CSF and Blood Plasma Liquid Biopsy in Patients With Metastatic Solid Tumours and CNS Metastases or no CNS Metastases

This study is collecting spinal fluid (CSF) and blood samples from patients with metastatic cancer — particularly those whose cancer has spread to the brain or the lining of the brain and spinal cord (called leptomeningeal disease) — to develop and test liquid biopsy tools. These tests look for DNA shed by tumors in body fluids, which could help doctors track cancer activity without needing invasive procedures. **You may be eligible if...** - You have a solid tumor cancer that has spread (metastatic), AND you fall into one of these groups: - Your cancer has spread to the lining of the brain or spinal cord (leptomeningeal disease) - Your cancer has spread to the brain itself (brain metastases) - Your cancer has spread elsewhere in the body but NOT to the brain or spinal cord - You are able to safely have a lumbar puncture (spinal tap) or have an accessible Ommaya reservoir **You may NOT be eligible if...** - You are unable or unwilling to give written consent (translators are allowed) - You have conditions that make a lumbar puncture unsafe (e.g., blood clotting problems, low platelet count, infection at the puncture site, significant brain pressure) Talk to your doctor to see if this trial is right for you.