Focal myositis: a clinicopathological study.

Focal myositis is a rare, benign inflammatory condition that may clinically simulate a soft tissue sarcoma. It was first described in 1977 and around 30 cases have been reported to date. We report two further cases on which we have performed immunocytochemistry and electron microscopy. Histology of both lesions was identical, showing a destructive inflammatory myopathy with evidence of regeneration. Stains for micro-organisms were negative and no viral particles were seen on electron microscopy. The immunocytochemical profile of our two cases differed from that of polymyositis: with a panel of T- and B-cell markers the cellular infiltrate was found to be composed of T-lymphocytes and variable numbers of macrophages: sub-typing in one case revealed the T-cells to be predominantly CD4+ cells. Use of antibodies to MHC class 1 and 2 antigens showed occasional positive inflammatory cells only. Clinicopathological correlations and the differential diagnosis are discussed.