Napsin-A, TTF-1, EGFR, and ALK Status Determination in Lung Primary and Metastatic Mucin-Producing Adenocarcinomas

Pulmonary mucin-producing adenocarcinomas may be indistinguishable on conventional histology from a metastasis, as TTF-1 expression often is lacking and KRAS mutations are widely present even in extra-pulmonary sites. Few data have been reported on the diagnostic role of napsin-A and EGFR and ALK gene alterations in this challenging differential diagnosis. Seventy-seven surgically resected cases, including 53 primary and 24 metastatic tumors from different sites, were evaluated for Napsin-A, TTF-1 and ALK by immunohistochemistry and for EGFR mutations by direct sequencing. Overall, napsin-A expression in primary lung mucin-producing adenocarcinomas was 36% (8% mucinous, 17% colloid, 87.5% solid, and 100% signet ring cell) and TTF-1 expression reached an overall figure of 42% (12.5% mucinous, 33% colloid, 87.5% solid, and 100% signet ring cell). Metastatic mucinous adenocarcinomas did not react with napsin-A or with TTF-1. All primary and metastatic tumors lacked EGFR mutations, while a single case of signet ring cell lung adenocarcinoma showed ALK expression and rearrangement at FISH analysis. Napsin-A has a lower sensitivity compared to TTF-1 in primary mucin-producing adenocarcinomas of the lung. However, both antibodies have an absolute specificity, being always negative in metastatic mucinous adenocarcinomas. EGFR mutations and ALK translocation or expression are exceedingly rare in mucin-producing adenocarcinomas of the lung, resulting unnecessary as diagnostic tool in this setting.