Mesenchymal hamartoma of chest wall in infancy: natural history of two cases.

Two cases of mesenchymal hamartoma of chest wall in infancy are reported. These distinctive and rare lesions arise in the antenatal period, present at birth or in early life as chest wall masses with marked rib deformation, and may produce respiratory compromise through encroachment on the chest cavities. Histologically composed of chondroid and primitive mesenchymal elements with giant-cell formation, endochondral ossification, and maturation to trabecular bone, they exhibit some features of aneurysmal bone cyst. Because of their cellularity and proliferative appearance, they are often misinterpreted as sarcomas; however, they pursue a benign course and can be well managed surgically. Our cases possessed multiple chest wall masses that were clinically apparent at birth and sequentially resected over a period of several months, creating an opportunity to document their hitherto unreported histologic evolution. Their pattern of maturation is in keeping with a hamartomous malformation.