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

Abstract

<jats:p>Introduction: This prospective study is the first report of daily adaptive radiotherapy (ART) for head &amp; neck cancers (HNC) using a 1.5T MR-linac, with particular focus on safety &amp; 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 (Σ), 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; Σ = 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.</jats:p>

Delineation uncertainties of tumour volumes on MRI of head and neck cancer patients

“…The use of MR-guided RT has evolved worldwide in recent years with more centres gaining access to MRI and the use of MR treatment systems for daily cancer treatment, including HNC [16,18,34]. Inter-observer variability in tumour delineation on CT image datasets is apparent despite existing international guidelines [21,23].…”

“…An example of prostate cancer is very appropriate: compared to CBCT, onboard MRI can potentially reduce the daily uncertainties in identifying the interface between the posterior part of the gland and the anterior rectal wall or between the prostate apex and penile bulb. But since all the stages in treatment using MRLinac result in a quite prolonged time period, lasting approx.30 min or more, patient inclusion is still a considerable issue and not all patients can be treated despite good indications.GTV changes during a long treatment period, as well as the possibility to decrease actual delivered dose to organs at risk, may justify the use of MR Linacs for some head and neck cancer patients despite long total treatment times [12,18,46,47]; however, these aspects are outside of the scope of this paper. Moreover, guidelines and additional training are needed to improve the consistency of target delineation for the clinical introduction of adaptive radiotherapy based on MRI [19].…”

“…MR-guided treatment is implemented in several centres using an MR-Linac [15][16][17], however, there are still problems to be solved (e.g. treatment time in fixation masks, definite contouring guidelines, the real benefit of treatment in MR Linac concerning the reduction of RT doses to many organs at risk; human resources) before MR-adapted treatment can be routinely implemented in clinical practice [12,18,19].…”

MR-Guided Radiotherapy for Head and Neck Cancer: Current Developments, Perspectives, and Challenges

“…The number of running prospective trials for HNC cancer is limited so far and the existing studies do not implement identical approaches regarding the frequency and modality of adaptation, i.e., daily versus weekly, or online versus offline adaptive radiotherapy. Up to the present day, no results of prospective trials or registries have been published as a full paper but there were several presentations on congresses ( 1 , 53 – 55 ).…”