Poorly differentiated medullary carcinoma of the colon with an unusual phenotypic profile mimicking high grade large cell lymphoma - a unique case report and review of the literature.

Medullary carcinoma (MC) of the colon and rectum is a rare entity, accounting for less than 0.1% of colonic adenocarcinoma that poses a diagnostic challenge for the practicing pathologist. Poorly differentiated or undifferentiated MC with an unusual histological appearance and immunoprofile in addition to heavy lymphoid infiltrate could make it problematic when differentiating it from a high grade lymphoma, in particular anaplastic large B- or T-cell lymphoma, plasmablastic lymphoma, and other undifferentiated neoplasms. Here we reported a unique case of an 81 y/o woman presenting with a 7.0 cm colon mass detected by computed tomography (CT) scan. A partial transverse and ileum resection with appendectomy were performed. Microscopic examination revealed sheets of large, pleomorphic, mitotically-active cells with abundant eosinophilic cytoplasm and multiple prominent nucleoli, growing with a pushing border and poor glandular formation in a background of intratumoral lymphocytes. The neoplastic cells were only focally positive for keratins (<10%); diffusely and strongly positive for vimentin and CD10 with high proliferative index (Ki-67, 90%). The tumor cells were also aberrantly positive for CD30, CD79a and CD43 (diffusely or focally), resulting in a diagnostic dilemma between colonic MC and high grade lymphoma. Careful examination and additional immunohistochemical stains performed proved there was no evidence of T or B-cell lymphoma, melanoma, or other types of primary colon or metastatic carcinomas. This case highlights the difficulty in distinguishing a high grade lymphoma and poorly differentiated colonic MC, and, also the aberrant expression of CD10 and a significant loss of pancytokeratin could result in a diagnostic pitfall.