'Bronchioloalveolar Carcinoma': is the Term Really Dead? A Critical Review of a New Classification System for Pulmonary Adenocarcinomas

'Bronchioloalveolar carcinoma' (BAC) is a designation that has been in use for over 50 years. Recently, the International Association for the Study of Lung Cancer, in association with the American Thoracic Society and the European Respiratory Society (IASLC/ATS/ERS) has recommended dropping the term altogether. It is argued that 'BAC' has no clear conceptual meaning, has been used in conjunction with invasive tumours of various types, and was applied to mucinous and non-mucinous tumours that are pathogenetically distinct. In addition to replacing 'BAC' with the standard pathological 'adenocarcinoma in situ', the IASLC/ATS/ERS panel also attempted to tackle the more substantial problem of standardising the terminology of lung adenocarcinomas that are not clearly invasive. By identifying 'minimally invasive adenocarcinomas' with <5 mm invasion, the IASLC/ATS/ERS has attempted to classify invasive tumours with little metastatic potential. Problems remaining with the new classification include use of the term 'lepidic predominant adenocarcinoma' (LPA), which replaces BAC with invasive components, and lack of clarity regarding what constitutes true invasion in well-differentiated lung adenocarcinomas. Specifically, the distinction between acinar growth pattern and in situ growth is not standardised. The basis for the new classification, as well as the plethora of previous attempts at classifying lung adenocarcinomas that are neither clearly invasive nor clearly in situ, are reviewed. Time will determine if the use of a 5 mm limit is workable or if the term 'LPA' has traction.