Exercise in Older Women With Breast Cancer During Systemic Therapy

Inclusion Criteria:

Participants must:

* Be operated for primary breast cancer within 12 weeks or
* Be diagnosed with locally advanced or metastatic breast cancer. Be treated with (neo)adjuvant or first or second-line palliative therapy defined as chemotherapy ± HER2 directed treatment, ± antihormonal treatment, antihormonal treatment ± HER 2, directed treatment ± CDK 4/6 inhibitor.
* Be ≥ 65 years of age at the time of signing the informed consent form
* Have an Eastern Cooperative Oncology Group (ECOG) performance status score ≤ 2
* Be able to speak and read Danish, and to provide a signed informed consent form.

Exclusion Criteria:

Participants with:

* Any physical condition that hinder the execution of physical exercise training
* Other types of cancer
* Documented and uncontrolled brain metastases that hinder participation in an exercise-based trial, based on the referring oncologist's assessment
* Dementia, psychotic disorders, or other cognitive diseases or conditions that hinder written consent
* Unstable medical disease or history of serious or concurrent illness; any medical condition that might be aggravated by exercise training or that cannot be controlled, including, but not restricted congestive heart failure (NYHA class III-IV), unstable angina pectoris, implantable cardioverter defibrillator (ICD), or myocardial infarction within 6 months, based on the referring oncologist's assessment.

In patients with documented bone metastases:

\- A bone metastatic burden or location that poses a risk of injury in the performance of exercise training, as assessed by the referring oncologist.