What is the best treatment for synovial sarcoma?

It is not: A common dx, unlike its name, does not always occur at joints, but is lumped as a "soft part sarcoma" so it can metastasize to lungs, but unlike many sarcomas, it can also travel to lymph nodes. Resection free margins is key. Even 1 node conveys higher risk of relapse. Statistics don't help. You have a chance!

Still under study: Everyone agrees that surgery is foremost. Whether local radiation and/or adjuvant chemotherapy (and if so what kind) are helpful is still up for grabs, with different studies giving different results for different stages of tumor. Unless the disease has spread to the lungs, the chances of long-term survival are generally better than even. Good luck.