High‐grade fetal adenocarcinoma of the lung is a tumour with a fetal phenotype that shows diverse differentiation, including high‐grade neuroendocrine carcinoma: a clinicopathological, immunohistochemical and mutational study of 20 cases

High-grade fetal adenocarcinoma (H-FLAC) is a rare variant of pulmonary adenocarcinoma; this study aims to elucidate its clinicopathological features and genetic abnormalities.Clinicopathological, immunohistochemical and mutational analyses were performed on 20 surgically resected lung cancers that showed H-FLAC histology in various proportions. These tumours predominantly occurred in elderly males and in 10 patients who were heavy smokers. Four cases were pure H-FLAC, and 16 cases were mixed H-FLAC, which were found to be combined with conventional-type adenocarcinoma (15 cases), large-cell neuroendocrine carcinoma (three cases), small-cell carcinoma (one case), enteric adenocarcinoma (two cases), choriocarcinoma (two cases), and a solid-clear cell pattern (seven cases). The fetal phenotype and diverse differentiation were supported by the immunoexpression of α-fetoprotein (95%), thyroid transcription factor-1 (TTF-1) (50%), neuroendocrine markers (30-45%), proneural markers (50-69%), and CDX2 (40%). Except for TTF-1 expression (pure H-FLACs, 0%; mixed H-FLACs, 63%), there were no significant differences in histological or immunohistochemical findings between pure and mixed H-FLACs. EGFR, KRAS, BRAF and PIK3CA mutations were identified in 20%, 0%, 0% and 7% of the tumours, respectively.Lung adenocarcinomas with H-FLAC features possess the potential for multidirectional differentiation, and are not strongly associated with known major driver gene mutations.