Quality assurance for the Leksell gamma unit: considering magnetic resonance image-distortion and delineation failure in the targeting of the internal auditory canal.

Our aim in this study was to distinguish quantitatively between the localization accuracy of a commercially available stereotactic fixation device as claimed by the manufacturer and the target accuracy as measured by a user, applying neuroradiologic imaging in Gamma Knife planning and phantom irradiation. Missing the target is the most serious possible failure in Gamma Knife and Linac therapy. To reduce this risk, we developed a quality control algorithm and designed a phantom. To evaluate the accuracy of the targeting procedure with a Leksell Gamma unit, and to experience the possible errors in all procedural steps, irradiations of phantoms were performed, using the so-called "unknown" targeting method. Accuracy is defined by the extent of spatial deviation of the irradiated target from the calculated target. Digital imaging was used for therapy planning. GafChromic films, which had been irradiated while affixed to a specially developed phantom, were used for measuring the precision of the radiation unit. A series of MR images (in two plains: transverse and coronal) was acquired sequentially to image the three-dimensional (3-D) volume of the phantom. The results obtained for isocentric accuracy of the Leksell Gamma unit, model B, were in good agreement to the calculated position. The observed spatial deviations between calculated and irradiated targets is less than 1 mm. The newly designed phantom and quality control algorithm are useful in quality assurance measurements of stereotactic radiation therapy.