The tumor tissue consists of a fibromatous component and of ill-defined large nests of spindle-chaped, epitheloid and undifferentiated cells. On account of this and the typical localization I consider an adamantinoma of the tibia with infiltration of the adjacent soft tissue as potential diagnosis. An atypical variant of a Ewing-sarcoma with spindle cells cannot be excluded. The easiest way to the diagnosis would be an immunostain for panCK.

Adamantinomas can indeed show a broad morphological spectrum and I absolutely agree to this differential diagnosis. However, the pictures shown here seem to mainly consist of soft tissue which would imply a soft tissue infiltration. This can occur in adamantinoma but is rather rare. To better classify the lesion the conventional XRays would be extremely helpful. From all the information we have here I wouldn´t go beyond a low-grade tumor which needs complete resection. Ewing sarcoma seems rather unlikely in my eyes.