Buttock mass noticed by parents when changing nappy - no fluctuance. No underlying infection.
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There is a large, irregular soft tissue mass in medial aspect of the right buttock that is centered on muscle, but crosses into the subcutaenous fat. It is T2 hyperintense, diffusion restricts and enhances peripherally following contrast. It crosses the midline and there are abnormal ileofemoral nodes suggesting local nodal metastasis.
Features are of an aggressive soft tissue mass, most likely a soft tissue sarcoma. In a 2 year old, the most likely cause is a rhabdomyosarcoma and biopsy of the lesion confirmed this to be an embryonal rhabdomyosarcoma.
Sacrococcygeal teratomas are another lesions that can occur in the buttock region in young children, but the imaging findings here do not fit.