Colloid adenocarcinoma of lung is a subtype that shows abundant extracellular mucin. The native alveolar architecture is destroyed by pools of mucin. Note the clusters of tumor cells floating in the mucin near the center of the image. Like other adenocarcinomas, it also usually occurs in the periphery of the lungs. Grossly, it appears as soft, gelatinous, well-circumscribed nodule which may show cystic change.