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

Inclusion Criteria: Inclusion criteria for the study: * patients \> 18 years of age * with a left-sided breast cancer diagnosis or DCIS * who are to receive RT with DIBH technology * with curative intention * preparatory CT visit minimum 7 days to a maximum of 4 weeks * able to speak and understand good enough Swedish to be able to assimilate content in the mobile app and answer surveys and interview questions * have access to a smart mobile phone. Exclusion Criteria: * patients receiving palliative RT or neoadjuvant chemotherap * Planning CT starting \< 7 days * people with some form of orthopedic, rheumatological or neurological injury or disease that limits the possibility of carrying out the intervention.