Thymoma with osseous metaplasia in an adolescent.

A 13-year-old girl was incidentally found to have a partially calcified anterior mediastinal mass during the work-up for a left humeral lesion. The resected specimen demonstrated a lymphocyte predominant World Health Organization type B1 thymoma with nodules of metaplastic bone consistent with osseous metaplasia. In addition, she had multiple osteochondromas, a periosteal chondroma of the humerus, a metaplastic osseous pseudotumor of the thigh, and other benign-appearing sclerotic foci, all manifestations of what is likely an autosomal dominant disorder.