Naltrexone is a drug which blocks some effects of chemicals called beta-endorphins that are made in the body. Beta-endorphins can be made in response to stress, injury, and also pleasurable activities. In previous studies, it has been shown that levels of beta-endorphins in the blood go up during radiation therapy, and that this increase is linked to fatigue. This suggests that naltrexone may help to reduce fatigue in people who are getting radiation therapy In this research study, the investigators are looking to see whether naltrexone works better than a placebo in reducing fatigue during radiation therapy.