Breast cancer is the most common cancer and a leading cause of death in women worldwide, with Belgium having the highest incidence rate in Europe. Radiotherapy (RT) is an important component of breast cancer treatment, with hypofractionated RT emerging as a new standard of care. Hypofractionation schedules have reduced treatment time and side effects while maintaining efficacy. Reducing the number of treatment days can increase the quality of life for the patient, reduce the cost for the health care system and allow to treat more patients in the same period.However, fractionation schedules do not take the variability of patient's response and tumor characteristics into account. This is important in particularly in triple negative breast cancer (TNBC) patients who have been shown to be potentially radioresistant. Experimental studies on TNBC 2D cell cultures mostly used single radiation doses and fractionation schedules were not investigated. Our study aims to explore the impact of various fractionation schedules on the radioresistance of TNBC cells. A breast cancer organoid model, which closely mimics the complexity of in vivo tumors, will be used to monitor the radiation effects on functional and molecular levels. The results of this study will provide evidence-based guidelines for fractionated radiotherapy planning for triple negative patients.