This patient went on to have a biopsy. 

Sections show multiple cores of ?brous tissue with a small amount of fat, skeletal muscle and lamellar bone. There is in?ltration by a highly cellular and viable tumour comprising sheets of small round cells. The tumour cells have ovoid nuclei, inconspicuous nucleoli and scant cytoplasm. The periodic acid-Schiff stain shows copious amounts of cytoplasmic glycogen. ImmunoMstochemical studies have been performed and the tumour cells show membranous staining for CD99. The tumour cells are negative for desmin, myogenin, CD45, TdT and chromogranin. The normal nuclear staining for INI-1 is retained.

The features are consistent with Ewing's sarcoma.