Bone tumor Pathology: Ewing Sarcoma

Ewing sarcoma is a diaphysial bone tumor which comes under the differential of "Small round cell tumors". The tumor is aggressive , occurs in children less than 20 years and often presents as knee pain with bone mass.
X ray shows lamellate periosteal reaction called as Onion skin appearance in around 50% of cases. On microscopy there are sheets of small round cells and the cytoplasm is clear due to presence of glycogen.
Confirmation is done by IHC CD99, FLI1. Genetic translocation is between chromosome t(11:22) in 95% of cases. Also you could use PAS stain for confirmation of glycogen.

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