The absence of INI1 (SMARCB1, hSNF5, BAF47) immunohistochemical reactivity is a central feature of malignant rhabdoid tumor, renal medullary carcinoma, and epithelioid sarcoma. We characterized INI1 immunoreactivity in synovial sarcoma (49 cases) in comparison with its closest histologic mimics (68 cases). We observed a unique pattern of decreased INI1 immunoreactivity with a high specificity (100%) and sensitivity (86%) for synovial sarcoma and particular sensitivity for poorly differentiated subtypes of synovial sarcoma (94%; 16/17 cases). Decreased INI1 immunoreactivity was not seen in any of the other lesions we examined, including 14 cases of Ewing sarcoma and 22 cases of malignant peripheral nerve sheath tumor. Furthermore, decreased INI1 immunoreactivity is distinct from the complete absence of INI1 immunoreactivity seen in malignant rhabdoid tumor or other INI1-negative neoplasms. We propose that this distinct INI1 immunohistochemical pattern serves as a useful diagnostic tool to provide preliminary results before molecular test results are available, especially in cases of poorly differentiated synovial sarcoma and in cases where limited material precludes confirmatory molecular studies. Awareness of this unique pattern is critical to avoid misinterpreting decreased INI1 immunoreactivity as a complete absence of INI1 and, consequently, misdiagnosing synovial sarcoma as an INI1-negative neoplasm.
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