Room: ePoster Forums
Purpose: The purpose of this study was to evaluate the performance of the CBCT-based patient positioning system as a tool for Gamma Knife radiosurgery.
Methods: Daily QA diode test results from a 6-month period were analyzed for the geometric accuracy and the stability of the CBCT imager. The performance of the image acquisition and registration modules was evaluated using an anthropomorphic head phantom and a XYZR axis manual linear positioning stage. The head phantom was fixed on a mask adaptor and manually translated or rotated along one of the X, Y, Z axes in the range of Â±10mm or Â±10Âº. A CBCT scan was performed after each manual position setup followed by an image registration to the reference scan. To assess the overall setup uncertainties in fractionated treatment, two cylindrical Presage phantoms of 15cm diameter and 10cm height were irradiated with identical prescription dose and shot placement following standard mask-based treatment workflow according to two different fraction schedules: a single fraction treatment of 7.5Gy and a 5-fraction treatment with 1.5Gy fraction dose.
Results: The averaged radial coordinates of the 4 marks are 0.087mm, 0.085mm, 0.095mm, and 0.079mm from their preset values from the daily QA tests. The averaged differences in the X, Y, Z coordinates and the pitch, yaw, roll angles from the image registration process are 0.28mm, 0.42mm, 0.2mm, 0.33Âº, 0.25Âº, 0.44Âº from the manual setup respectively. Compared to the treatment plan using a 2mm&1% criteria, the averaged 2D Gamma passing rate is 98.01 for the measured dose distribution from the Presage phantom with 1-fraction irradiation and 94.71 for the 5-fraction irradiation.
Conclusion: The CBCT imager and the image registration algorithm can reproduce phantom position with less than 0.5mm/0.5Âº uncertainty. A systematic contribution from the inter-fraction phantom repositioning procedure was observed in the Gamma analysis over the irradiated volume.