Skeletal Muscle Tumors

Abstract

The diagnosis of soft tissue tumors with skeletal muscle differentiation may be challenging for multiple reasons. First, benign tumors are much less common than aggressive neoplasms, rhabdomyomas accounting for less than 2% of all skeletal muscle tumors. Second, the morphology of tumor cells is extremely variable, ranging from primitive round cells to epithelioid cells with prominent nucleoli and eosinophilic cytoplasm - reminiscent of myoblasts -, to spindle cells, to well differentiated mature striated muscle cells, ending with very pleomorphic multinucleated cells. Third, the growth patterns are very variable, ranging from patternless entities, to poorly cellular myxoid lesions, to neoplasms with alveolar growth patterns, to solid variants, to sclerosing entities, ending with fascicular patterns reminiscent of leiomyosarcoma. Finally, cytogenetics or FISH analyses are of help in a few entities like the alveolar subtype of rhabdomyosarcoma, the majority of which is characterized by a fusion gene including PAX3 or PAX7 and FOXO1A. Some variants of the recently defined spindle cell variant of rhabdomyosarcoma also carry well defined gene fusions. Here, the most relevant clinical, morphological immunohistochemical and cytogenetic features of benign and malignant skeletal muscle tumors are reported, with the aim that a drawing-based key representation of their most important morphological features might facilitate pathologists in their recognition and differential diagnosis with other soft tissue tumors.