Mucinous bronchioloalveolar carcinoma with K-ras mutation arising in type 1 congenital cystic adenomatoid malformation: a case report with review of the literature.

Congenital cystic adenomatoid malformation (CCAM) of the lung is a rare hamartomatous cystic lesion, characterized by the presence of large cysts, which are histopathologically lined by pseudostratified ciliated cells. It has been recognized that rare cases of type 1 CCAM show malignant transformation, usually bronchioloalveolar carcinoma (BAC) or adenocarcinoma. Herein, we describe a case of BAC arising in type 1 CCAM with K-ras mutation. A 9-year-old Japanese girl presented with fever. Computed tomography demonstrated large cystic lesions in her right lower lung. Histopathological study of the resected specimen revealed multiple cysts, which were lined by pseudostratified ciliated cells, and occasionally interspersed with mucous cells without atypia. A small focus of proliferation of columnar cells showing lepidic growth pattern was present. These columnar cells had abundant mucin in the cytoplasm and mildly to moderately enlarged nuclei. Accordingly, a diagnosis of BAC arising in type 1 CCAM was made. Polymerase chain reaction analysis revealed K-ras mutation at codon 12 in the BAC component. The presence of mucous cell/goblet cell hyperplasia and atypical adenomatous hyperplasia is a well known phenomenon in type 1 CCAM. A recent study clearly demonstrated K-ras mutation in these lesions, which are thought to be precursors of BAC. Therefore, the concept of malignant transformation in the sequence from type 1 CCAM to mucous cell hyperplasia to atypical adenomatous hyperplasia to BAC and invasive adenocarcinoma due to K-ras mutation has been proposed. Careful histopathological analysis is important for evaluation of malignant lesions in type 1 CCAM.