ESTRO 2025 - Abstract Book

S3970

Radiobiology - Tumour radiobiology

ESTRO 2025

shown that MRT substantially spares normal tissue, while being equally effective in tumor ablation. The aim of this study was to determine the tumor control probability in an in vivo mouse xenograft model comparing MRT and MBRT with conventional broadbeam radiotherapy (BB). Material/Methods: 8-12 week old immunocompromised CD-1 Foxn1nu mice from Charles River were injected subcutaneously into the right flank with A549 cells (human non-small cell lung carcinoma). Once the tumors reached a volume of ≥60mm³, the mice were randomly assigned to specific groups and irradiated accordingly. All animals were irradiated with X rays using a self-developed setup within the small animal irradiation device, XenX by X-Strahl. Tumors were irradiated by MRT with microbeams of 100 μm in width, with a CTC (center to center distance) of about 416 μm and a PVDR of 25. Tumors were irradiated by MBRT with minibeams of 525 μm in width, with a CTC (center to center distance) of about 2100 μm and a PVDR of 24. The irradiation time for MRT and MBRT was determined using the concept of the equivalent uniform dose (EUD) based on the linear quadratic model (LQM). Tumor volumes after treatment were constantly measured with a caliper for a total follow-up time of 120 days. The volumes obtained were analyzed to determine tumor control or regrowth. The percentages of tumor control among all the dose groups for MRT, MBRT and BB were fitted with a logistic regression and the corresponding TCD50 values were obtained. Results: The TCD50 (dose needed to control 50% of the tumors) values obtained for MRT and MBRT were similar with TCD50 values of 20+/-2 Gy and 21+/-1 Gy, respectively. For animals irradiated with conventional broad beam irradiation (BB), the TCD50 value was significant higher (32+/-3 Gy; p< 0.001). Conclusion: Our results demonstrate that both microbeams and minibeams perform significantly better in controlling tumors than conventional treatment. The decrease in dose broadens the therapeutic window of MRT and therefore reduces normal tissue side effects. This suggests a potential benefit of MRT and MBRT for the treatment of human lung cancer. Furthermore, we observed no significant differences between MRT and MBRT in terms of tumor control probability. Proffered Paper Spatial identification of favorable molecular modulations after SBRT in pancreatic cancer with a practical staining-based classifier. Christelle Bouchart 1,2 , Ellis Michiels 3 , Julie Navez 4,1 , Oier Azurmendi Senar 1 , Tatjana Arsenijevic 1 , Ilse Rooman 3 , Jean Luc Van Laethem 1,5 1 Laboratory of Experimental Gastroenterology, Université Libre de Bruxelles, Brussels, Belgium. 2 Department of Radiation Oncology, HUB Institut Jules Bordet, Brussels, Belgium. 3 Laboratory of Medical and Molecular Oncology, Vrije Universiteit Brussel, Brussels, Belgium. 4 Department of Hepato-biliary-pancreatic surgery, HUB Erasme University Hospital, Brussels, Belgium. 5 Department of Digestive Oncology, HUB Erasme University Hospital, Brussels, Belgium Purpose/Objective: Consensus have been recently made regarding the existence of two main molecular subtypes in pancreatic ductal adenocarcinoma (PDAC) named Classical and Basal-like, associated with opposite prognosis and different chemosensitivity. However, the determination of molecular subtype is currently performed through transcriptomic gene profile analysis which is inadequate for daily practice. We previously developed a practical RNAScope staining- Keywords: SFRT, MRT, MBRT, tumor control, TCD50 1102