The PRIMROSE CSF Study aims to study cerebrospinal fluid (CSF) in patients with breast cancer that has spread to the brain (brain metastasis) or the lining surrounding the brain (leptomeningeal disease). Research into breast cancer that has spread to the brain or the lining of the brain is limited due to difficulty in obtaining brain tissue containing cancer cells. Such tissue is typically only available when tumours are removed, which does not occur in all patients. Evidence indicates an increasing number of patients with breast cancer are developing disease that spreads to the brain or the lining of the brain as treatments improve. The PRIMROSE CSF Study aims to improve understanding of breast cancers that spread to the brain or brain lining by collecting and analysing the fluid that circulates around the brain and comparing these samples with other cancer and blood samples. Cancer cells that have spread to the brain or its lining shed genetic material into the surrounding fluid. Collection of this fluid enables purification of genetic material released by breast cancer cells. This approach allows examination of the genetic profile of breast cancer cells affecting the brain or its lining without the need for surgery. Differences between the original cancer and metastatic disease in the brain can then be analysed. This research will support improved understanding of why certain breast cancers spread to the brain and contribute to the development of new treatments for breast cancer that has spread to the brain or the lining of the brain.