Primary extramedullary malignant plasmacytoma mimicking carcinoma of rectum

Abstract

The author reports a rare case of extramedullary plasmacytoma (EP) of rectum. A 74-year-old man presented melena. Endoscopy revealed a polypoid tumor in rectum. Biopsy revealed poorly cohesive malignant cells resembling plasma cells. The biopsy was erroneously diagnosed by author as poorly differentiated adenocarcinoma. Resection of rectum was performed. Gross examination showed a polypoid tumor measuring 6 cm × 5 cm × 5 cm. Microscopically, the tumor was composed of poorly cohesive malignant cells resembling plasma cells. Three lymph nodes out of 29 lymph nodes dissected revealed similar malignant cells. Differential diagnosis was needed among plasmacytoid adenocarcinoma, signet-ring cell carcinoma, malignant lymphoma, EP, and poorly differentiated adenocarcinoma. Immunohistochemically, the tumor cells showed positive reaction for vimentin, p53, Ki-67 (labeling = 43%), CD45, CD20, CD38, CD138, CD79α, CD3, CD45RO, CD56, κ-chain and λ-chain. The tumor cells were negative for CK AE1/3, CK CAM5.2, CK WSS, CK34BE12, CK5/6, CK7, CK8, CK14, CK18, CK19, CK20, EMA, p63, CEA, CA19-9, CDX-2, NSE, synaptophysin, chromogranin, MUC1, MUC2, MUC5AC, and MUC6. The final pathological diagnosis was primary extra-medullary malignant plasmacytoma of rectum. Imaging techniques showed no other tumors in the body. The bones were normal, excluding multiple myeloma. The blood leukocytes and their segments were normal. The patient was treated by chemotherapy, and the disease cured. He died of other cause (pneumonia) five years after the operation.