Frequent appearance of club cell (Clara cell)‐like cells as a histological marker for ALK ‐positive lung adenocarcinoma

Anaplastic lymphoma kinase‐rearranged ( ALK + ) lung cancers show characteristic histological features, such as solid signet ring cell patterns and mucinous cribriform patterns; however, these features are not always observed in ALK + lung cancers. We noticed that club cell (Clara cell)‐like cells (CLCs) were frequently present in the papillary portion of ALK + lung adenocarcinomas. In this study, we investigated the importance of CLCs in papillary patterns of ALK + lung cancers. We compared the histological features of 18 ALK + cases with 62 control cases (22 epidermal growth factor receptor‐positive ( EGFR + ) and 40 ALK ‐ and EGFR ‐negative ( ALK − / EGFR − ) cases). The present study analyzed presence of papillary pattern, proportion of papillary pattern area, presence of micropapillary pattern, frequency of CLCs and lengths of snout. The frequency of CLCs in ALK + cases was significantly higher than that in EGFR + cases and ALK − / EGFR − cases. Micropapillary pattern was more frequently observed in ALK + cases than that in ALK − / EGFR − cases ( P  < 0.001). The present study indicated that the high frequency of CLCs in papillary patterns was significantly associated with ALK + cases. When solid signet ring cell patterns and mucinous cribriform patterns are absent, the high frequency of CLCs in papillary adenocarcinoma could be a useful histological marker for ALK + lung cancers.