Skeletal muscle regeneration mimicking rhabdomyosarcoma: a potential diagnostic pitfall

We report three cases of skeletal muscle regeneration, of which two mimicked a small round cell tumour, especially a rhabdomyosarcoma.

One case presented as an intramuscular mass, located in the right quadriceps of a 12-year-old male; the second patient was a 25-year-old football player who complained of painful left peroneus muscles; the third patient was a 22-year-old male who underwent an amputation of the right thigh 5 days after right leg amputation due to limb crush. Histologically, muscle biopsy specimens showed a proliferation of small round cells, either infiltrating the striated muscle in a diffuse manner or growing within and around necrotic myofibres. Immunohistochemically and ultrastructurally, the cellular population was composed of two types of cells: phagocytic cells the nuclei of which occasionally showed a wreathlike arrangement around necrotic myofibres resulting in structures resembling Langhans-type multinucleated giant cells, and proliferating satellite cells showing enlarged nuclei, prominent nucleoli, mitotic figures, myogenic differentiation and fusion features in order to form regenerating myotubes.

Muscle regeneration is a benign process that may occasionally mimic a small round cell proliferation resembling a lymphoma or an alveolar rhabdomyosarcoma with which it should not be confused.