<qa>  What is the RT regimen?Subjects will be randomized to receive either melatonin or placebo. Subjects will receive 20-mg oral melatonin or placebo the night before their first RT treatment, each night throughout the course of RT treatment, and for 2 weeks following the completion of RT. Patients with localized breast cancer will receive standard-of-care RT as determined by the treating physician. The RT regimens include: 1 1 week of APBI; 2 3-4 weeks of an accelerated hypofractionation RT schedule; and 3 6-8 weeks of a standard RT schedule. No additional concomitant medication or supportive care guidelines are required for this study. Subjects will receive daily melatonin or placebo beginning the night before their course of RT and for an additional 2-week period that extends beyond the conclusion of their RT. Subjects will be given a Study Diary to record their use of study medication. Patients will be followed for 60 days after removal from the study treatment or until death, whichever occurs first. Patients removed from the study treatment for unacceptable AEs will be followed until resolution or stabilization of the adverse event. 