Repositioning accuracy of fractionated stereotactic irradiation: assessment of isocentre alignment for different dental fixations by using sequential CT scanning

Background and purpose: To quantify the accuracy and reproducibility of patient repositioning in fractionated stereotactic conformal radiotherapy (SCRT) using dental fixations in conjunction with a stereotactic head mask. Patients and methods: One hundred and fourteen verification CT scans were performed on 57 patients in order to check set-up alignment. The first scan was done immediately after the first treatment. Twelve patients were checked for alignment accuracy with weekly CT scans over a period of 3-6 weeks, all others had 1-2 scans. Two different dental fixations were used in combination with a non-invasive mask system: an upper jaw support (35 patients) and a customised bite-block (17 patients). Five patients were treated with no additional fixation. Co-registration to the planning CT was used to assess alignment of the isocentre to the reference markers. Additionally, the intra-operator variability of image co-registration was assessed. Results: There was a significant improvement of the overall alignment in using the bite-block instead of the upperjaw support (P < 0.001). The mean deviation was for the bite-block 2.2 +/- 1.1 mm (1 SD), for the upperjaw support 3.3 +/- 1.8 mm and 3.7 +/- 2.8 mm for the mask alone. Overall isocentre deviations independent of the method of fixation were 2.8 mm (1.7 mm, 1 SD). Displacements in CC direction were significantly less for the bite-block compared to the upper jaw support (P = 0.03). The addition of an upper jaw support significantly reduced lateral rotations compared to the mask system alone (P = 0.03). The intra-operator variability of image co-registration was 1.59 +/- 0.49 mm (1 SD). Conclusion: The reproducibility of patient positioning using a re-locatable head mask system combined with a bite-block is within the reported range for similar devices and is preferable to a simple upper jaw support. In order to further reduce the margin for the planning target volume an intra-oral dental fixation is recommended. (C) 2004 Elsevier Ireland Ltd. All rights reserved.