Cystic lesion in a muscle suggests a parasitic infection at first glance especially since leukemia and neuroblastoma and a retinoblastoma seems less likely at this stage. Does the child have similar swellings else where on his body? Does he have a contact with pets? I think we do need a CT scan and if negative a surgical biopsy of the lesion. I would put some money on Cysticercosis or a toxocara like disorder!
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