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