specific anatomical (3D) models were created from registered preoperative (PET-)CT and MR imaging using 3D Slicer v5.6.1 [2,3] (Figure 1A). Intraoperatively, electromagnetic navigation (Aurora® hardware [NDI, Germany] with Laterna™ software [Bcon Medical, The Netherlands]) was used to rigidly register each model to the patient anatomy using ultrasound mapping of the pelvic bones [4]. After tumour resection, the pose
Conclusion: HD neoadjuvant CRT within a TNT framework improves pathological and clinical response rates in LARC without increasing significant toxicity or postoperative complications. Keywords: Locally advanced rectal cancer, Chemoradiotherapy
of the IOERT bolus was recorded using an electromagnetic-tracked pointer, enabling
reconstruction of the IOERT field within the model and imaging based on applicator diameter, angle, and electron energy (90%-isodose line) (Figure 1B-C). Neoadjuvant PTVs were transformed to the same coordinate system using bone-to-bone registration.
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