Τρισδιάστατη επιβεβαίωση ακτινοθεραπείας με δοσιμετρία πολυμερισμού και απεικόνιση μαγνητικού πυρηνικού συντονισμού

Abstract

This work describes a method for quality control of stereotactic radiosurgery with Leksell Gamma Knife model C®. The Leksell Gamma Knife utilizes 201 intersecting 60Co beams and four helmets with different size collimators to form four standard clinical beam sizes of 18, 14, 8 and 4 mm nominal diameter and a total geometrical accuracy in dose delivery of the order of 0.5 mm (Leksell Gamma Unit User’s Manual Vol. 2). Quality assurance checks were made for every step of the entire planning and treatment procedure, including mechanical accuracy, precise dose delivery, Automatic Positioning System (APS) accuracy, as well as imaging and target localization accuracy, in order to guarantee a high quality standard. The polymer gel (Vipar and Pabig) - magnetic resonance imaging (MRI) dosimetry method is being used due to its inherent three-dimensional feature and linear dose response over the range of gamma knife applications. Percentage relative dose results are presented not only in the form of one dimensional profiles but also planar isocontours and isosurfaces in three dimensions. Experimental results are compared with corresponding Gammaplan treatment planning system calculations as well as radiochromic film measurements. A good agreement, within the experimental uncertainty, has been observed between measured and expected dose distributions. Accuracy of the experimental data depends on the scanning resolution in MRI. Pituitary adenoma treatment by using four 8 mm and one 14 mm collimator helmet shots delivers to the 50% isodose line, a prescription dose of 15 Gy (30 Gy maximum dose) to the target. The comparison of experimental and theoretical data in terms of percentage isodose contours on axial, coronal and sagittal planes, as well as 3D plan evaluation criteria such as dose-volume histograms for the target volume, target coverage and conformity indices, were in good argument. The multiple metastasis treatment plan includes planning system calculations in different dose matrices which are be combined in the final stage of the calculations. The accuracy of APS, maximum dose and increased intra-target distances has been verified. Measured dose distributions were found to agree with corresponding treatment planning calculations, besides statistical fluctuation of experimental results. On average, distance to agreement between experimental and calculated dose distributions was satisfied within the experimental uncertainty of the order of one pixel in the MRI gel readout session (< 1 mm). The measured and calculated DVH results were in good agreement.