This study is a prospective, non-randomized clinical study aimed at investigating the potential benefits of continuous positive airway pressure (CPAP) support during radiotherapy for breast cancer. CPAP is a device commonly used to support breathing, for example in patients with sleep apnea. The investigators expect a reduction in radiation doses to the heart and/or lungs with CPAP-supported radiotherapy compared to standard radiotherapy (without CPAP), which may also lead to a decrease in radiation-induced heart and/or lung conditions in the long term. The study will also examine how the use of a CPAP device can be implemented in daily radiotherapy practice.

Patients undergoing breast conserving surgery for invasive or in situ BC and requiring adjuvant RT are prospectively enrolled -either left-sided cases with or without regional nodal irradiation (RNI), or right-sided cases requiring RNI. Each patient undergoes an additional CT simulation with continuous positive airway pressure (CPAP) (15cm H20) -in free breathing (FB) for right-sided cases and in deep inspiration breath hold (DIBH) for left-sided cases. Left-sided patients unable to perform DIBH are evaluated in FB. Two RT plans (with and without CPAP) are dosimetrically compared. If CPAP reduces mean heart or CLB dose by ≥0.5 Gy, or mean lung dose by ≥1 Gy, treatment is delivered with CPAP. Patient comfort is assessed through surveys.

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