A Case of Papillary Lung Adenocarcinoma Derived from Malignant Transformation of Papillary Adenoma

Background. Pulmonary papillary adenoma may malignantly transform into lung adenocarcinoma, although this is very rare. We herein report a case of papillary lung adenocarcinoma derived from malignant transformation of papillary adenoma. Case. A 45-year-old man was referred to our department for the examination and treatment of a nodular shadow in the right lower lobe on computed tomography (CT) performed by a primary care physician because of right back pain. Eight years previously, a nodular shadow was incidentally found in the right lower lung lobe on CT, and surgical biopsy was considered. However, the patient had refused to address it. Bronchoscopy raised the suspicion of lung adenocarcinoma, and right lower lung lobe resection and mediastinal lymph node dissection were performed. Grossly, the tumor was well-defined, round, and 20 mm in diameter. A pathological examination revealed papillary proliferation and lepidic growth at the tumor margin. Although the clinical course, imaging and gross findings were suggestive of papillary adenoma, the diagnosis of papillary lung adenocarcinoma was made based on the relatively high degree of nuclear atypia and the presence of lepidic growth. In addition, CTNNB1 mutation was detected, which suggested malignant transformation. Conclusion. We report a very rare case of papillary adenoma with malignant transformation into lung adenocarcinoma.