The influence of MRI scan position on patients with oropharyngeal cancer undergoing radical radiotherapy

BACKGROUND The purpose of this study was to demonstrate how magnetic resonance imaging (MRI) patient position protocols influence registration quality in patients with oropharyngeal cancer undergoing radical radiotherapy and the consequences for gross tumour volume (GTV) definition and radiotherapy planning. METHODS AND MATERIALS Twenty-two oropharyngeal patients underwent a computed tomography (CT), a diagnostic MRI (MRI(D)) and an MRI in the radiotherapy position within an immobilization mask (MRI(RT)). Clinicians delineated the GTV on the CT viewing the MRI(D) separately (GTV(C)); on the CT registered to MRI(D) (GTV(D)) and on the CT registered to MRI(RT) (GTV(RT)). Planning target volumes (PTVs) were denoted similarly. Registration quality was assessed by measuring disparity between structures in the three set-ups. Volumetric modulated arc therapy (VMAT) radiotherapy planning was performed for PTV(C), PTV(D) and PTV(RT). To determine the dose received by the reference PTV(RT), we optimized for PTV(C) and PTV(D) while calculating the dose to PTV(RT). Statistical significance was determined using the two-tailed Mann-Whitney or two-tailed paired student t-tests. RESULTS A significant improvement in registration accuracy was found between CT and MRI(RT) versus the MRI(D) measuring distances from the centre of structures (geometric mean error of 2.2 mm versus 6.6 mm). The mean GTV(C) (44.1 cm3) was significantly larger than GTV(D) (33.7 cm3, p value = 0.027) or GTV(RT) (30.5 cm3, p value = 0.014). When optimizing the VMAT plans for PTV(C) and investigating the mean dose to PTV(RT) neither the dose to 99% (58.8%) nor 95% of the PTV (84.7%) were found to meet the required clinical dose constraints of 90% and 95% respectively. Similarly, when optimizing for PTV(D) the mean dose to PTV(RT) did not meet clinical dose constraints for 99% (14.9%) nor 95% of the PTV (66.2%). Only by optimizing for PTV(RT) were all clinical dose constraints achieved. CONCLUSIONS When oropharyngeal patients MRI scans are performed in the radiotherapy position there are significant improvements in CT-MR image registration, target definition and PTV dose coverage.