This study looks at a new way to watch hormone receptor positive HER2-negative metastatic breast cancer. The study checks if using blood tests called *serum tumor markers* (STMs) to monitor cancer spread works as well as the usual way of using scans. If you join the study, you'll be put in one of two groups. One group will carry on with regular scans every 12 weeks, while the other group will have blood tests every 4-8 weeks and scans only if those tests show something might be wrong. **Key Points:** - The study lasts up to 312 weeks, and you'll need regular check-ups. - You must have certain blood tests before joining and during the study. - The study's goal is to see if the new method is just as good but costs less and causes less stress. If you have certain health conditions like untreated B12 deficiency or brain tumors, you can't join. Also, if you're pregnant or have had other cancers recently, you might not be eligible. You'll need to agree to the study rules and give consent to participate.