A method for MR imaging of head and neck cancer patients in the radiotherapy treatment position without loss of image quality

Introduction: Modern radiotherapy treatment planning (RTTP) requires volumetric localisation of tumour and critical structures. 3dimensional computed tomography (CT) data is the preferred imaging modality in RTTP due to: spatial accuracy; electron density information; the speed of acquisition; the ability to image the patient in the treatment position and the visualisation of bony anatomy. However, magnetic resonance imaging (MRI) offers superior soft tissue contrast over CT and can be used to discriminate tumour extension in most (>80%) brain tumour patients [1]. It is therefore desirable to integrate MR data into the RTTP process. MR suffers from geometric distortion and lack of electron density information, so many hospitals fuse MRI and CT data. The fusion process introduces errors as a result of changes in patient positioning, potentially giving inaccuracies in the delineation of the target volume. Patient positioning problems can be overcome by imaging MR diagnostic scans in the same position as the radiotherapy treatment. For head and neck cancers this requires immobilisation, by way of a thermoplastic head and neck mask and a head rest to tilt the head in the sagittal plane. Patients are not routinely positioned in this way in MR because clinicians do not wish to compromise diagnostic information for positional accuracy. This study investigates a method for maintaining satisfactory diagnostic information in MR while positioning the patient in the RTTP position using a customised flat table and a suitable imaging coil.