Dear Grandparent. I understand your concern about amputation. I will explain the line of treatment first. Since Synovial sarcoma is an aggressive malignant tumour the current line of treatment is surgery. The type of surgery -that is how far should we operate depends on the site or location of the tumour. As you have mentioned behind the knee, this means that the popliteal fossa is involved. Synovial sarcomas in such locations spread rapidly through the blood and lymph channels to the lungs. Therefore depending on the location most oncosurgeons prefer amputation in order to save the life of the patient. Please remember that synovial sarcoma is very aggressive, grows and spreads fast . I do not have the biopsy report of your grandchild but from the description you have given I presume that after the first operation, the pathological report and surgical findings may how shown involvement of the popliteal fossa and fascial planes. You may request another specialist opinion if you wish but please do not delay the operation. You may also post the details of the histopathology report for further assistance and guidance. You may also refer websites such as Merck for patients or WebMD which explains further details of this disease in simple English. You may use google translate to convert such explanations into French Best regards