Insulinoma‐associated protein 1 is a robust nuclear immunostain for the diagnosis of small cell lung carcinoma in cytology smears

Background In a significant percentage of patients with small cell lung carcinoma (SCLC), cytology samples represent the only source of tumor tissue. Ancillary immunocytochemistry (ICC) for neuroendocrine markers is an important adjunct for the diagnosis of SCLC. Insulinoma‐associated protein 1 (INSM1) is a novel neuroendocrine marker proposed as an economical single‐marker alternative to the traditional 3‐marker panel of chromogranin, synaptophysin, and CD56. To the authors’ knowledge, limited studies have evaluated INSM1 immunohistochemistry (IHC) for the diagnosis of SCLC and reported high sensitivities and specificities. The objective of the current study was to evaluate the sensitivity and specificity of INSM1 ICC on direct smears (DS) from patients with SCLC in comparison with IHC on small biopsies (SBs). Methods All available DS and SBs from patients with SCLC who were diagnosed over the previous year were retrieved. Immunostaining for INSM1 was performed on alcohol‐fixed DS and formalin‐fixed SBs wherever available. A total of 10 DS and SBs from patients with non–small cell lung carcinoma were included for comparison. Nuclear staining for INSM1 in ≥1% tumor cells was interpreted as positive. Results Among a total of 60 patients with SCLC who were included in the current study, a total of 37 underwent INSM1 IHC on SBs and 36 underwent INSM1 ICC on DS. ICC was noninterpretable in 3 DS due to necrosis. The sensitivity of INSM1 IHC was 97% (36 of 37 cases) whereas the sensitivity of INSM1 ICC was 91% (30 of 33 cases) for the diagnosis of SCLC. Among matched IHC and ICC results available for 11 patients, 91% of cases (10 of 11 patients) demonstrated concordant IHC‐ICC staining. All cases of non–small cell lung carcinoma were negative for INSM1 (100% specificity). Conclusions INSM1 appears to be a robust and reliable ICC marker for the confirmation of SCLC diagnosis on cytology smears. Immunocytochemistry for insulinoma‐associated protein 1 (INSM1) can be performed reliably on alcohol‐fixed direct smears with high sensitivity and specificity for the confirmatory diagnosis of small cell lung carcinoma. The nuclear localization of INSM1 allows for the easy and objective interpretation of staining results in tumor cells.