Primary Colonic Medullary Carcinoma With Exclusive Squamous Differentiation.

BACKGROUND/AIM: Medullary carcinoma (MC) of the colon is a rare subtype of colorectal adenocarcinoma (CRC) with unique histomorphology and frequent mismatch repair (MMR) deficiency. MC with exclusive squamous differentiation has not been reported. We report an unusual case of MC with squamous differentiation and tested this differentiation potential in other MMR-deficient CRC cases. CASE REPORT: A 68-year-old woman presented with a large ascending colon mass and biopsy showed squamoid tumor morphology with immunoprofile concerning for squamous cell carcinoma (SCC). She underwent right hemicolectomy. Immunohistochemistry and next-generation sequencing (NGS) were performed for tumor classification. Macroscopically, the tumor was large and locally advanced. It metastasized to the lung without lymph node metastasis. Microscopically, the tumor cells were monotonous with cytological features of both MC and SCC. Immunostains were diffusely positive for p40 and CK5/6, but negative for other lineage markers including CDX2, CK20, and SATB2. The tumor was MMR deficient with loss of MLH1 and PMS2. NGS confirmed BRAF V600E mutation. In comparison, a tissue microarray comprising 64 previously diagnosed MMR deficient CRC was tested for squamous differentiation, and only 1 case showed focal CK5/6 expression, but none was positive for p40. CONCLUSION: MC with exclusive squamous differentiation not only posed significant diagnostic challenges, but also unveiled unrecognized differentiation plasticity in this tumor type.