• Matthias G Hautmann, Ernst-Michael Jung, Lukas P Beyer, Christoph Süß, Felix Steger, Markus Weber, Fabian Pohl, Oliver Kölbl, and Franz Josef Putz.
• Department of Radiotherapy, University of Regensburg, Regensburg, Germany. matthias.hautmann@ukr.de.
• Strahlenther Onkol. 2019 Jan 1; 195 (1): 69-76.

PurposeOsteoarthritis of the knee is a common disease, often associated with a Baker's cyst. Besides osteoarthritis, also other joint pathologies of the knee can be causative for a Baker's cyst. Radiotherapy is known to be an effective treatment for osteoarthritis, with an anti-inflammatory effect. As the excessive production of synovia usually is associated with intraarticular inflammation, our hypothesis was that radiotherapy might positively influence the synovial production and reduce the volume of a Baker's cyst.Materials And MethodsWe performed a prospective trial, including 20 knees receiving radiotherapy for knee arthritis. Besides documentation of NRS (numeric rating scale), WOMAC (Western Ontario and McMaster Universities Osteoarthritis Index) score and Knee Society Score, the volume of the Baker's cyst was calculated for a short- (6 to 12 weeks) and long-term (9 to 12 months) follow-up. Ultrasonic volumetry was performed using an ultrasound device with a high-resolution multifrequency linear probe (6-9 MHz).ResultsLow-dose radiotherapy improved NRS, WOMAC score and Knee Society Score significantly. The mean volume of Baker's cyst decreased from 22.3 ml to 10.7 respectively 3.1 ml during follow-up. A decrease in volume of more than 25% compared to the baseline could be achieved for 75% of the patients in the short-term and 79% of the patients in the long-term follow up.ConclusionRadiotherapy of knee osteoarthritis is an effective treatment that decreases the volume of a Baker's cyst. Most patients respond to the treatment. Whether radiotherapy is an effective treatment for Baker's cyst without associated osteoarthritis has to be further examined.

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