A method for MR imaging of prostate 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) provides superior soft tissue contrast over CT and it has been shown that improved prostate and rectal volume delineation from MRI has the potential to improve target coverage and rectal sparing [1]. It is therefore desirable to integrate MR data into the treatment planning process. MR suffers from geometric distortion and gives no electron density information and so most hospitals rely on the CT scans to plan treatment. CT-MR fusion introduces errors as a result of changes in patient positioning from diagnostic and planning scans, potentially giving inaccuracies in the delineation of the target volume. Patient positioning problems can be overcome by taking the MR diagnostic scans in the same position as the radiotherapy treatment. For pelvic tumour patients a flat table is required since image registration with a curved table introduces fusion inaccuracies, as shown in figure 1. Patients are not routinely scanned on a flat table 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 CT and radiotherapy treatment positions using a customised flat table, patient immobilisation, a laser positioning system and a suitable imaging coil.