Case 27 Answer

Parosteal Osteosarcoma

Dense ossified mass arising from the posterior metaphysis of the distal femur in a 20-ish yo person. 

Classic age, Classic location, Classic appearance for parosteal type

Parosteal arises from the outer fibrous periosteum so sometimes you can see a slight cleavage plain between the mass and the cortex (not here). 

Actually low grade so better than the classic osteosarcoma and the rarer periosteal type

ref: https://radiopaedia.org/articles/parosteal-osteosarcoma-1?lang=us