OBJECTIVETo study the clinicopathologic features, diagnosis and differential diagnosis of extramedullary infiltration of acute monocytic leukemia/monoblastic sarcoma.

METHODSFive cases of extramedullary infiltration of acute monocytic leukemia/monoblastic sarcoma were selected from 102 cases of myeloid sarcoma diagnosed during the period from 1990 to 2006. The clinicopathologic findings and followup data were retrospectively analyzed. Immunohistochemical study was also carried out with SP method.

RESULTSAmong the 5 cases studied, 3 were males and 2 were females, including 2 children and 3 adults. Generalized lymphadenopathy was found in 4 patients and skin lesions were observed in 2 patients. The tumor cells in all cases were positive for CD68 (KP1), CD68 (PGM1), lysozyme and CD45. They were negative for MPO, CD15, CD163, TdT, CD117, T and B cell markers. The Ki-67 index ranged from 40% to 80%. Follow-up data were available in all the 5 patients. Four of the 5 patients died of the disease, with the average survival time being 6.25 months.

CONCLUSIONSMonoblastic sarcoma is a rare disease with poor prognosis. It is almost impossible to distinguish monoblastic sarcoma from granulocytic sarcoma and other types of small round cell tumors on the basis of morphologic examination alone. Immunohistochemistry is mandatory for a correct diagnosis.