Due to better treatment options and earlier detection, survival rates of patients with breast cancer continue to increase. As such, (late) treatment toxicity, (long-term) quality of life and the cosmetic outcome are becoming more important. Also, many competing experimental interventions (e.g. treatment, lifestyle interventions) for breast cancer are being developed, all in need to be properly evaluated before being implemented in routine clinical care. Randomized Controlled Trials are the gold standard to do so, but they have shown many challenges, especially when applied in a cancer setting. The 'cohort multiple Randomized Controlled Trial (cmRCT)' design is a promising design for multiple (simultaneous) randomized evaluations of experimental interventions, with potential for increased recruitment, comparability and long-term outcomes as a standard.

By setting up UMBRELLA, as a prospective cohort according to the cmRCT design, the investigators aim to:

• provide an infrastructure for efficient, fast and pragmatic evaluation and implementation of experimental interventions
• gain insight into short and long-term treatment response, toxicity, complications, quality of life and survival of patients with breast cancer.

Inclusion Criteria:

• Age ≥ 18 years
• Patients with breast cancer who undergo irradiation in the UMC Utrecht
• Informed consent - at least - for use of routinely collected clinical data and to fill out questionnaires.

Exclusion Criteria:

• Inability to understand the Dutch language

Breast cancer patients undergoing radiation treatment