INSM1 Expression in Angiosarcoma A Potential Diagnostic Pitfall

Objectives: Aberrant expression of neuroendocrine markers has been reported in angiosarcomas and can occasionally result in diagnostic confusion. The aim of this study was to evaluate the expression of insulinoma-associated protein 1 (INSM1), a marker for neuroendocrine differentiation, in angiosarcomas as well as other sarcomas. Methods: Tissue microarrays, including angiosarcoma, Ewing sarcoma, desmoplastic small round cell tumor (DSRCT), clear cell sarcoma, synovial sarcoma, leiomyosarcoma, alveolar soft part sarcoma, epithelioid sarcoma, and undifferentiated pleomorphic sarcoma, were evaluated for expression of INSM1. The extent of immunoreactivity was graded according to the percentage of positive tumor cell nuclei (0, no staining; 1+, <5%; 2+, 5%-25%; 3+, 26%-50%; 4+, 51%-75%; and 5+, 76%-100%), and the intensity of staining was graded as weak, moderate, or strong. Results: INSM1 expression was found in a subset of angiosarcomas (n = 24/94, 26%; majority 5+, weak to moderate), as well as DSRCTs (n = 7/62, 11%; 2+, weak to strong) and rarely synovial sarcomas (n = 3/76, 4%; 2+, moderate to strong). No INSM1 expression was detected in the other sarcomas. Conclusions: Aberrant expression of INSM1 can be seen in a subset of angiosarcomas often with diffuse labeling Other sarcomas that can rarely demonstrate small cell morphology and focal INSM1 expression include DSRCT and synovial sarcoma.