Chao-Pin Hsiao1,2, Mei-Kuang Chen,3 Martina L Veigl,4 Rodney Ellis,5 Matthew Cooney,6 Barbara Daly,1 Charles Hoppel71The Frances Payne Bolton School of Nursing, Case Western Reserve University, Cleveland, OH, USA; 2School of Nursing, Taipei Medical University, Taipei , Taiwan; 3Department of Psychology, University of Arizona, Tucson, AZ, USA; 4Gene Expression & Genotyping Facility, Case Comprehensive Cancer Center, Case Western Reserve University, Cleveland, OH, USA; 5Department of Radiation Oncology and Urology, School of Medicine, Case Western Reserve University, Cleveland, OH, USA; 6Department of Medical Oncology, School of Medicine, Case Western Reserve University, Cleveland, OH, USA; 7Center for Mitochondrial Disease, Department of Pharmacology and Medicine, School of Medicine, Case Western Reserve University, Cleveland, OH, USAIntroduction: Cancer-related fatigue (CRF) is the most debilitating symptom with the greatest adverse side effect on quality of life. The etiology of this symptom is still not understood. The purpose of this study was to examine the relationship between mitochondrial gene expression, mitochondrial oxidative phosphorylation, electron transport chain complex activity, and fatigue in prostate cancer patients undergoing radiotherapy (XRT), compared to patients on active surveillance (AS).Methods: The study used a matched case–control and repeated-measures research design. Fatigue was measured using the revised Piper Fatigue Scale from 52 patients with prostate cancer. Mitochondrial oxidative phosphorylation, electron-transport chain enzymatic activity, and BCS1L gene expression were determined using patients’ peripheral mononuclear cells. Data were collected at three time points and analyzed using repeated measures ANOVA.Results: The fatigue score was significantly different over time between patients undergoing XRT and AS (P