[Non-traumatic myositis ossificans circumscripta].

BACKGROUND: Myositis ossificans circumscripta is a benign lesion with an acute course and may simulate a malignant tumor. It usually follows trauma to soft tissue.

CASE REPORTS: Case n. 1. A 13 year-old girl was admitted because of a painful inflammatory tumour in the left thigh. Initial X-rays were normal. Ultrasound imaging showed a heterogeneous echogenic mass with several extending shadow cones in the distal part of the vastus medialis muscle resembling a calcifying hematoma. Twenty days later, X-rays showed a vague calcified peripheral rim in the medial distal part of the thigh. White blood count was normal, and blood sedimentation rate was 46 millimeters in the first hour. CT scan showed a transparent zone between the lesion and the adjacent bone and a lucent central area, surrounded by a dense outer area consistent with myositis ossificans. Histological examination of the excised mass confirmed myositis ossificans. Two years later, the patient was asymptomatic and X-rays showed no ossification. Case n. 2. A 14 year-old girl suffered from pain in the right anterior hip area since 10 days. She denied any trauma. A firm mass was palpable in the anterior superior iliac spine area and X-rays revealed a calcific density. CT scan showed a dense bony mass in the right gluteus medius muscle clearly separated from the adjacent bony pelvis by a soft tissue plane. Histological examination of the excised mass confirmed myositis ossificans. One year later, the patient was asymptomatic and X-rays of the pelvis showed no ossification.

CONCLUSION: Myositis ossificans circumscripta is rare in children. CT scan suggests the benign nature of the lesion by demonstrating integrity of bony cortex and characteristic disposition of calcifications. The biopsy is not necessary if the diagnosis is certain. Surgery permits to reduce the evolution.