This double-blind, placebo-controlled, cross-over study will explore the effect of a single, intravenous dose of ketamine in providing immediate reduction of fatigue following radiation therapy. The primary objective of the study is to determine the immediate effect of a single intravenous dose of ketamine in reducing clinically-significant worsening of fatigue following radiation therapy. The secondary objectives of this study are to investigate the levels of cytokines (i.e., tumor necrosis factor-alpha (TNFalpha), insulin-like growth factor 1 (IGF-I), interleukin (IL)-6, IL-8, transforming growth factors (TGF)alpha and beta), neurotrophic factors (i.e., BDNF, GDNF, SNAP), metabolic (i.e., apoliprotein, arginine, arginase), and mitochondrial (i.e., oxygen consumption rate, glycolysis rate) markers from peripheral blood before and after treatment with ketamine or placebo and relate these levels to self-reported fatigue, depression, and health-related quality of life (HRQOL) scores. This study also aims to measure cognitive function and skeletal muscle strength of patients before and after treatment with ketamine or placebo and relate these findings with self-reported fatigue, depression, and HRQOL scores.