Ultra Low Dose Radiation Delivered Before or After Chemotherapy-Free Targeted Therapy in Treating Patients With Relapsed or Refractory Mantle Cell Lymphoma

This trial combines ultra-low dose radiation — a very small, targeted dose — with a targeted cancer drug called venetoclax (which blocks a protein that cancer cells use to survive) for patients with mantle cell lymphoma (MCL), a type of non-Hodgkin lymphoma. The idea is that even very small amounts of radiation can 'sensitize' tumor cells to make venetoclax work better. **You may be eligible if...** - You have been diagnosed with mantle cell lymphoma (MCL), either newly diagnosed or previously treated - You are 18 or older in generally good health (ECOG performance status 2 or less) - You have measurable disease by CT scan - Women of childbearing potential must use reliable contraception and have a negative pregnancy test **You may NOT be eligible if...** - You are unable to swallow capsules - You are pregnant or breastfeeding - You have serious medical conditions affecting your participation - You have had a prior allergic reaction to any components of venetoclax Talk to your doctor to see if this trial is right for you.