Focal myositis of the thigh: report of two cases

Focal myositis is a benign inflammatory pseudotumor of skeletal muscle, presenting as a localized painful swelling within the soft tissue of an extremity. Histological examination reveals lymphocytic infiltration, scattered muscle fiber necrosis and regeneration, and interstitial fibrosis. Complete recovery follows surgical removal of the lesion. Two cases are presented of young active males with a localized painful swelling of the thigh. Clinical examination, standard laboratory tests, electromyography (EMG), magnetic resonance imaging (MRI) and surgical biopsies were performed. Surgical biopsies were formalin-fixed, paraffin-embedded and sections were stained with hematoxylin and eosin, van Gieson, modified Gomori trichrome and Mallory's method for iron. Clinical examination in both cases revealed an indolent lump in the quadriceps femoris muscle. Standard laboratory tests and electromyography were normal. MRI prior to surgery visualized the focal nature of the lesions. Histological examination of repeated surgical biopsies showed scattered muscle fiber necrosis, regeneration, and interstitial fibrosis but few lymphocytes. Both patients recovered after surgical removal of the lesion. Two years after surgery there was no recurrence of the lesions and repeated EMGs were normal.