Monitoring for Cancer Spread to the Central Nervous System (CNS) in People With Breast Cancer

MRI Brain (unless already done as standard of care within 2 months on enrollment to evaluate for CNS disease) MRI Brain at 2 timepoints 6 months apart (+/- 8 weeks).

LP at 2 timepoints 6 months apart (+/- 8 weeks). LP will be performed to analyze cerebrospinal fluid for: cytology, circulating tumor cells and cell-free DNA.