[Myositis ossificans circumscripta in the axilla].

Myositis ossificans circumscripta (MOC) is a rare benign hereditary disorder characterized by heterotopic bone formation in soft tissue and muscle. This disorder is rarely seen in childhood. MOC is most commonly located in the extremities. We describe a 15-year-old girl with nontraumatic MOC in the axilla. Imaging studies (radiographic, ultrasound) did not provide the correct diagnosis and did not rule out malignancy. Incisional biopsy of the mass and histopathologic examination gave the diagnosis of MOC. Some reports suggest that magnetic resonance imaging may be the first-choice diagnostic option and may avoid unnecessary biopsy. However, in many cases diagnosis is still histological. Differential diagnosis between these lesions and malignant sarcomas and fibrodysplasia ossificans progressiva is required. This entity has a good prognosis in childhood.