Description: Publication date: Available online 23 December 2017 Source: Radiotherapy and Oncology Author(s): Lauren Henke, Rojano Kashani, Clifford Robinson, Austen Curcuru, Todd DeWees, Jeffrey Bradley, Olga Green, Jeff Michalski, Sasa Mutic, Parag Parikh, Jeffrey Olsen Purpose/objectives SBRT is used to treat oligometastatic or unresectable primary abdominal malignancies, although ablative dose delivery is limited by proximity of organs-at-risk (OAR). Stereotactic, magnetic resonance (MR)-guided online-adaptive radiotherapy (SMART) may improve SBRT’s therapeutic ratio. This prospective Phase I trial assessed feasibility and potential advantages of SMART to treat abdominal malignancies. Materials/methods Twenty patients with oligometastatic or unresectable primary liver ( n  = 10) and non-liver ( n  = 10) abdominal malignancies underwent SMART. Initial plans prescribed 50 Gy/5 fractions (BED 100 Gy) with goal 95% PTV coverage by 95% of prescription, subject to hard OAR constraints. Daily real-time online-adaptive plans were created as needed, based on daily setup MR-image-set tumor/OAR “anatomy-of-the-day” to preserve hard OAR constraints, escalate PTV dose, or both. Treatment times, patient outcomes, and dosimetric comparisons between initial and adaptive plans were prospectively recorded. Results Online adaptive plans were created at time of treatment for 81/97 fractions, due to initial plan violation of OAR constraints (61/97) or observed opportunity for PTV dose escalation (20/97). Plan adaptation increased PTV coverage in 64/97 fractions. Zero Grade ≥ 3 acute (〈6 months) treatment-related toxicities were observed. Discussion SMART is clinically deliverable and safe, allowing PTV dose escalation and/or simultaneous OAR sparing compared to non-adaptive abdominal SBRT.