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

Abstract

Purpose: This prospective study is, to our knowledge, the first report of daily adaptive radiation therapy (ART) for head and neck cancer (HNC) using a 1.5T magnetic resonance imaging-linear accelerator (MR-linac) with particular focus on safety and feasibility and dosimetric results of an online rigid registration-based adapt to position (ATP) workflow.

Methods and materials: Ten patients with HNC received daily ART on a 1.5T/7MV MR-linac, 6 using ATP only and 4 using ATP with 1 offline adapt-to-shape replan. Setup variability with custom immobilization masks was assessed by calculating the mean systematic error (M), standard deviation of the systematic error (Σ), and standard deviation of the random error (σ) of the isocenter shifts. Quality assurance was performed with a cylindrical diode array using 3%/3 mm γ criteria. Adaptive treatment plans were summed for each patient to compare the delivered dose with the 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; Σ = 3.3, 2.6, 1.4; and σ = 1.7, 2.9, 1.0. The median (range) γ pass rate was 99.9% (90.9%-100%). The differences between the reference and summation plan doses were -0.61% to 1.78% for the clinical target volume and -11.74% to 8.11% for organs at risk (OARs), although an increase greater than 2% in OAR dose only occurred in 3 cases, each for a single OAR. All cases had at least 2 fractions with 1 or more constraint violations. However, in nearly all instances, constraints were still met in the summation plan despite multiple single-fraction violations.

Conclusions: Daily ART on a 1.5T MR-linac using an online ATP workflow is safe and clinically feasible for HNC and results in delivered doses consistent with planned doses.

Figure 1:

The MR-linac simulation and on-line adaptive re-planning workflow. a) Custom head & neck immobilization mask with the coil on the MR-linac; b) indexing bar on the CT simulation MR-linac table overlay; c) CT sim hoop device used to test clearance in MR-linac bore; d) planning CT with contours; e) reference treatment plan on planning CT; f) daily setup MRI from MR-linac (2-minute T2 protocol); g) rigid registration of planning CT and setup MRI; h) adaptive plan using ATP workflow; i) dose volume histogram (DVH) of reference plan; j) DVH comparison between reference and adaptive plans.

Figure 2:

Percent differences between the summation plan and reference plan doses for each plan quality metric specified in Appendix D. Positive values mean that the dose in the summation plan exceeded the dose in the reference plan. The number of treatments delivered on the MR-linac (n) for each case is provided in the legends. In cases where no dosimetric criterion was used for an OAR, that case was omitted from the legend of the graph. “Ipsilateral” and “contralateral” are abbreviated “ipsi” and “contra,” respectively.

Figure 3:

The absolute difference between the cumulative delivered dose and the expected delivered dose if the reference plan had been delivered at each fraction for case 9 (treatment site: oral tongue). Positive values mean that the cumulative delivered dose exceeded the reference plan expected dose. An off-line ATS was performed between fractions 25 and 26. This case was chosen as a representative case because all fractions were delivered on the MR-linac.

Figure 4:

a) Dice similarity coefficient (DSC) and b) mean surface distance (MSD) between physician-drawn contours and contours propagated via deformable image registration from the CT and MR simulation images to the setup MRIs at fractions 1, 10, 20, and 30.