In the randomized DAI-BREAC trial, a reminder app will be prospectively tested that reminds breast cancer patients four times each day to perform the required skin care. This will likely contribute to the reduction of grade ≥2 radiation dermatitis in these patients. A total of 268 patients will be randomized to receive standard skin care supported by a reminder app (Arm A) or standard skin care alone (Arm B). Stratification will be done using the three factors treatment volume, radiation boost, and at least one risk factor of dermatitis. Secondary aims include pain (radiation fields), patient satisfaction with the reminder app (Arm A only), impact of the app on the use of health technology (Arm A only), and benefit from support by staff members of the treating Department of Radiation Oncology and/or the UKSH academy regarding the use of the app (Arm A only).