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.