Continuous Positive Airway Pressure (CPAP) Assisted Radiotherapy in Breast Cancer

Inclusion Criteria: 1. Voluntary written informed consent of the participant or their legally authorized representative has been obtained prior to any screening procedures 2. At least 40 years of age and 80 years or younger at the time of signing the Informed Consent Form (ICF) 3. Female patients 4. Patients that underwent breast conserving surgery (BCS) 5. Left-sided invasive BC / in situ carcinoma with indication for adjuvant RT (\<70 years old) 6. Left-sided invasive BC with indication for adjuvant locoregional RT (including RT of the regional lymph nodes) (70-80 years old) 7. Right-sided invasive BC with indication for adjuvant locoregional RT (including RT of the regional lymph nodes) 8. Prior chemotherapy allowed 9. Prior immunotherapy allowed 10. Prior / concomitant hormonal therapy allowed 11. Prior / concomitant HER2-targeted therapy allowed Exclusion Criteria: 1. Patient has active bullous lung disease, bypassed upper airway, pneumothorax, cerebral spinal fluid leaks, abnormalities of the cribriform plate (contra-indications for the use of CPAP) 2. Patient has history of major head trauma and/or pneumocephalus (contra-indications for the use of CPAP) 3. Any disorder, which in the investigator's opinion might jeopardise participant's safety or compliance with the CIP. 4. Female who is pregnant, breast-feeding or intends to become pregnant (which is a contra-indication for RT in general) 5. Male BC patients 6. Patients that underwent mastectomy Patients whose initial tumor was located just beneath the skin (defined as being less than 28mm below the breast surface), indicating the need for an electron boost 7. Patients requiring RT boost on positive lymph nodes 8. Distant metastasis 9. Breast implants in situ 10. Right-sided in situ carcinoma 11. Right-sided invasive BC only requiring local adjuvant RT (without irradiation of the regional lymph nodes, because the presumed benefit on cardiac doses of local right-sided breast irradiation is assumed to be rather small because of the left anatomical position of the heart) 12. Bilateral BC 13. Concomitant use of chemotherapy during RT 14. Substantial comorbidities, incompatible with RT or CPAP use, estimated by the treating radiation oncologist 15. Insufficient arm mobility to perform comfortable arm positioning in radiation treatment position, evaluated by the treating radiation oncologist 16. Other active oncological disease / treatment with the exception of non-melanoma skin cancer 17. Previous RT with overlapping RT fields with actual target volume