Head and neck radiotherapy on the MR linac: a multicenter planning challenge amongst MRIdian platform users

Abstract

Purpose: Purpose of this study is to evaluate plan quality on the MRIdian (Viewray Inc., Oakwood Village, OH, USA) system for head and neck cancer (HNC) through comparison of planning approaches of several centers.

Methods: A total of 14 planners using the MRIdian planning system participated in this treatment challenge, centrally organized by ViewRay, for one contoured case of oropharyngeal carcinoma with standard constraints for organs at risk (OAR). Homogeneity, conformity, sparing of OARs, and other parameters were evaluated according to The International Commission on Radiation Units and Measurements (ICRU) recommendations anonymously, and then compared between centers. Differences amongst centers were assessed by means of Wilcoxon test. Each plan had to fulfil hard constraints based on dose-volume histogram (DVH) parameters and delivery time. A plan quality metric (PQM) was evaluated. The PQM was defined as the sum of 16 submetrics characterizing different DVH goals.

Results: For most dose parameters the median score of all centers was higher than the threshold that results in an ideal score. Six participants achieved the maximum number of points for the OAR dose parameters, and none had an unacceptable performance on any of the metrics. Each planner was able to achieve all the requirements except for one which exceeded delivery time. The number of segments correlated to improved PQM and inversely correlated to brainstem D_0.1cc and to Planning Target Volume1 (PTV) D_0.1cc. Total planning experience inversely correlated to spinal canal dose.

Conclusion: Magnetic Resonance Image (MRI) linac-based planning for HNC is already feasible with good quality. Generally, an increased number of segments and increasing planning experience are able to provide better results regarding planning quality without significantly prolonging overall treatment time.

Keywords: Low field magnetic resonance imaging; MR-adaptive treatment planning; MR-guided radiation therapy; Step-and-shoot IMRT; Treatment plan comparison.

Fig. 1

Planning MRI image and PTV contours of the clinical case used in the planning challenge. Three dose levels depicted in red (70 Gy), blue (59.4 Gy), and green (54 Gy). MRI magnetic resonance imaging, PTV planning target volume

Fig. 2

Scoring criteria as displayed on the ProKnow website created by ViewRay (Oakwood Village, OH, USA). PTV1 high-risk volume, PTV2 intermediate-risk volume, PTV3 low-risk volume, PTV planning target volume

Fig. 3

Descriptive statistics of all metrics evaluated in this study. PTV1 high-risk volume, PTV2 intermediate-risk volume, PTV3 low-risk volume, PTV planning target volume

Fig. 4

Relative difference of all evaluated metrics with respect to the value that results in maximum score. The +’and − signs on the right side indicate whether the median performance of all centers was better or worse than the ideal, respectively. Asterisks on the left indicate whether the scores were significantly different from the ideal score (*p < 0.05, **p < 0.01, ***p < 0.001). PTV1 high-risk volume, PTV2 intermediate-risk volume, PTV3 low-risk volume, PTV planning target volume

Fig. 5

Correlation matrix representing Spearman’s ρ correlation coefficients for all evaluated metrics in this study. Only correlations that are considered moderate (ρ < −0.5 or ρ > 0.5) are displayed. Asterisks indicate significance (*p < 0.05, **p < 0.01, ***p < 0.001). OAR organ at risk, PTV planning target volume, MU monitor units, PTV1 high-risk volume, PTV2 intermediate-risk volume, PTV3 low-risk volume

Fig. 6

Plots indicating the relationship between different PTV dose parameters (y-axis) versus the total number of points achieved for OAR dose parameters (x-axis). The dotted line represents the linear fit and the gray shaded areas represent 95% confidence intervals. OAR organ at risk, PTV planning target volume, PTV1 high-risk volume, PTV2 intermediate-risk volume, PTV3 low-risk volume