The impact of gadolinium-based MR contrast on radiotherapy planning for oropharyngeal treatment on the MR Linac.

PURPOSE: Gadolinium-based contrast agents (GBCAs) may add value to magnetic resonance (MR)-only radiotherapy (RT) workflows including on hybrid machines such as the MR Linac. The impact of GBCAs on RT dose distributions however have not been well studied. This work used retrospective GBCA-enhanced datasets to assess the dosimetric effect of GBCAs on head and neck plans. METHODS: Ten patients with oropharyngeal squamous cell carcinoma receiving RT from November 2018 to April 2020 were included in this study. RT planning included contrast-enhanced computed tomography (CT) and MR scans. A contrast agent "contour" was defined by delineating GBCA-enhanced regions using an agreed window/level threshold, transferred to the planning CT and given a standardized electron density (ED) of 1.149 in the Monaco treatment planning system (Elekta AB). Four plans were per patient calculated and compared using two methods: (1) optimized without contrast (Plan A) then recalculated with ED (Plan B), and (2) optimized with contrast ED (Plan C) then without (Plan D). For target parameters minimum and maximum doses to 1cc of PTVs, D95 values, and percent dose differences were calculated. Dose differences for organs-at-risk (OARs) were calculated as a percentage of the clinical tolerance value. For the purposes of this study, ±2% over the whole treatment course was considered to be a clinically acceptable dose deviation. Wilcoxon-signed rank tests were used to determine any dose differences within and between the two methods of optimization and recalculation (p < 0.05). Pearson's correlations were used to establish the relationship between gadolinium uptake volume in a structure (i.e., proportion of structure covered by a density override) and the resulting dose difference. RESULTS: The median percent dose differences for key reportable dosimetric parameters between non-contrast and simulated contrast plans were <1.2% over all fractions over all patients for reportable target parameters (mean 0.34%, range 0.22%-1.02%). The percent dose differences for maximum dose to 1cc of both PTV1 and PTV2 were significantly different after application of density override (p < 0.05) but only in method 2 (Plan C vs. Plan D). For D95 PTV1, there was a statistically significant effect of density override (p < 0.01), however only in method 1 (Plan A vs. Plan B). There were no significant differences between calculation methods of the impact of contrast in most target parameters with the exception of D95 PTV1 (p < 0.01) and for D95 PTV2 (p < 0.05). The median percent dose differences for reportable OAR parameters as a percentage of clinical planning tolerances were <2.0% over a full treatment course (mean 0.65%, range 0.27%-1.62%). There were no significant differences in dose to OARs within or between methods for contrast impact assessment. CONCLUSIONS: Dose differences to targets and OARs in oropharyngeal cancer treatment due to the presence of GBCA were minimal, and this work suggests that prospective in vivo evaluations of impact may not be necessary in this clinical site. Accounting for GBCAs may not be needed in daily adaptive workflows on the MR Linac.