This study is for patients with a specific type of breast cancer that is hormone receptor positive and HER2 negative. The study is testing a new way to monitor cancer using **serum tumor markers (STMs)**, which are substances in the blood that can indicate the presence of cancer. Patients are divided into two groups: one group will have regular scans every 12 weeks, and the other group will have blood tests every 4-8 weeks to decide if a scan is needed. The study lasts up to 312 weeks, unless the cancer progresses.

• Study Length: Up to 312 weeks
• Visits Needed: Every 4-8 weeks for blood tests or every 12 weeks for scans
• Eligibility: Must have hormone receptor positive, HER2 negative metastatic breast cancer

The aim is to see if using STMs is as effective as regular scans and how it affects healthcare costs and quality of life. Eligible participants must be able to give informed consent and must have specific blood tests and imaging done as part of the study.