IMC Is Rare Overall but Is Most Frequently Found in High-Risk CRC Patients

Background: Intramucosal carcinoma (IMC) is poorly defined histologically in the colon and has uncertain biological significance. Objective: To describe the incidence and identify any significant associations for colonic IMC using strict histologic criteria for diagnosis. Methods: The pathology database at a single institution was searched from 1990 to 2024 to identify cases diagnosed as IMC. Cases were re-reviewed using strict criteria for IMC: unequivocal infiltrative growth of neoplastic cells through the basement membrane into the lamina propria, including intramucosal budding and/or signet ring cells. Mismatch-repair (MMR) protein immunohistochemistry was performed on all cases. Additionally, the authors examined the risk of lymph node metastasis based on the depth of invasion and MMR status in a national database. Results: 14 cases of IMC were identified using strict criteria. Most cases arose in conventional adenomas (n=11), whereas 2 arose in traditional serrated adenomas and more...