mediastinally or pleurally based, as well as intrapulmonary. Tumours are classified as type I (cystic 14% of cases), type II (cystic and solid 48% of cases) and type III (solid 38% of cases). The multilobated masses measure 8–23 cm in diameter and weigh up to 1,100 g. Microscopically, the cystic areas are composed of small-to-large cysts lined with respiratory-type epithelium overlying a loose-to-dense fibrous stroma that, in areas, is composed of primitive small cells in a cambium-like arrangement beneath the epithelium (fig. 11). The solid areas of types II and III PPB consist of blastematous islands of loose mesenchyme blending into fibrosarcoma-like foci, nodules of benign appearing to overtly Fig. 9. – Congenital pulmonary airway malformation type 4. Cross-section of the cyst walls displaying thick mesenchymal tissue containing thick muscularised arteries (haematoxylin and eosin stain). Fig. 10. – Bronchioloalveolar carcinoma. Alveoli are filled with mucin and polypoid clusters of mucogenic cells. The nuclei of the cells retain their basal orientation. Note the absence of any apparent invasion (haematoxylin and eosin stain). J.T. STOCKER, A.N. HUSAIN 14