What Causes Numbness In Leg After Chemotherapy For Osteo Sarcoma Of The Sacrum?

I have an osteo sarcoma of the sacrum. I have had chemo, radiation, proton beam therapy and more chemo.They think the cancer is dead. The problem is from the proton beam therapy. I having a lot of numbness in my leg and where the treatment was I have some pain there. The tumoe is pressing on my siatic nerve, is there anything I can do for the pain. I take gabapentin and oxycodone