Medullary Carcinoma of the Ascending Colon in an Older Woman: A Case Report.

INTRODUCTION: Medullary carcinoma of the colon is a rare histological subtype of poorly differentiated or undifferentiated adenocarcinoma, with a reported incidence of 0.05%-0.08%. It typically affects older women, is more frequently located in the right colon, and is characterized by a high prevalence of mismatch repair deficiency, particularly microsatellite instability (MSI). In comparison to other poorly differentiated adenocarcinomas, medullary carcinoma generally exhibits a lower rate of lymph node or distant metastasis and carries a relatively favorable prognosis. Due to its recent classification and rarity, reported cases remain limited. We herein describe a case of surgically resected medullary carcinoma of the ascending colon in an older woman, along with a brief review of the relevant literature. CASE PRESENTATION: An 88-year-old woman presented with unintentional weight loss of 4 kg over 3 months. Colonoscopy revealed a type 2 tumor in the ascending colon, and biopsy demonstrated poorly differentiated adenocarcinoma. Imaging showed asymmetric wall thickening without lymph node or distant metastasis, and the clinical diagnosis was ascending colon cancer, cT3N0M0, Stage IIA. She underwent an elective laparoscopic right hemicolectomy with D3 lymphadenectomy. The postoperative course was uneventful. A histopathological examination showed sheets of atypical cells with enlarged nucleoli, expansive growth, a Crohn's-like lymphoid reaction, and focal signet-ring-like features. Immunohistochemistry demonstrated positivity for MUC5AC, partial positivity for MUC2, focal positivity for CK20 and CDX-2, retained expression of MSH2 and MSH6, and loss of MLH1 and PMS2 expression, indicating deficient mismatch repair (dMMR), which supports the diagnosis of medullary carcinoma. The final pathological staging was pT3N0M0, Stage IIA. Given the absence of lymph node metastasis and the patient's advanced age, adjuvant chemotherapy was not administered. She has remained recurrence-free for 4 years under active surveillance. CONCLUSIONS: Colonic medullary carcinoma is a rare tumor that predominantly occurs in older patients and generally demonstrates a more favorable prognosis than other poorly differentiated adenocarcinomas. Treatment decisions should be based primarily on tumor stage. Although preoperative diagnosis is often difficult, awareness of its characteristic features may help avoid excessive therapeutic de-escalation.