Focal myositis, a benign inflammatory pseudotumour: CT appearances

Focal myositis is a rare benign pseudotumour of skeletal muscle, of unknown cause. Clinically it presents as an enlarging mass within muscle, usually of an extremity, and is often mistaken for a soft tissue neoplasm. The diagnosis is made by biopsy which reveals characteristic histological changes of inflammation, focal degeneration and regeneration, and some evidence of denervation. The CT findings include irregularity and enlargement of the muscles involved, with diffuse, poorly defined fatty infiltration of the muscle planes, but no evidence of an associated mass. We present two cases of focal myositis of the calf, both of which mimicked a soft tissue neoplasm, and in which CT was helpful in determining the nature and extent of the abnormality, for needle biopsy, and follow-up.