Summary: The transcription factor ISL1 (islet-1) has emerged as a useful marker for metastatic pancreatic well differentiated neuroendocrine neoplasms, but recent studies showed wider expression in poorly differentiated neuroendocrine carcinomas from different sites as well as poorly differentiated neuroblastoma. Expression of ISL1 in soft tissue sarcomas has not been studied before.
We evaluated ISL1 expression in 249 soft tissue tumour specimens from 249 patients and 17 precursor cell lymphoblastic lymphomas (ALL).
ISL1 was not detected in any of 63 liposarcomas of different subtypes, 55 leiomyosarcomas, 22 solitary fibrous tumours, 20 undifferentiated pleomorphic/spindle cell sarcomas, 13 small cell synovial sarcomas and 17 ALL cases. Variable nuclear expression was detected in rhabdomyosarcoma (15/25, 60%), rhabdomyoblastic areas of malignant müllerian mixed tumours (5/5), Ewing sarcoma (2/12, very weak) and monophasic fibrous synovial sarcoma (2/29). More extensive staining (moderate to strong) was restricted to rhabdomyosarcoma. Taken by histological subtype, ISL1 was expressed more frequently in alveolar (9/11, 82%) versus non-alveolar (6/14, 43%) rhabdomyosarcoma.
ISL1 is commonly expressed in rhabdomyosarcoma, particularly the alveolar subtype and should be distinguished from poorly differentiated neuroendocrine and neuroblastic neoplasms. Awareness of this finding helps to avoid misinterpretation as neuroendocrine neoplasms that would result in inappropriate therapeutic and prognostic consequences.