The primary aim of the trial is to investigate if a simple outdoor walking intervention, which is practicable under real-life conditions, beginning at the start of adjuvant aromatase inhibitor (AI) therapy, can prevent the occurrence of muscle or joint pain/stiffness in breast cancer patients.

After tumor removal, patients with hormone receptor positive breast cancer tumors often receive adjuvant endocrine treatment, with the use of an aromatase inhibitor (AI) being standard of care in the population of postmenopausal women. Common side effects of AI therapy are joint pain, muscle pain, stiffness, fatigue, hot flashes, and weight gain. Arthralgia and/or myalgia can result in lower physical activity and can negatively influence quality of life (QoL). In addition, muscle or joint pain/stiffness are among the main reasons for non-compliance and discontinuation of AI therapy. Because AI therapy is usually administered for 5 and sometimes even 10 years, this is a major clinical challenge.

For breast cancer patients undergoing AI therapy, physical activity can provide potential benefit by reducing muscle/joint pain and fatigue and can thus improve QoL. The preventive effect of physical activity on AI side effects, however, remains elusive. In addition, activity programs to reduce AI side effects have so far mostly been rather complex. The primary aim of the trial is to investigate if a simple outdoor walking intervention, which is practicable under real-life conditions, beginning at the start of adjuvant AI therapy, can prevent the occurrence of muscle or joint pain/stiffness in breast cancer patients.

Furthermore, this trial will assess the effect of physical activity on symptom burden in general and quality of life in patients receiving adjuvant AI therapy. During the follow-up phase, the trial will assess whether this intervention leads to a sustained change in lifestyle regarding activity, less pain, and better treatment adherence in the intervention group.

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.