Neuroendocrine Differentiation in Synovial Sarcoma—An Immunohistochemical Study of 357 Tumors With 4 Markers: Chromogranin A, Synaptophysin, Insulinoma-associated Protein 1 and Insulin Gene Enhancer Protein ISL-1

Abstract

Expression of neuroendocrine differentiation (NE) markers in synovial sarcoma may result in misdiagnosis as a neuroendocrine tumor. This investigation evaluated 357 synovial sarcomas to determine the presence of NE markers—chromogranin A (CgA), synaptophysin (SYP), insulinoma-associated protein 1 (INSM1), and insulin gene enhancer protein ISL-1 (ISLET1)—using immunohistochemistry. The cohort included 248 primary tumors, 28 metastases, 4 recurrences, and 77 undetermined lesions. The female-to-male ratio was equal (128/124), and patient ages ranged from 7 to 80 years (median=32). Lower (n=107) and upper (n=50) limbs represented the most frequent sites for primary tumors, while the lung (n=18) was the main site of metastasis. Tumors were classified as monophasic (n=289), biphasic (n=57), and poorly differentiated (n=11). Immunostainings were categorized as diffuse, patchy (≥10% and