AB105. SOH26AB_0468. A decade of early-onset colorectal cancer in the Mid-West of Ireland: molecular, sociodemographic, and clinicopathological landscape

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Background: Early-onset colorectal cancer (EOCRC), diagnosed <50 years old, is increasing globally and in Ireland. This study sought to comprehensively characterise the sociodemographic profile, clinicopathological patterns, and targeted mutational landscape of EOCRC in the Mid-West of Ireland. Methods: All EOCRC cases diagnosed at University Hospital Limerick from 2015 to 2024 were identified. Social deprivation was classified using the Pobal HP Deprivation Index. Standard clinical and pathological definitions of disease stage were used. Mismatch repair (MMR) proteins were assessed with immunohistochemistry (IHC), and microsatellite instability (MSI) was defined as ≥4 microsatellites. Targeted next-generation sequencing (TruSight Oncology 500) was interpreted using PierianDx CGW. Results: A total of 114 EOCRC cases were included in the final analysis (n=65 colon and n=49 rectal). Median age was 43.1 (interquartile range, 38.95–46.97) years, 51% (n=58) were female, and 17% (n=19) were socially deprived. Fifty-seven percent were colon cancer (n=65) and 43% (n=49) were rectal cancer. Thirty-seven percent (n=24) of colon and 22% (n=11) of rectal cancer cases were diagnosed as an emergency. Deficient DNA MMR (dMMR) was tested in 32% (n=43) and MSI in 27% (n=31), with 72% (n=26) performed in the past 5 years. Loss of hMSH1, PMS2 MMR protein expression was most commonly seen. Where tested (n=28 colon and n=16 rectal), 43% demonstrated KRAS mutations compared to 25% rectum and 14% of colon cancers. Conclusions: A significant proportion of EOCRC present as an emergency, further highlighting the need for increased education/awareness of the risk of colorectal cancer in this cohort. MSI testing and targeted genomic analysis is increasing, but reflex testing may identify more familial colorectal cancers and emerging actionable alterations.

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