Phase I trial of stereotactic MR-guided online adaptive radiation therapy (SMART) for the treatment of oligometastatic or unresectable primary malignancies of the abdomen

TLDR

SBRT treats oligometastatic or unresectable primary abdominal malignancies, but ablative dose delivery is limited by proximity of organs‑at‑risk. This prospective Phase I trial evaluated the feasibility and potential advantages of stereotactic MR‑guided online‑adaptive radiotherapy (SMART) to improve SBRT’s therapeutic ratio for abdominal malignancies. Twenty patients with liver or non‑liver abdominal malignancies received SMART with initial 50 Gy/5 fractions (BED 100 Gy) plans targeting 95 % PTV coverage, and daily real‑time adaptive plans were generated as needed based on MR imaging to preserve OAR constraints or boost PTV dose. Adaptive plans were created for 81 of 97 fractions, increased PTV coverage in 64 fractions, and no Grade ≥ 3 acute toxicities were observed, indicating that SMART is clinically deliverable, safe, and improves dose coverage and OAR sparing.

Abstract

Purpose/objectivesSBRT 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/methodsTwenty 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.ResultsOnline 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.DiscussionSMART is clinically deliverable and safe, allowing PTV dose escalation and/or simultaneous OAR sparing compared to non-adaptive abdominal SBRT.