Study aim: Investigating the Effect of Oral Whey Protein on Radiation-Induced Dermatitis in Breast Cancer Patients Design: This double-blind randomized clinical trial will include 100 breast cancer patients undergoing adjuvant radiotherapy at Imam Reza Clinic and Namazi Hospital in Shiraz. Patients will be randomized into intervention and control groups (50 each). The intervention group will receive 30 g/day of whey protein while the control group will receive a matching placebo, from the start to the end of radiotherapy. Radiation-induced dermatitis will be assessed weekly using the EORTC/RTOG criteria. Settings and conduct: Imam Reza Clinic and Namazi Hospital Participants/Inclusion and exclusion criteria: Inclusion Criteria: Pathologically confirmed breast cancer; previous breast-conserving surgery and eligibility for adjuvant radiotherapy; absence of systemic or skin diseases affecting wound healing or preventing radiotherapy; no concurrent chemotherapy or trastuzumab, with at least 3 weeks since the last chemotherapy cycle; no systemic corticosteroid use; no lactose intolerance or dairy allergy; and provision of informed consent. Exclusion Criteria: Withdrawal from the study; previous breast radiotherapy; development of a medical condition affecting study participation or radiotherapy tolerance; consumption of less than 70% of the prescribed supplement; serious adverse effects related to whey protein; discontinuation of radiotherapy for any reason; diagnosis of metastasis; or requirement for a tumor-bed boost or bolus. Intervention groups: Whey protein Main outcome variables: Reduction in the Incidence and Severity of Radiation-Induced Dermatitis