A 25-year-old man complains of pain in the lower third of his left thigh, which occurs both with and without physical exertion. Possibility of trauma is denied by the patient. Objectively: skin colour is normal; pastosity and pain can be detected with deep palpation; knee joint mobility is reduced. X-ray of distal femoral metaphysis shows an area of destruction and spicule. Blood test: immature cells, no signs of inflammation. The most likely diagnosis is: