Background: Fatigue is common in patients undergoing radiotherapy (RT). Melatonin, an inexpensive natural supplement, may improve symptoms and attenuate the side effects of RT. The effect of melatonin for prevention of fatigue and other symptoms was evaluated in a double-blind placebo-controlled trial. Methods: Early-stage or ductal carcinoma in-situ breast cancer patients ≥ 18 years, female, Eastern Cooperative Oncology Group performance status (ECOG PS) < 3, hemoglobin ≥9 g/dL. RT with curative intent, randomized 1:1 to melatonin 20 mg or oral placebo, starting night before RT initiation until 2 weeks post-RT. Sample size of 142 evaluable patients in each arm for 80% power and interim analysis at mid recruitment using the unified family method rho = 0.3. Randomization stratified to RT duration (< 3 weeks, ≥3 weeks) and prior chemotherapy. Primary outcome : Functional Assessment of Chronic Illness Therapy (FACIT)-Fatigue subscale after completion of RT with additional scores measured at baseline, 2 and 8 weeks post-RT. Secondary outcomes : FACIT-F subscales, Edmonton Symptom Assessment System (ESAS) and Patient Reported Outcomes Measurement Information System (PROMIS) Fatigue-Short Form 8a. Secondary analyses reported using an F-test at a 5% significance level. Results: For this interim analysis, 85 patients screened; 80 randomized; 39 received melatonin and 38 placebo. 72 included in the analysis as 5 patients had no post baseline FACIT score. Characteristics of age, race, and ECOG PS similar in both arms. Treatment X time for FACIT-Fatigue not significant for melatonin compared to placebo (p .83). FACIT physical, emotional, and functional well-being scores not significantly different (p .35, .62, and .71) but showing a trend for social well-being (p .06). PROMIS scores not changed over time (p .34). ESAS individual symptoms of anxiety, well-being, drowsiness, poor appetite, nausea, pain, shortness of breath, sleep and tiredness not significant, except for depression (p.04). However, a decrease of 0.01 unit in depression score is not considered clinically significant. No grade 3 or 4 adverse events. No participants died during study, 2 died after study completion from breast cancer recurrence. 16 withdrew prior to study completion because of adverse events, unrelated hospitalizations, RT discontinuation, and COVID-19 precautions. Trial was stopped based on statistical analysis demonstrating no difference for primary outcome and imminent expiry of available drug. Discontinuation was approved by Data Safety Monitoring Committee. Conclusions: Melatonin did not prevent fatigue in patients with early stage breast cancer undergoing RT. Melatonin also demonstrated no benefit for other symptoms, except depression. Analysis showed little evidence of an effect, and the trial was terminated early. Clinical trial information: NCT02332928.