Assessment of invasion in lung adenocarcinoma classification, including adenocarcinoma in situ and minimally invasive adenocarcinoma.

Classification of adenocarcinoma has undergone recent evaluation to better align histological classification with clinical outcomes. One terminology, in particular, that of bronchioloalveolar carcinoma (BAC), has been debated for many decades. Although initial discussion surrounded the cell-of-origin of this tumor, more recent confusion has been generated from the use of this term both as a pattern of growth within an otherwise invasive adenocarcinoma and as a term for a pre-invasive tumor synonymous with adenocarcinoma in situ. As a result, adenocarcinomas with quite different radiology, gross morphology and metastatic potential have been associated with the BAC term. Focusing on invasion and using an illustrative case, we will explore the current recommendations that incorporate assessment of invasion to clarify the confusion caused by the different uses of the historical term 'BAC'.