Safety and Efficacy of Whole Brain LDRT+ICI+Intrathecal Chemotherapy in Refractory Meningeal Metastasis of Lung Cancer

This study is testing a combination of low-dose whole-brain radiation, immunotherapy, and chemotherapy injected directly into the spinal fluid for people with lung cancer that has spread to the lining of the brain and spinal cord (a condition called leptomeningeal or meningeal metastasis). **You may be eligible if...** - You are between 18 and 75 years old - You have been diagnosed with lung cancer that has spread to the brain/spinal cord lining, confirmed by spinal fluid testing or a combination of imaging and clinical findings - Standard treatments have stopped working - Your disease outside the brain is stable - Your general health allows you to receive radiation, immunotherapy, and spinal fluid chemotherapy - Your expected survival is at least 3 months and your performance score is acceptable **You may NOT be eligible if...** - You have conditions that prevent spinal taps or brain radiation - You have certain uncontrolled infections or severe organ problems - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.