Activity Program During Aromatase Inhibitor Therapy

Inclusion Criteria:

* Written informed consent according to Swiss law and ICH/GCP regulations before registration and prior to any trial specific procedures.
* Histologically confirmed hormone-receptor-positive newly diagnosed breast cancer, AJCC (American Joint Committee on Cancer) stage I-III
* Patient had tumor removal by breast conserving surgery or mastectomy, followed by chemotherapy (if indicated) and/or radiotherapy (if indicated)
* Patient is starting adjuvant first-line endocrine treatment with an AI alone (in postmenopausal women) or combined endocrine treatment with an AI and ovarian suppression with an LHRH-agonist (in premenopausal women)
* Patient completed the PRO Form Eligibility before registration
* Patient is fluent in German, Italian, or French
* Patient is willing to wear a wrist worn activity tracker for 24 weeks
* Female patient, age ≥ 18 years
* WHO performance status 0-2

Exclusion Criteria:

* Pre-existing severe medical conditions such as heart or lung problems or musculoskeletal conditions precluding participation in the physical activity program of moderate walking a total of 150 minutes per week as determined by the local investigator
* Mild, moderate, or severe pain (other than post-operative pain) in the last 24 hours due to muscle/joint pain on the BPI-SF single item "worst pain" ("worst pain" ≥3) within 7 days prior to registration
* Inoperable, locally advanced and/or metastatic breast cancer
* Active rheumatoid arthritis
* Neoadjuvant endocrine treatment with an AI
* NSAIDs, acetaminophen or opioids on a regular basis (\> 1 time per week)
* Concurrent participation in other clinical trials or observational studies
* Any other serious psychological, familial or geographical condition, which in the judgment of the investigator may interfere with the intervention and follow-up or affect patient compliance with trial procedures.