Korean researchers found that low-dose radiation therapy eased knee pain and improved movement in people with mild to moderate osteoarthritis. The treatment, far weaker than cancer radiation, showed real benefits beyond placebo. With no side effects and strong trial results, the approach could provide a middle ground between painkillers and joint surgery.
It is a pretty old and well known treatment. The effect is caused by slowing down the immune systeme The question is more like: if it is so good, why does nobody use it?