ESTRO 2025 - Abstract Book

S3971

Radiobiology - Tumour radiobiology

ESTRO 2025

based classifier of the two main molecular subtypes in PDAC, allowing to spatially visualize intraductal heterogeneity. Here, our aim is to better characterize in situ the molecular modulations following neoadjuvant treatments, including isotoxic high-dose stereotactic body radiotherapy (iHD-SBRT).

Material/Methods:

Using differential gene expression from two published RNAseq PDAC databases, two RNAScope multiplex panels were created to differentiate Classical and Basal molecular subtypes and were previously validated on an independent PDAC cohort. The Classical panel (CLP) included GATA6 , CTSE and EPS8L3 while the Basal panel (BLP) included AHNAK2 , MUC16 and S100A2 . To spatially study molecular modulations after neoadjuvant treatments, paraffin-embedded residual tumoral tissues of 23 localized PDAC resected between 2014 and 2020 were used: eight patients had surgery first, eight received an induction chemotherapy with FOLFIRINOX (FFX) only and seven with FFX followed by an iHD-SBRT designed to individually maximize the dose prescribed to the tumor and vessels interfaces up to D max(0.5cc) <53Gy in 5 fractions. Quantitative analysis was carried out using the HALO™ software in 10 ducts per sample randomly chosen within the tumoral area (Figure 1).