Cynthia, have your dad checked. My husband had rectal cancer also and chemo and radiation. About 30 months later he started having back pain that radiated down the back of his leg. The X-Rays, CT scan, and PET scan report only stated lymph node involvement. He finally had an MRI that showed cancer in one of his vertebrae in the lumbar spine area. He is now taking treatment again. Hopefully, your dad's is just a side effect of previous treatment. Best wishes to you and your family.