Because of the polymorphism of osteogenic sarcomas, examination of a biopsy specimen, which is necessarily small, cannot give an accurate evaluation of the differentiation of the whole tumor. Therefore, a test which provides an overall assessment of the functional capacity of the tumor is needed. For the last ten years, we have been using 85 strontium, which has a metabolism similar to that of calcium and radioactive characteristics that allow external measurements. With this isotope, classification of osteogenic sarcomas is more accurate, ensuring better therapeutic trials.