Spontaneous pancreatic undifferentiated pleomorphic sarcoma in a laboratory rat: A case report

We present a case of spontaneous undifferentiated/unclassified sarcoma, of a pleomorphic subtype formerly known as malignant fibrous histiocytoma (UPS/MFH), arising from the pancreas of a laboratory rat. The mass was excised after laparotomy from a 6‐month‐old female laboratory Wistar rat. It presented a giant multilobulated mass of irregular shape, which had arisen from the pancreas and occupied almost the entire peritoneal cavity. Histologically the tumor was characterized by a highly variable morphological pattern, with frequent transitions from storiform to pleomorphic areas. An extensive immunohistochemical examination revealed no specific lines of differentiation. Immunohistochemical positivity was observed only to MIB‐1 (high Ki‐67 proliferation index), vimentin and CD68 antibodies. The diagnosis was compatible with UPS/MFH. To the best of our knowledge, the present case is the first report of a spontaneous primary UPS/MFH arising from the pancreas of a laboratory rat.