Deep Inspiration Breath Hold (DIBH) technology is increasingly used with radiation therapy to protect healthy organs from unwanted absorbed dose. Using deep breaths, this technique creates as larger distance between the heart and the chest wall. DIBH has shown good results, but requires a well-prepared, involved patient who has learned the correct breathing technique so that optimal position and breathing patterns can be reproduced during each treatment session. There is no evidence regarding which type of inhalation is optimal or how to best train this. However, a person-centered model for DIBH training has been developed in co-design with relevant stakeholders, and this will be integrated into a digital information and instruction tool enabling training undertaken at home. The purpose of this project is to describe and evaluate a person-centered model to train the most optimal breathing technique for breath-adapted postoperative radiotherapy of women affected by left-sided breast cancer.