Initial Feasibility and Clinical Implementation of Daily MR-guided Adaptive Head and Neck Cancer Radiotherapy on a 1.5T MR-Linac System: Prospective R-IDEAL 2a/2b Systematic Clinical Evaluation of Technical Innovation

Introduction This prospective study is the first report of daily adaptive radiotherapy (ART) for head & neck cancers (HNC) using a 1.5T MR-linac, with particular focus on safety & feasibility and dosimetric results of an on-line rigid registration-based adapt-to-position (ATP) workflow. Methods Ten HNC patients received daily ART on a 1.5T/7MV MR-linac, six using ATP only and four using ATP with one off-line adapt-to-shape re-plan. Setup variability with custom immobilization masks was assessed by calculating the average systematic error (M), standard deviation of the systematic error (∑), and standard deviation of the random error (σ) of the isocenter shifts. Quality assurance was performed with a cylindrical diode array using 3%/3mm γ criteria. Adaptive treatment plans were summed for each patient to compare delivered dose with planned dose from the reference plan. The impact of dosimetric variability between adaptive fractions on the summation plan doses was assessed by tracking the number of optimization constraint violations at each individual fraction. Results The random errors (mm) for the x, y, and z isocenter shifts, respectively, were M = − 0.3, 0.7, 0.1; ∑ = 3.3, 2.6, 1.4; and σ = 1.7, 2.9, 1.0. The median γ pass rate was 99.9% (range: 90.9%-100%). The differences between the reference and summation plan doses were within [-0.61%, 1.78%] for the CTV and [-11.74%, 8.11%] for organs at risk (OARs), though percent increases in OAR dose above 2% only occurred in three cases, each for a single OAR. All cases had at least two fractions with one or more constraint violations. However, in nearly all instances, constraints were still met in the summation plan despite multiple single-fraction violations. Conclusion Daily ART on a 1.5T MR-linac using an on-line ATP workflow is safe and clinically feasible for HNC and results in delivered doses consistent with planned doses.