This unique subtype of adenocarcinoma is primarily characterized by its distinctive architectural growth pattern. In this type of growth pattern, called lepidic pattern, the tumor cells spread along intact pre-existing alveolar walls. Mucinous, non-mucinous, and mixed variants are recognized. In addition to this architectural growth pattern, demonstration of vascular, pleural or stromal invasion is required for a definite diagnosis of bronchioloalveolar carcinoma. Current WHO criteria indicate that in the absence of invasion, this lesion is essentially an in-situ tumor.