The aim of the present randomized double-blinded placebo-controlled clinical trial is to investigate if melatonin can protect against acute radiation dermatitis in patients with early breast cancer receiving radiation therapy, and whether this has an impact on the patients' quality of life.

The study will be a randomized, placebo-controlled, double-blinded clinical pivotal trial. Patients will be allocated in a ratio of 1:1 to the melatonin or placebo group. Patients will be stratified according to the type of surgery (lumpectomy or mastectomy). Randomization will be performed in blocks of randomized sizes. The study will be performed in the Department of Oncology, Rigshospitalet, Denmark.

Eligible patients with early breast cancer receive adjuvant radiation therapy over 15 to 30 daily fractions (5 fractions per week) of ionizing megavoltage photon radiation to a total of 40 - 60 Gy within 3-5 weeks according to the guidelines of the Danish Breast Cancer Cooperative Group. In this study, the patients will administer approximately 1 g of cream containing melatonin (25 mg/g) and dimethyl sulfoxide (DMSO) (150 mg/g) or a placebo cream topically twice daily on the irradiated skin area. The patients are scheduled to do this every day from the first to the last fraction of radiation therapy, including the days where they do not receive radiation therapy. The investigators have chosen placebo as the comparator due to it being safe and the most reasonable method of evaluating any effects of the intervention. On days where the patients receive radiation, the melatonin/DMSO or placebo cream will be applied no less than 2 hours prior to radiation. Throughout the study, the patients will meet with an investigator once weekly who will monitor compliance and assess outcomes.

Inclusion Criteria:

• Diagnosed with early breast cancer
• Over 49 years old
• Have had radical tumor resection surgery
• Follows treatment regimens and follow-up at Rigshospitalet
• Written informed consent after written and verbal information

Exclusion Criteria:

• Inability to understand Danish, written or spoken
• Mental illness (Defined as having a diagnosis and being in medical treatment, or if anticipated poor compliance)
• Previous therapy with ionizing radiation in the thoracic or neck area
• Use of bolus for radiation therapy (A bolus is a material which has dose absorption properties equivalent to tissue. It is placed on the irradiated area to alter dosing or target of the radiation therapy)
• Pregnancy