[The differential diagnosis of malignant soft tissue tumors: pyomyositis].

The case of a seven-year-old boy with an enlarging woody-hard mass in the upper thigh is described and the related literature is reviewed. In absence of conclusive signs of inflammation on both clinical and radiological findings a malignant soft tissue tumour was initially suspected. On operation the mass was found to contain multiple loculated abscesses, and turned out to be a subacute staphylococcal myositis (pyomyositis). Such lesions are quite common in subtropical and tropical climates, and a review of the literature indicates that the incidence of this formerly rare entity is increasing in temperate climates. A variety of factors play a role in pathogenesis, and a history of previous aseptic trauma can be found in about 50% of all cases. The most frequent location is the proximal lower limb or buttock. The clinical history and physical findings are often non-specific. Plain radiographs are non-diagnostic; ultrasound, CT and/or MRI may in some cases be equivocal and angiography sometimes is even misleading. It is important to keep this differential diagnosis in mind, especially in children.