A component of high‐grade fetal lung adenocarcinoma diagnosed as the cause of lymph node metastasis

Abstract High‐grade fetal lung adenocarcinoma (H‐FLAC) is a rare type of tumor. There have been no reports demonstrating the degree of metastatic susceptibility of this tumor type. In this report, we describe a case in which 15% of the adenocarcinoma components were H‐FLAC diagnosed as the cause of lymph node metastasis. A 75‐year‐old man presented with suspected primary lung cancer (clinical stage IIA, T2bN0M0) and underwent left upper lobectomy and superior mediastinal lymph node dissection. Postoperative histopathology revealed lung cancer with only lobar bronchial lymph node (#11) metastasis. Approximately 60% of the invasive adenocarcinoma showed a papillary morphology, 25% showed a lepidic morphology, and 15% showed a fetal morphology. The histomorphological and immunohistological features of #11 metastasis were similar to those of H‐FLAC. Herein, we report a rare and important case of H‐FLAC with proven lymph node metastasis, showing that even a small amount of H‐FLAC tissue can cause metastasis.