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 stops inflammation for a while. This leads to pain relief. It does not stop further cartilage damage, AFAIK (though, in theory, should slow it down a bit?)