Repositioning accuracy of a commercially available thermoplastic mask system Martin Fuss a, * , Bill J. Salter a,b , Dennis Cheek c , Amir Sadeghi b , James M. Hevezi b , Terence S. Herman a a Department of Radiation Oncology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA b Cancer Therapy and Research Center, 7979 Wurzbach, San Antonio, TX 78248, USA c Division of Radiological Sciences, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA Received 23 September 2003; received in revised form 25 February 2004; accepted 5 March 2004 Abstract Background and purpose: To evaluate the repositioning accuracy of a commercially available thermoplastic mask system for single dose radiosurgery treatments and fractionated treatment courses. Patients and methods: The repositioning accuracy of the Raycast q -HP mask system (Orfit Industries, Wijnegem, Belgium) was analyzed. Twenty-two patients that were treated by intensity-modulated radiation therapy (IMRT) or intensity modulated radiosurgery (IMRS) for 43 intracranial lesions, underwent repeated CT imaging during their course of treatment, or as a positional control immediately before radiosurgery. We evaluated multiple anatomical landmark coordinates and their respective shifts in consecutive repeated CT-controls. An iterative optimization algorithm allowed for the calculation of the x, y and z-components of translation of the target isocenter(s) for each repeated CT, as well as rotation in the respective CT data sets. In addition to absolute target isocenter translation, the total magnitude vector (i.e. sum-vector) of isocenter motion was calculated along with patient rotations about the three principle axes. Results: Fifty-five control CT datasets were analyzed for the target isocenter’s respective position relative to the original treatment planning CT simulation. Mean target isocenter translation was 0.74 ^ 0.53, 0.75 ^ 0.60 and 0.93 ^ 0.78 mm in x, y and z-directions, respectively. Mean rotation about the x, y and z-axes was 0.67 ^ 0.66, 0.61 ^ 0.63 and 0.67 ^ 0.61 degrees, respectively. The respective median and mean magnitude vectors of isocenter translation were 1.28 and 1.59 ^ 0.84 mm. Analysis of the accuracy of the first setup control, representative of setup accuracy for radiosurgery treatments, compared with setup accuracy throughout a fractionated radiation treatment course were statistically equivalent ðP ¼ 0:15Þ; thus indicating no measurable deterioration of setup accuracy throughout the treatment course. Conclusions: The analyzed Orfit thermoplastic mask system performed favorably compared with other mask immobilization systems for which peer-reviewed repositioning data exist. While the performance of the system for fractionated treatment courses was considered to be excellent, use of this mask system for radiosurgery immobilization in our clinic is subject to additional quality assurance measures to prohibit the delivery of treatments with target dislocations larger than 2 mm. The measured data in the present study should enable the users of this system to assign appropriate margins for the generation of planning target volumes. q 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Immobilization; Mask system; Repositioning accuracy; Radiosurgery; Fractionated radiotherapy 1. Introduction Effective immobilization and reliable alignment and repositioning accuracy are of paramount importance to the effective and safe delivery of fractionated and, most critically, single-dose radiosurgery treatments. For fractio- nated radiotherapy of cranial lesions, a variety of non- invasive mask-type immobilization devices have been developed and used. While the technical specifications and method of implementation of the various commercially available mask systems often differ significantly, reliable data regarding their individual repositioning accuracies are seldom reported in peer-reviewed literature. Mask immobilization systems for cranial radiotherapy must strike a tenuous balance between an effective degree of rigidity of the immobilization device and acceptable 0167-8140/$ - see front matter q 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2004.03.003 Radiotherapy and Oncology 71 (2004) 339–345 www.elsevier.com/locate/radonline * Corresponding author.