I have not heard about radiation therapy used for arthritis, although I must say I am a diagnostic radiologist who reads CTs, MRIs, plain films, etc. I do not do radiation therapy, which is practiced by radiation oncologists.
From a holistic medicine standpoint, I did not use pain pills when I broke and dislocated my foot (Lisfranc fracture requiring surgery, cast, 3 months in bed, 3.5 months of physical therapy). I ate 1 tablespoon of extra virgin organic coconut oil with food every 8 hours, which worked like a charm to decrease the pain so I can do physical therapy exercises. The brand is "Garden of Life" and you can buy it online at Vitaminshoppe.com. It comes in a glass jar. Do not buy coconut oil that comes in plastic jar, because plastic leaches toxins out, especially when in contact with oil. Coconut oil is a wonderful substance. Bruce Fife ND has a book on Amazon.com about coconut oil and arthritis: "The New Arthritis Cure: Eliminate Arthritis and Fibromyalgia Permanently, 2012 Kindle edition. Buy it and read it. Being on a vegan organic diet will also reduce inflammation. If you don't believe in all this, forgive my info and advice. I am just sharing what I do for myself, not what traditional textbook medicine teaches.