[Development and clinical application of MR simulation system for radiotherapy planning: with reference to...

To obtain the optimal radiation field, an MR simulation system (MRSS) has been developed. It basically depends upon the higher soft tissue contrast resolution of MRI than of CT. The system consists of an MR unit, an image processing work-station and a laser marking system. In brief, the procedures are as follows: [1] marking the reference point on the patient's skin out of the MR gantry, and automatic table shift to set the reference point at the center of the magnetic field (CMF); [2] MR imaging (T1W1: SE, TR:500 msec, TE: 20 msec); [3] transfer of MR data to a work-station through floppy disc; [4] postprocessing of MR data using a work-station to perform radiotherapy planning; delineation of ROI for irradiation, calculation of the contour of radiation field along with iso-center; [5] corresponding the reference point correspond with the base point of the laser marking system on the CT table; [6] reproducing the calculated iso-center on the patient's skin using the laser marking system. A phantom study of geographic distortion and the total accuracy of MRSS revealed that the former was less than 1 mm within a 90 mm distance between MR slices and CMF, while the latter showed maximal errors of 2 mm in field size and 3 mm in iso-center. This system was applied to 15 patients with intracranial or head and neck lesions, and all procedures were smoothly performed. In order to evaluate the usefulness of MRSS, 6 experienced radiation oncologists compared the difference between MRSS and a CT simulation system in setting the radiation field. The results were satisfactory in all cases, especially in cases in which the tumor extent was unclear on CT images. In spite of some limitations of MRI such as distortion of image and impossibility of iso-dose curve calculation, it was considered that this system could support radiotherapy planning for intracranial or head and neck regions.