MRI alone simulation for conformal radiation therapy of prostate cancer: technical aspects.

The value of MRI in defining target volumes and organs at risk is established. Numerous difficulties appear to stand in the way of using MRI alone in dose planning, with the result that this imaging modality is used in most cases in conjunction with computerized X-ray tomography (CT). The aim of this paper is to appreciate these difficulties: geometrical distortion, chemical shifts, dosimetric accuracy. Geometrical distortion measurements were carried out on two 1.5 T MR scanners and the effect of chemical shift and magnetic susceptibility were evaluated in volunteers. The effect on dosimetric calculations of uncertainty in determining electron densities was evaluated too. Geometrical distortion remained at small values: less than 2 mm and 3 mm for field of view of 20 cm and 45 cm. The chemical shift and magnetic susceptibility values obtained, ranging from 0.3 to 3 mm, were well below the theoretical values. The assignment of relative electron densities to only two structures in MR images seems to permit dose planning that is identical with that obtained with CT. None of the technical obstacles mentioned represents a stumbling block. The access to MRI facility could represent a persisting problem.