Inclusion Criteria:  Female patients Non-metastasized breast carcinoma Multidisciplinary decision of adjuvant radiotherapy after lumpectomy for breast cancer Prone WBI without LNI indicated according to the department's guidelines, treatment schedule of at least 15 fractions Age ≥ 18 years Informed consent obtained, signed and dated before specific protocol procedures  Exclusion Criteria:  < 18 years old Mastectomy Need for LNI Bilateral breast irradiation Partial breast irradiation Unable to be treated in prone position Less than 15 treatment fractions planned Situs inversus Pregnant or breastfeeding Mental condition rendering the patient unable to understand the nature, scope and possible consequences of the study Patient unlikely to comply with the protocol; i.e. uncooperative attitude, inability to return for follow-up visits, and unlikely to complete the study