Room: ePoster Forums
Purpose: We present an analytic solution to determine shutter dose for the Leksell Gamma Knife ICON system. For the purpose of this abstract, shutter dose is the dose contribution at isocenter of a solid water phantom while the collimators move from completely-blocking to completely-exposing the 192 Co-60 sources. The classical definition of transit dose does not take into account partial volume effects of the ion chamber or diode detector and should include the dose in moving from the home position to the treatment position and back.
Methods: Current collected by a microDiamond detector was measured as a function of time with a high frequency electrometer, which recorded charge and current at 0.04s intervals. Shutter times for each collimator: 4mm, 8mm, and 16mm were determined and the time to traverse the 4mm collimator. Shutter times were determined from a visual inspection of the current vs. time graph. A Matlab routine was then developed to determine the accumulated doses by integrating the differential plot areas as the collimators transitioned from blocked to full exposure assuming constant velocity.
Results: The measured shutter times for the 16mm, 8mm and 4mm collimators were 0.28s, 0.24s, and 0.24s+/-0.4s respectively. Collimator shutter doses were derived by integrating the instantaneous exposed areas as the collimators move into full source exposure position over the measured shutter time intervals. The ratio of this integral to the area that would have been irradiated if the shutter time was zero is 0.4244. This method resulted in shutter doses of 0.65cGy, 0.50cGy and 0.46cGy for the 16mm, 8mm and 4mm collimators, respectively, assuming a nominal source doserate of 3.3Gy/min or 5.5cGy/s. Transit dose includes sources accelerating and returning to blocked position.
Conclusion: Shutter dose constancy can be accurately determined each month with this method in a single acquisition for each collimator.