Universal Tumor Screening in Colorectal Cancer: Role of MMR Immunohistochemistry for Lynch Syndrome and Early-Onset CRC.

Background: Mismatch repair deficiency (MMRd) is a central molecular determinant of colorectal cancer (CRC) biology, prognosis, and treatment response, and Universal Tumour Screening (UTS) is advocated for Lynch syndrome (LS) detection; yet real-world performance across clinical subgroups remains limited. We evaluated MMRd distribution and UTS-based LS detection in a large consecutive surgical cohort. Methods: We retrospectively analyzed 1022 consecutive CRC patients undergoing surgical resection at the University Hospital of Padua (2015-2023). MMR status was assessed by immunohistochemistry; MMRd cases underwent reflex BRAF mutation testing and, when available, MLH1 promoter methylation analysis, followed by germline multigene panel testing for suspected LS. Clinicopathological features were compared by MMR status, age at onset, and tumour location. Results: MMR testing was performed in 875 patients (85.6%), rising from 67.0% (2015-2017) to 97.4% (2021-2023). MMRd was identified in 139 tumors (15.9%) and was independently associated with age ≥ 70 years, colonic location, and stage 0-II. Of 22 patients with confirmed LS, 13 (59.1%) were newly identified through UTS; family history showed no significant univariate association with LS status and was not independently associated with MMRd after multivariable adjustment. MMRd prevalence was numerically higher in early- than late-onset CRC (20.0% vs. 15.4%), approaching significance after multivariable adjustment (OR 1.90, 95% CI 0.99-3.64; p = 0.054); hereditary syndromes were also more frequent in early-onset disease. MMRd was markedly rarer in rectal than colonic cancer (4.1% vs. 22.5%; p < 0.0001), though MMRd rectal cancers arose in younger patients. Conclusions: UTS identified a substantial proportion of LS carriers missed by age- or family-history criteria. The relationship between age and MMRd prevalence proved more nuanced than a simple comparison would suggest, reinforcing the value of universal over selective testing across the age spectrum, while MMRd rectal cancer shows a distinct younger profile relevant to immunotherapy-based organ preservation.