BACKGROUND:A malignant fibrous histiocytoma,recently referred to as an undifferentiated pleomorphic sarcoma (UPS),is very rare in the liver,and only 34 cases of primary hepatic malignant fibrous histiocytoma have been reported in the English literature. METHODS:We report a rare case of a hepatic UPS presenting as a unilocular cystic lesion with respect to histopathologic features,the newly revised diagnostic criteria,and the differential diagnosis. RESULTS:A 60-year-old man presented for evaluation of epigastric pain of 7 months duration.Abdominal computed tomography revealed a hypodense,unilocular cystic mass in the right lobe of the liver,measuring 14.0×11.1 cm.A right lobectomy was performed.Grossly,the cut surface showed a single,well-circumscribed unilocular cystic tumor mass containing dark red-brown necrotic debris and blood clots, which occupied most of the mass.Microscopically,the tumor consisted of haphazardly arranged mononuclear pleomorphic tumor cells,admixed with abundant osteoclast- like multinucleated giant cells.Immunohistochemically,the tumor cells expressed vimentin only.The histopathologic and immunohistochemical findings were compatible with a UPS.The patient is alive and well 41 months after surgery without recurrence. CONCLUSIONS:Clinically,most of the hepatic UPSs are solid masses.Only two cases have presented as multilocular cystic masses.A primary hepatic UPS presenting as a unilocular cyst has never been reported.A UPS should be included in the differential diagnosis of unilocular cystic lesions in the liver. BACKGROUND: A malignant fibrous histiocytoma, recently referred to as an undifferentiated pleomorphic sarcoma (UPS), is very rare in the liver, and only 34 cases of primary hepatic malignant fibrous histiocytoma have been reported in the English literature. METHODS: We report a rare case of a hepatic UPS presenting as a unilocular cystic lesion with respect to histopathic features, the newly revised diagnostic criteria, and the differential diagnosis. RESULTS: A 60-year-old man presented for evaluation of epigastric pain for 7 months duration. tomography revealed a hypodense, unilocular cystic mass in the right lobe of the liver, measuring 14.0 × 11.1 cm. A right lobectomy was performed. Grossly, the cut surface showed a single, well-circumscribed unilocular cystic tumor mass containing dark red-brown necrotic debris and blood clots, which occupy the most of the mass. Microscopically, the tumor consisted of haphazardly arranged mononuclear pleomorphic tumor cells, admixed with abundan The tumor cells expressed vimentin only. The histopathologic and immunohistochemical findings were compatible with a UPS. The patient is alive and well 41 months after surgery without recurrence. CONCLUSIONS: Clinically, most of the hepatic UPSs are solid masses. Of the two cases have presented as multilocular cystic masses. A primary hepatic UPS presenting as a unilocular cyst has never been reported. A UPS should be included in the differential diagnosis of unilocular cystic lesions in the liver.