We will enroll 40 subjects who completed radiation therapy for cancer within at least 3 months. The primary outcome measure of the study is the change in self-reported fatigue score after receiving a single intravenous dose (0.5 mg/kg) of ketamine or placebo. The secondary outcomes of this study include: the cytokine profile (e.g. TNFalpha, IGF-I, IL-6, IL-8, TGFalpha and TGFbeta), neurotrophic factors (e.g. BDNF, GDNF), metabolic (i.e., apoliprotein, arginine, arginase), and mitochondrial markers (i.e., Complex I-V, manganese superoxide dismutase (MnSOD), oxygen consumption rate, glycolysis rate) from blood samples; cognitive function test scores; depression scores; HRQOL scores; and skeletal muscle strength of study participants before and after a dose of ketamine or placebo.