Dosimetric and geometric evaluation of an open low-field magnetic resonance simulator for radiotherapy treatment planning of brain tumours.

Abstract

Magnetic resonance (MR) imaging is superior to computed tomography (CT) in radiotherapy of brain tumours. In this study an open low-field MR-simulator is evaluated in order to eliminate the cost of and time spent on additional CT scanning. Eleven patients with brain tumours are both CT and MR scanned and the defined tumour volumes are compared. Image distortions and dose calculations based on CT density correction, MR unit density and MR bulk density, bone segmentation are performed. Monte Carlo simulations using 4 and 8 MV beams on homogeneous and bone segmented mediums are performed. Mean MR and CT tumour volumes of approximately the same size (V MR =55+/-34 cm3 and V CT =51+/-32 cm3) are observed, but for individual patients, small intersection volumes are observed. The MR images show negligible distortion within radial distances below 12 cm (<1.5 mm). On unit density mediums, dose errors above 2% are observed in low dose areas. Monte Carlo simulations with 4 MV photons show large deviations in dose (>2%) just behind the skull if bone is not segmented. It is feasible to use an MR-simulator for radiotherapy planning of brain tumours if bone is segmented or a careful choice of beam energy (>4 MV) is selected.