ESTRO 2025 - Abstract Book

S1176

Clinical – Lower GI

ESTRO 2025

Conclusion: Combination treatment of radiotherapy, systemic therapy and Tislelizumab demonstrated a high ETS rate and a manageable safety profile in MSS unresectable mRC. Translational study to identify predictive biomarkers is ongoing. Clinical trial information: NCT05359406.

Keywords: rectal cancer, MSS, immunotherapy

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Digital Poster FAPI PET/CT in anal and rectal cancer – first clinical experience with an early imaging PET protocol for radiotherapy planning Simone Ferdinandus 1,2 , Dominic Ufton 3,2 , Karina Claus 1,2,4 , Philipp Linde 1,2 , Joel Schmitz 1,2 , Johannes Rosenbrock 1,2 , Karolina Jablonska 1,2 , Thomas Zander 5,2 , Christiane Bruns 6,2 , Georg Dieplinger 6,2 , Felix Popp 6,2 , Dolores T. Krauss 6,2 , Thomas Fischer 3 , Klaus Schomaecker 3 , Carsten Kobe 3,2 , Alexander Drzezga 3,2,7 , Katrin Sabine Roth 3,2 , Emmanouil Fokas 1,2 , Hendrik A. Dapper 1,2 1 Department of Radiation Oncology, Cyberknife and Radiotherapy, Faculty of Medicine and University Hospital Cologne, Cologne, Germany. 2 Center for Integrated Oncology (CIO), Faculty of Medicine and University Hospital, University of Cologne, Cologne, Germany. 3 Department of Nuclear Medicine, Faculty of Medicine and University Hospital Cologne, Cologne, Germany. 4 Department of Radiation Oncology, Olivia Newton-John Cancer Wellness & Research Centre, Austin Health, Melbourne, Australia. 5 Department I of Internal Medicine, Faculty of Medicine and University Hospital, University of Cologne, Cologne, Germany. 6 Department of General, Visceral, Cancer and Transplant Surgery, University Hospital of Cologne, University of Cologne, Cologne, Germany. 7 Institute of Neuroscience and Medicine, Molecular Organization of the Brain, Forschungszentrum Jülich, INM-2, Cologne, Germany Purpose/Objective: Positron-emission tomography targeting fibroblast activation protein (FAPI PET) features high tumor-to-background ratios and accuracy for localizing rectal cancer (RC) and anal cancer (AC). However, the renal excretion of tracers together with close proximity of tumor to the bladder poses a significant challenge, especially for image-guided treatment. In this pilot study, we investigate a PET-protocol adjusted for RC/AC imaging for primary tumor and lymph node detection prior to planned (chemo)radiation. Material/Methods: In this pilot study, fifteen patients with histologically confirmed tumors (9 RC, 6 AC) received FAPI PET prior to (chemo)radiation for clinical indication at our institution. The imaging protocol consisted of prone positioning of the patients and an abbreviated delay time between tracer injection and image acquisition (5 min) to ensure lower overall vesical levels and ventral sedimentation of renally excreted tracer. We analyzed standard uptake values (SUVs) of primary tumors and nodal metastasis and demarcation against the urinary bladder. Results: Mean delay time between tracer injection and image acquisition was 5.93 min (SD 1.28). Primary tumor was detected in FAPI PET in 14/15 patients (93.3 %). In two cases, primary tumor was not detected in conventional imaging (MRI), but in PET (see Figure 1). Demarcation of primary tumors against the bladder was possible in 14/15 cases (93.3%). 3/15 patients (20%) had positive lymph nodes which were concordant to conventional imaging. The smallest PET positive node detected was 0.55cm in diameter. Mean SUVmax of primary tumor was higher for patients with AC (20.33, SD 5.79) compared to RC (11.64, SD 1.58, see Figure 2) but was generally higher compared to mean background SUVs ranging between 1.64 and 2.55. Mean SUVmax for lymph nodes was 13.48 (SD 2.67). 5/15 patients (33%) had proof of distant metastasis according to FAPI PET.