Daily Online Adaptation Versus Localization for MRI-Guided SBRT for Unresectable Primary or Oligometastatic Abdominal Malignancies

In light of this new technology and preliminary findings of low toxicity of online, adaptive, magnetic resonance (M)-guided stereotactic radiation on a single arm prospective study, the investigators propose to compare this technique to online MR-guided stereotactic body radiation therapy (SBRT) without adaptation. Online plan adaptation increases treatment times for patients and comprises an increased burden on technical and clinical staff. Although preliminary trial results are encouraging, it remains unclear if the dosimetric benefits of online-adaptive planning studies will translate to measurable improvements in clinical outcomes that merit its routine use. In our preliminary study, plan adaptation was most often required when tumors were adjacent to the gastrointestinal tract (the esophagus to the sigmoid colon), as those structures were most commonly the dose-limiting structures and were noted to change in location on a day-to-day basis. For these reasons, abdominal disease sites have historically highlighted the limitations of SBRT. Specifically, the investigators will enroll patients with oligometastatic or unresectable primary disease of the non-liver abdomen to a randomized, prospective trial. 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.