Supporting Women With Breast Cancer to Practice DIBH at Home Before Radiation Therapy

The following questions will be posted: * Are there any differences in levels of distress (primary outcome) between the control and intervention group at baseline and over time? * Are there any differences in preparedness for RT (secondary outcomes) between the control and intervention group at baseline and over time? * Are there any correlations between Health Literacy (HL) levels (secondary outcome) and levels of distress (primary outcome) in the control and intervention group? * Are there any differences in objective clinical variables (chest mobility and respiratory movements and planned absorbed dose to the heart) between the control and intervention group before and after the intervention (baseline and at planning CT)? * Which patterns of practice and use of the application (when, how, how often and for how long) can be discerned from the intervention group? * Are there any correlations between different levels of practice and use of the digital tool and primary and secondary outcomes? * How do patients in the different groups (control and intervention group) describe their experience of care and information process during RT with DIBH? * Are there differences regarding health economic variables in terms of costs and health care usage (e.g., estimated time for planning and conducting computed tomography (CT), number of scheduled CT examinations) and patient effects (e.g. co-morbidities (heart related), sick leave, and quality-adjusted life year (QALYs) between patients in control- or interventions group? * Are there any differences regarding effects on environmental aspects and costs in the different groups (control and intervention)?