Immune and stromal tumour biology associated with neoadjuvant radiotherapy response in locally advanced rectal cancer.

BACKGROUND: Response to neoadjuvant chemoradiotherapy in locally advanced rectal cancer (LARC) is heterogeneous. The Neoadjuvant Rectal (NAR) score quantifies treatment response via tumour downstaging and nodal status and correlates with survival outcomes. This work set out to explore whether low NAR and high NAR tumours represent biologically distinct groups pre-treatment. METHODS: Transcriptome microarrays quantified RNA expression within pre-treatment biopsies from LARC patients in GSE87211 and S:CORT consortium Grampian datasets. A validation cohort from the Beatson West of Scotland Cancer Centre was analysed using multiplex immunofluorescence and H&E-derived tumour-stromal ratios. RESULTS: Seventeen MSigDB Hallmarks were enriched in high NAR tumours; 'Epithelial-Mesenchymal Transition' was notably enriched (padj < 0.001). Immune-related signatures were enriched in low NAR tumours (p < 0.25). Both cohorts showed higher Microenvironment Cell Population counter 'Cytotoxic Lymphocytes' in low NAR (p < 0.05) and more stromal cells ('Endothelial Cells', 'Fibroblasts'; p < 0.05) in high NAR tumours. CD8 densities confirmed higher immune infiltration in low NAR tumours. DISCUSSION: Pre-treatment biopsies show consistent microenvironmental differences between high and low NAR tumours. While immune and stromal features are associated with response, the observed differences are insufficient to serve as standalone predictive biomarkers.