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 MRIRT (GTV_RT). Planning target volumes (PTV_s) 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, PTVD 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 MRIRT versus the MRID 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 PTVRT 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 PTVRT 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. [ABSTRACT FROM AUTHOR]