Low-Dose radiotherapy for painful knee osteoarthritis: a multicenter, interdisciplinary, prospective Single-Arm study

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Age 60 years or older. * Clinical and radiographic diagnosis of primary knee osteoarthritis. * Kellgren-Lawrence grade II or III osteoarthritis in the index knee. * Pain in the index knee for at least 6 months. * Baseline average index-knee pain intensity greater than 4 on a 0-to-10 * Numerical Rating Scale (NRS), referring to the previous 7 days. * Persistent clinically relevant knee pain despite documented prior conservative management. Conservative management must include pharmacological treatment appropriate to the participant's clinical profile and therapeutic exercise and/or physiotherapy. If any of these measures were not undertaken because of contraindication, intolerance, or clinical inability, the reason must be documented. * Radiograph of the index knee performed within 6 months before inclusion and adequate to confirm the diagnosis and Kellgren-Lawrence grade. A new radiograph will be obtained if no valid recent examination is available. * Ability to understand the study procedures and reliably complete the required clinical assessments and questionnaires. * Written informed consent to participate in the study. Exclusion Criteria: * Kellgren-Lawrence grade 0, I, or IV in the index knee. * Knee prosthesis in the index knee. * Previous radiotherapy involving the treatment region. * Body mass index greater than 39 kg/m². * Active connective tissue disease with clinically relevant skin or soft-tissue involvement in the treatment region, including active systemic lupus erythematosus or rheumatoid arthritis when considered clinically relevant for radiotherapy. * Fibromyalgia or a psychiatric disorder that, in the investigator's judgment, would substantially compromise reliable assessment of pain or patient-reported outcomes. * Inability to provide informed consent or to reliably complete the required study assessments. * Intra-articular corticosteroid injection, intra-articular hyaluronic acid injection, or radiofrequency procedure involving the index knee within the 2 months before inclusion.