Answer: Bronchioalveolar carcinoma, mucinous type

Histology: The alveoli of the lung are expanded and distorted by massive pooling of mucin. There is a paucity of cells lining the alveolar walls, but after careful examination, clusters of lining epithelial cells can be appreciated. These cells are tall, columnar, non-ciliated and contain abundant intracytoplasmic mucin. Significant pleomorphism and mitotic activity is not appreciated. There is no frank invasion of the lung with associated desmoplastic stromal reaction. The mucin dissects into the adjacent lung parenchyma – it is not confined by a limiting fibrous wall.

Discussion: Bronchioloalveolar carcinoma (BAC) is a distinct subtype of lung adenocarcinoma that is defined by its growth along the walls of pre-existing alveoli without frank invasion and destruction of lung parenchyma. They are typically rather bland at the cytologic level. Two types of BAC are recognized, mucinous and non-mucinous. In the mucinous variant of broncioloalveolar carcinoma, the walls of the alveoli are lined by tall columnar cells with intracytoplasmic mucin. Extracellular mucin often fills the alveoli. Sometimes the massive production of extracellular mucin may dominate the histologic picture. Rendering a diagnosis of carcinoma may evoke anxiety when multiple sections are required to uncover the presence of even a few neoplastic cells. In these instances, one might consider the possibility of other mucinous neoplasms of the lung including mucinous cystadenoma and mucinous tumor of borderline malignancy. According to the AFIP Atlas of Tumor Pathology (Tumors of the Lower Respiratory Tract), both mucinous cystadenoma and mucinous tumor of borderline malignancy are well circumscribed nodules that are surrounded by a partial or complete fibrous wall.

Distinction of BAC from other types of lung adenocarcinoma is important. In its pure form (i.e. no invasive foci), the ability of BAC to disseminate by hematogenous or lymphatic routes is limited (perhaps non-existent). Instead, BAC – particularly the mucinous variant – has a propensity to spread aerogenously through the airways. Over time this pattern of spread may give rise to multiple parenchymal metastasis and ultimately extensive bilateral consolidation.