Female with mass on sternum. Biopsied 8 years ago benign. I had a pet scan it's hypermetabolic. I have MS and borderline lupus. What are my options?

Sternal mass. Leave it alone unless it has begun enlarging rapidly. I suspect it is one of the several benign hamartomas / developmental anomalies that can happen there and the "hypermetabolism" is marrow. You also have a right and duty to know what the old biopsy showed and what the implications are.
Complex. Your rheumatologist and neurologist would answer you better, border line lupus with MS, medicines would overlap for both conditions , good communication between treating physicians is the key, goodluck, .