Bronchiolar adenoma with unusual presentation: Two case reports.

Background: The clinicopathological features, immunohistochemical characteristics, and genetic mutation profile of two unusual cases of distal bronchiolar adenoma are retrospectively analyzed and the relevant literature is reviewed.

Case Summary: Case 1 was a 63-year-old female patient who had a mixed ground-glass nodule, with mild cells in morphology, visible cilia, and bilayer structures in focal areas. Immunohistochemical staining for P63 and cytokeratin (CK)5/6 revealed the lack of a continuous bilayer structure in most areas, and no mutations were found in epidermal growth factor receptor, anaplastic lymphoma kinase, ROS1, Kirsten rat sarcoma, PIK3CA, BRAF, human epidermal growth factor receptor-2 (HER2), RET, and neuroblastoma RAS genes. Case 2 was a 58-year-old female patient who presented with a solid nodule, in which most cells were observed to be medium sized, the nuclear chromatin was pale and homogeneous, local cells had atypia, and cilia were found locally. Immunohistochemical staining for P63 and CK5/6 showed no expression of these proteins in mild cell morphology whereas the heteromorphic cells showed a bilayer structure. The same nine genes as above were analyzed, and HER2 gene mutation was identified.

Conclusion: Some unresolved questions remain to be answered to determine whether the lesion is a benign adenoma or a part of the process of malignant transformation from benign adenoma of the bronchial epithelium. Furthermore, whether lesions with atypical bilayer structures are similar to atypical hyperplastic lesions of the breast remains to be elucidated. Moreover, clarity on whether these lesions can be called atypical bronchiolar adenoma and whether they are invasive precursor lesions is needed. Future studies should examine the diagnostic significance of HER2 gene mutation as a prognostic indicator.