David is right, ask your doctors, BOT can be difficult to treat, pesky, and go deep into the musculature, more likely to metastasize. I don't know what depth Cyberknife goes, and don't think it covers a broad area. Disappearing primary can happen, one if it's HPV related, the body could have fought it off, but they metastasize to the nodes quickly, another is that it moved on, usually to lymph nodes, and took all the blood, nutrients with it, and primary dies off. Another guesstimate is that there could be a co-existent factor, like infection, inflammation, cyst, that was part of the disease process, and not differentiated from the tumor. Good luck with your decision.