Patients will be randomized to one of two treatment arms, in which they will receive either online-adaptive, MRI-guided SBRT or non-adaptive MRI-guided SBRT. Both patient groups will undergo MRI simulation and MRI treatment localization with online MR monitoring and/or gating. All patients will be treated in five fractions over one to two weeks. By adhering to strict normal tissue constraints, the investigators expect toxicity to be within the current standard of care for the non-adaptive arm, with reduction in toxicity in the arm of patients who undergo adaptation based on daily anatomic changes.