The tumour is characterised by fascicles of spindle cells with palely eosinophilic cytoplasm and mainly quite uniform tapering nuclei. There are multifocally scattered mitotic figures. There are small foci of stromal hyalinisation with a rather sclerotic appearance. In some areas, the tumour is irregularly infiltrating skeletal muscle but, elsewhere, even within the spindle cell areas, a very small minority of the lesional cells has more discernible eosinophilic cytoplasm.
By immunohistochemistry, the tumour cells were very strongly and diffusely positive for desmin and negative for S-100 protein. Additional stains showed multifocally scattered cells positive for myf-4, perhaps accounting for 5% of the neoplastic population. By contrast, staining for MYOD1 revealed strong positivity in more than 80% of the tumour cells.