show early severe respiratory distress due to associated pulmonary hypoplasia of the uninvolved lung. Grossly, the lesion is large, somewhat firm and bulky, and involves an entire lobe or even an entire lung, producing mediastinal shift and compression of the adjacent lung, accounting for its hypoplasia (fig. 6). However, if aerated, the lesion may display randomly scattered thin-walled cysts of 0.5–1.5 cm in diameter. Microscopically, the entire lesion is composed of randomly scattered bronchiolar/ alveolar duct-like structures lined with low cuboidal epithelium and surrounded by alveoli, also lined with cuboidal epithelium. Arteries and even arterioles are notably absent from the lesion. Survival is possible with resection of the lobe or lung if hypoplasia of the remaining lung is mild enough (i.e. i30–40% of the expected weight remains) to permit adequate oxygenation. CPAM type 4 CPAM type 4 is composed of large cysts with no readily discernible lining and thus resembles distal acinar structures, i.e. alveolar saccules/alveoli. Clinically, the lesion may be seen in infants or young children (age range birth to 4 yrs), and they may present with mild respiratory distress, sudden respiratory distress due to tension pneumothorax, pneumonia or, on occasion, an incidental finding with no symptoms. Radiographically, the large air-filled cysts are readily identified, along with mediastinal shift and an occasional pneumothorax [4]. Grossly, large thin-walled cysts, often 8–10 cm in diameter in older infants and young children, are present at the periphery of the lobe (fig. 7). Their internal surface appears to be a smooth membrane, and vessels can often be seen beneath the surface. In newborns, the cysts are much smaller (0.3–2.0 cm). Microscopically, both small and large cysts are lined with flattened epithelial cells (type 2 alveolar lining cells) over most of the wall, with only rare areas in which low cuboidal epithelium may be present. The character of the lining cells can be determined with the use of surfactant and other immunohistochemical stains associated with alveolar lining cells (fig. 8). A capillary bed of varying density is present beneath the epithelial lining and may have a few lymphocytes and macrophages scattered throughout it. The walls of the larger cysts may be 0.1–0.3 cm thick and are composed of loose mesenchymal tissue containing prominent arteries and arterioles. These large arteries may display a multilayered muscular media far thicker than that seen in normal peripheral acinar vessels (fig. 9). Dense connective tissue may be present in older patients in some cases. The loose mesenchyme must not be confused with similar tissue seen in the cystic type of PPB (see below), in which mononuclear cells (rhabdomyoblasts) may be present in areas beneath strips of cuboidal/columnar epithelium. For this reason, multiple sections of cyst walls should be taken when examining a suspected case of CPAM type 4 in order to exclude the diagnosis of purely cystic PPB. With resection of the involved lobe, the survival of children with CPAM type 4 is excellent. CPAM type 1 and bronchioloalveolar carcinoma Bronchioloalveolar carcinoma (adenocarcinoma BAC) is the most common form of bronchogenic carcinoma in children (fig. 10), although the lesion as a whole is most frequently seen in the elderly [11, 12], and may rarely be familial [13]. It occurs with cough, pneumonia and chest pain, but, if peripheral, may be asymptomatic. Initial CLASSIFICATION OF CHILDHOOD CYSTIC LUNG LESIONS 11