The utility of napsin‐A in the identification of primary and metastatic lung adenocarcinoma among cytologically poorly differentiated carcinomas

BACKGROUND: New developments in the treatment of lung cancer have necessitated the further histologic and cytologic subtyping of nonsmall cell lung carcinomas. Thyroid transcription factor‐1 (TTF‐1) long has served as the predominant marker for demonstrating lung origin. However, it is also expressed in a variety of other tumors, particularly neuroendocrine neoplasms and, to a much lesser degree, squamous cell carcinoma of the lung. Napsin‐A, which is expressed in lung tissue, is a relatively new marker for lung adenocarcinoma. In this study, the authors examined the utility of napsin‐A compared with TTF‐1 in cytologic specimens of primary and metastatic, poorly differentiated lung adenocarcinomas. METHODS: The archives of the Department of Pathology at The Johns Hopkins Hospital were searched for cytologic cases of poorly differentiated lung adenocarcinoma that were histologically confirmed. In total, 75 patients (cases) along with 95 controls were included, each of whom had adequate cell block material for TTF‐1 and napsin‐A staining. Tissue microarrays of lung adenocarcinoma also were examined. RESULTS: TTF‐1 and napsin‐A were detected in 61 of 75 cases (81.3%) and in 49 of 75 cases (65.3%), respectively. The sensitivity and specificity of TTF‐1 were 81% each; and napsin‐A had a greater specificity of 96%, and sensitivity of 65%. Napsin‐A was not detected in small cell carcinomas or in other carcinomas of nonlung origin except for renal cell carcinoma. CONCLUSIONS: Although TTF‐1 had a higher sensitivity, napsin‐A was useful as a surrogate marker when encountering a poorly differentiated lung adenocarcinoma or an unknown primary tumor, particularly in cytologic specimens and difficult cases. The current results indicate that the dual use of both markers may be necessary to improve diagnostic accuracy. Cancer (Cancer Cytopathol) 2010. © 2010 American Cancer Society. The results from this study indicated that, although thyroid transcription factor‐1 has higher sensitivity, napsin‐A is a useful surrogate marker when encountering a poorly differentiated lung adenocarcinoma or unknown primary tumor, particularly in cytology specimens and difficult cases.