Insulinoma-associated protein 1 (INSM1) is a sensitive and highly specific marker of neuroendocrine differentiation in primary lung neoplasms: an immunohistochemical study of 345 cases, including 292 whole-tissue sections.

Recent evidence suggests a role for the nuclear marker INSM1 in the diagnosis of neuroendocrine lung neoplasms. The aim of this study was to determine the utility of INSM1 as a marker of neuroendocrine differentiation using a large series of whole-tissue sections of primary lung neoplasms. We stained 345 primary lung neoplasms with INSM1, including 292 whole-tissue sections. Most cases were also stained with synaptophysin, chromogranin, and CD56. The tumors included 64 small cell lung carcinomas, 24 large cell neuroendocrine carcinomas, 64 carcinoid tumors (48 typical, 16 atypical), 130 adenocarcinomas, and 33 squamous cell carcinomas. For small cell lung carcinoma, the sensitivity of INSM1 (98%) was similar to synaptophysin (100%) and CD56 (95%) but considerably higher than chromogranin (83%). For large cell neuroendocrine carcinoma, CD56 (92%) and synaptophysin (88%) were more sensitive than INSM1 (75%), while chromogranin was less sensitive (46%). All markers stained 100% of carcinoid tumors, except one atypical carcinoid tumor, which was negative for INSM1. The sensitivity of INSM1 for neuroendocrine lung neoplasms as a group (95%) was similar to synaptophysin (98%) and CD56 (97%), but higher than chromogranin (84%). The specificity of INSM1 for neuroendocrine lung neoplasms (97%) was similar to chromogranin (98%) but higher than synaptophysin (90%) and CD56 (87%). INSM1 staining was concordant in primary tumors and matched metastases. In conclusion, INSM1 is a reliable marker of neuroendocrine differentiation in primary lung neoplasms, with sensitivity similar to synaptophysin and CD56, and specificity similar to chromogranin.