Yoichi Watanabe1, Bruce J Gerbi. 1. Department of Therapeutic Radiology, University of Minnesota, 420 Delaware Street, Minneapolis, MN 55455, USA. watan016@umn.edu
Abstract
PURPOSE: To measure radiation exposure to a patient during head repositioning with the automatic positioning system (APS) for Gamma Knife radiosurgery. METHODS AND MATERIALS: A 16-cm diameter spherical solid phantom, provided by the manufacturer, was mounted to the APS unit using a custom-made holder. A small-volume ionization chamber (0.07-cm(3) volume) was placed at the center of the phantom. We recorded the temporal variation of ionization current during the entire treatment. Measurements were made for 3 test cases and 7 clinical cases. RESULTS: The average transit time between successive shots, during which the APS unit was moving the phantom for repositioning the shot coordinates, was 20.5 s for 9 cases. The average dose rate, which was measured at the center of the phantom and at a point outside the shot location, was 0.36 +/- 0.09 cGy/min when the beam output was approximately 3.03 Gy/min for the 18-mm collimator helmet. Hence, the additional intracranial radiation dose during the APS-driven head repositioning between two successive shots (or APS transit dose) was 0.12 +/- 0.050 cGy. The APS transit dose was independent of the helmet size and the position of shots within the phantom relative to the measurement point. CONCLUSION: The head repositioning with the APS system adds a small but not negligible dose to the dose expected for the manual repositioning method.
PURPOSE: To measure radiation exposure to a patient during head repositioning with the automatic positioning system (APS) for Gamma Knife radiosurgery. METHODS AND MATERIALS: A 16-cm diameter spherical solid phantom, provided by the manufacturer, was mounted to the APS unit using a custom-made holder. A small-volume ionization chamber (0.07-cm(3) volume) was placed at the center of the phantom. We recorded the temporal variation of ionization current during the entire treatment. Measurements were made for 3 test cases and 7 clinical cases. RESULTS: The average transit time between successive shots, during which the APS unit was moving the phantom for repositioning the shot coordinates, was 20.5 s for 9 cases. The average dose rate, which was measured at the center of the phantom and at a point outside the shot location, was 0.36 +/- 0.09 cGy/min when the beam output was approximately 3.03 Gy/min for the 18-mm collimator helmet. Hence, the additional intracranial radiation dose during the APS-driven head repositioning between two successive shots (or APS transit dose) was 0.12 +/- 0.050 cGy. The APS transit dose was independent of the helmet size and the position of shots within the phantom relative to the measurement point. CONCLUSION: The head repositioning with the APS system adds a small but not negligible dose to the dose expected for the manual repositioning method.
Authors: Tuan-Anh Tran; Vincent Wu; Harish Malhotra; James P Steinman; Dheerendra Prasad; Matthew B Podgorsak Journal: Radiol Oncol Date: 2011-04-23 Impact factor: 2.991
Authors: Tuan-Anh Tran; Thomas R Stanley; Harish K Malhotra; Steven F De Boer; Dheerendra Prasad; Matthew B Podgorsak Journal: J Appl Clin Med Phys Date: 2010-01-28 Impact factor: 2.102