I consider a chondroblastoma as d/d. The localisation in the epiphysis and the cystic appearance in the x-ray are characteristic of a chondroblastoma and unusual for an osteosarcoma. Moreover, it's a cellular tumor tissue with rather uniform polygonal or round cells and a fibrous and chondroid matrix. There are only few osteoclasts. But taken as a whole the histology is in my view not well compatible with a high grade osteosarcoma.

I´d also consider chondroblastoma the first differential diagnosis. Although it would be very helpful to see more XRays (conventional XRays, more CT pics) the lesion seems to be located in the epiphysis which absolutely is in line with chondroblastoma. The cells are eosinophilic and polygonal, don´t have "high-grade" atypia and I can´t identify unequivocal osteoid production. An easy way to proof chondroblastoma would be an immunohistochemistry against cytokeratins that would typically stain at least a subset of cells.

I have added 6 images of our immunohistochemistry: Cytokeratin is negative (however, also the control is weak), S-100 protein is positive. I think this confirms the diagnosis of chondroblastoma. Thanks to All !