Congenital cystic adenomatoid malformation is caused by anomalous fetal development of terminal respiratory structures [1]. This lesion is usually unilateral, involving all or part of a lung [2]. It has both cystic and solid tissue. It can be classified on the basis of clinical, radiographic, and histological features [1,3]. Type I has cysts of varying sizes with at least one cyst with a diameter of more than 2 cm. Type II has smaller cysts of more uniform size with diameter of 2 cm or less. Type III has microcysts, which appear solid. Most patients with this condition present with respiratory distress in the neonatal period or shortly thereafter. However, those with small lesions may present later with symptoms of pulmonary infection [4].
         The radiographic appearance of congenital cystic adenomatoid malformation depends on type, age of the patient, and presence of complications such as infection [1-3]. Immediately after birth it appears as a water density mass, which  shifts the mediastinum toward the opposite side. A short time later fluid leaves the mass, which then appears reticulated or bubbly. Type I lesions have one or more dominant cysts with adjacent smaller cysts and solid elements. Type II lesions have smaller, more evenly sized cysts. Type III lesions appear as a solid mass.

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.Figure 1. Note the irregular cystic mass with associated
soft-tissue elements in the right thorax, which had
displaced the mediastinum to the left.
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Figure 2. A lateral film of the same patient shows an
irregular cystic mass with associated soft-tissue
elements.

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