Bronchiolar adenoma-like tumour with monolayered component: Represent malignant transformation of bronchiolar adenoma? A series of five cases

Abstract

Pulmonary bronchiolar adenoma (BA) is a rare lung tumour, it is unclear whether BA can develop into a malignancy. We presented five cases of BA-like tumour with monolayered components. This type of tumour may represent the malignant transformation of BA. Histologically, these tumours showed acinar and lepidic growth patterns. The acinar components were well-differentiated. The glandular tumour cells in these tumours contained cuboidal to columnar cells resembling type II pneumocytes or club (Clara) cells. A small number of mucinous cells were found in two cases. A few scattered ciliated cells were detected in three cases. The ciliated cells only existed in the bilayered components. The basal cells were highlighted by CK5/6 and p40 in a partial region of the tumour rather than in the entire tumour. The glandular tumour cells, including those in the bilayered component, were diffusely positive for TTF-1 and napsin-A. EGFR Exon19 deletions were found in four cases, and BRAF V600E mutation was found in one case. These BA-like tumours have biphasic morphological and molecular characteristics of BA and lung adenocarcinoma, suggesting distal-type BA may develop into a malignancy. More cases should be studied and especially cases with metastasis should be searched to further prove the malignant transformation.