Radiation exposure during head repositioning with the automatic positioning system for gamma knife radiosurgery.

Abstract

To measure radiation exposure to a patient during head repositioning with the automatic positioning system (APS) for Gamma Knife radiosurgery. 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. 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. The head repositioning with the APS system adds a small but not negligible dose to the dose expected for the manual repositioning method.