Radiotherapy treatment planning of basal meningiomas

A localization technique, based on 3-D CT and MR imaging data for precision radiotherapy of basal meningiomas, is presented. Indications for radiotherapy included unresected tumors, gross disease remaining despite surgery, and recurrences. The patient's head was fixed in a stereotactic localization system which is usable at the CT, MR and the linear accelerator installations. The geometrical distortion of MR imaging data was evaluated in 3 dimensions by phantom measurements. Geometrical distortion was 'corrected' (reducing displacements to the size of a pixel) by calculations based on modelling the distortion as a fourth order 2-D polynomial. The target volume was defined in 3-D MR imaging data after application of 0.1 mmol/kg b.w. Gd-DTPA solution and transferred precisely from MR onto CT data to provide a map of radiation attenuation coefficient for dose calculation. The superior soft tissue contrast of MR showed an excellent tumor delineation especially when the bony base of the skull obscured the target in CT images. Target volume, calculated dose distribution, and critical structure could be transferred between CT and MR imaging data and displayed as 3-D shade structures for better assessment for matching of target volume and dose distribution. With the described planning system a more precise target definition of basal meningiomas was possible by integration of the superior tumor delineation in MR compared with CT. (author). 15 refs., 5 figs