(A) Low power microphotograph demonstrating a multicystic tumor without any distinct solid areas, partially rimmed by a fibrous capsule. Hematoxylin and eosin, 4×. (B) Benign areas in the tumor consisting of a simple columnar mucinous epithelial lining, with frequent goblet cells. The nuclei are regular, basally oriented; without stratification, atypia or mitoses. Hematoxylin and eosin, 20×. (C) Areas of 'borderline' morphology with significant tufting, stratification and hyperchromasia of nuclei coupled with pleomorphism and mitotic activity. Hematoxylin and eosin, 20×. (D) Invasion into surrounding parenchyma in a 'lepidic/bronchioloalveolar' pattern. Mucinous tumor cells appear to line alveoli in a patchy fashion while respecting the alveolar septal framework. Hematoxylin and eosin, 40×.