Abstract B099: Spatial identification of favorable molecular modulations after stereotactic body radiotherapy in pancreatic cancer with a practical staining-based classifier

Abstract Purpose/Objective: Pancreatic ductal adenocarcinoma (PDAC) remains one of the deadliest tumors. Consensus have been recently made regarding the existence of two main molecular subtypes in 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-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 practical 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 Dmax(0.5cc)<53Gy in 5 fractions. Quantitative analysis was carried out using a dedicated software in 10 ducts per sample randomly chosen within the tumoral area. Results: Labelling of the treatment naïve cohort highlighted the high level of molecular intratumoral heterogeneity of PDAC regarding molecular subtypes, including within a single duct. After FFX alone, a shift was visually observed towards the BLP, whereas tumors treated with FFX + iHD-SBRT displayed a more enhanced CLP profile. After ductal quantification, tumors treated with FFX+SBRT were significantly associated with more Classical and less Basal-like cells than the tumors treated with FFX alone or treatment-naïve. Similarly, a nearly significant trend in decreased level of KRT17+ cells, a marker associated with Basal-like and poor prognosis, was identified in the FFX + SBRT cohort compared to FFX alone. Conclusions: We successfully applied a practical multiplex staining-based classifier allowing to easily display the molecular hetereogeneity of PDAC, including at the intraductal level. Contrary to the non-treated and FFX cohorts, tumors treated with FFX + iHD-SBRT were significantly enriched in cells displaying a Classical subtype, associated with a better prognosis, suggesting either a selection or a reprogramming effect of SBRT on the Basal-like cells. Citation Format: Christelle Bouchart, Ellis Michiels, Julie Navez, Oier Azurmendi Senar, Tatjana Arsenijevic, Ilse Rooman, Jean-Luc Van Laethem. Spatial identification of favorable molecular modulations after stereotactic body radiotherapy in pancreatic cancer with a practical staining-based classifier [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research—Emerging Science Driving Transformative Solutions; Boston, MA; 2025 Sep 28-Oct 1; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_3):Abstract nr B099.