Academy of Medical Sciences of I.R. Iran Archives of Iranian Medicine 1029-2977 29 4 2026 04 01 Clinical and Histopathological Differences in Microsatellite Instable (MSI) and Microsatellite Stable (MSS) Colorectal Tumors 233 236 10.34172/aim.36853 EN Özgecan Hamdemir https://orcid.org/0000-0001-7204-1627 Semin Ayhan Ömer Atmış Journal Article 10.34172/aim.36853 2026 02 28 2026 03 19 Introduction: Microsatellite instability (MSI) tumors represent an important molecular subtype of colorectal carcinoma with distinct clinicopathological features. This study aims to compare the clinical and histopathological characteristics of dMMR/MSI and microsatellite stable (MSS) colorectal tumors. Methods: We retrospectively analyzed a total of 580 colorectal carcinoma cases resected between 2001 and 2018 and archived at the Department of Pathology, Manisa Celal Bayar University Faculty of Medicine. Tumors were classified as dMMR/MSI or MSS based on immunohistochemical analysis of MLH1, MSH2, MSH6, and PMS2 protein expression. Clinical and histopathological parameters were compared between the two groups. Results: dMMR/MSI tumors accounted for 19.5% of cases. Compared with MSS tumors, dMMR/MSI tumors were significantly larger, more frequently right-sided, more commonly mucinous, and more poorly differentiated. Perineural invasion was more frequent in MSS tumors. No significant differences were found between the groups in terms of other parameters such as lymphovascular invasion, lymph node metastasis, tumor budding, or immune response. Conclusion: dMMR/MSI colorectal tumors demonstrate distinct clinicopathological features compared with MSS tumors, particularly regarding tumor location, morphology, and differentiation.