malignant cartilage, rhabdomyoblasts and areas of large bizarre pleomorphic multi- nucleate mesenchymal cells. Immunohistochemical staining is variable from one tissue type to another. Trisomy 2 and 8 have been noted in cytogenetic studies of the tumours. Most recently, Roque et al. [20] reported the comparative genomic hybridisation of PPB and found several chromosomal imbalances (e.g. 1q12–q23), whole chromosome gains of 2 and 7, and loss of genetic material at regions 6q130qter, 10pter–p13, 10q22–qter and 20p13. Local recurrence may develop inv15% of type I PPBs, but inw45% of type II and III PPBs. Metastatic disease occurs iny25% of patients (all with type II or III PPB), chiefly to the brain/spinal cord or bone. Patients with pleural or mediastinal involvement fare significantly worse than those without such involvement. In a series of 50 cases reported by Priest et al. [6], 5-yr survival was 83% for type I and 42% for types II and III. Can purely cystic pleuropulmonary blastoma arise in a pre-existing CPAM type 4? Although superficially resembling CPAM type 1 or 4, purely cystic PPB lesions are, in the experience of the present authors and others [21], distinctly different morphologically in that the CPAM type 1 lesion (lined with respiratory epithelium) does not have a loose mesenchymal component in its wall, and the CPAM type 4 lesion (with a loose-to-dense mesenchymal wall) is lined with thinned type 2 alveolar lining cells, not with cuboidal or columnar cells. Small patches of cuboidal and/or columnar epithelium may occasionally be noted along the wall of CPAM type 4 cysts, but these are usually associated with an adjacent component of a normal bronchiolar structure apparently entrapped in the developing lesion. Nevertheless, in the case of CPAM type 1 or 4, great care must be taken to exclude the diagnosis of purely cystic PPB. When in question, immunohisto- chemical stains for rhabdomyoblastoma cells may help in distinguishing the malignant Fig. 11. – Pleuropulmonary blastoma. Ciliated columnar epithelium overlies a dense layer of small round blue cells (rhabdomyoblasts on immunohistochemical staining), which, in deeper layers, become fewer in number and mixed with oval and strap-like cells and, eventually, densely eosinophilic skeletal muscle-like cells (haematoxylin and eosin stain). CLASSIFICATION OF CHILDHOOD CYSTIC LUNG LESIONS 15