There is physician in New York City at Sloan-Kettering, Dr. Michael Stubblefield who is a cancer rehabilitation specialist. His special interest is identifying and treating the long-term complications of cancer survivors and seems to be very knowledgeable about what he calls 'radiation fibrosis syndrome' that includes weakness and painful spasms of the muscles of the jaw, neck, back and extremities. More info including some treatment options at http://www.mskcc.org/prg/prg/bios/666.cfm

He was written up in Cure magazine if I remember correctly. It seems like cancer rehabilitation is a relatively new field, now that survivors are surviving long enough to have on-going health issues. Could be there's a comparable specialist in your area.