Room: AAPM ePoster Library
Purpose: To verify the transient time is compensated in Gamma Knife Icon
Method: For the 16mm cone in GK ICON, there are two types of transient time(shutter time). The source sectors pass through the 4mm cone going to and coming from the treatment positions. In addition to the shutter time in the 4mm and 8mm cones, there are the in-4mm-time and the out-4mm-time. The traditional method of measuring the transient time is to obtain the integrated dose and calculate the time. This is problematic because the two types of transient time cannot be separated. Further complicating the issue is that the chamber volume is too large and will under evaluate the 4mm cone dose. In this study, we introduce a novel method to measure directly the transient time for all three cones. A high-frequency sampling electrometer was used to collect the differential data of the charge and the current every 0.05 sec. The collected data was saved as a CSV file and analyzed by a MATLAB code. Chamber volume correction was considered.
Results: For the cones of 4mm, 8mm, and 16mm, the shots with the minimum time of 6 sec, 6 sec, and 9 sec are delivered. The in-4mm and out-4mm transient time for 16mm is the same as 0.15 sec. The up-shutter and the down-shutter times are the same as 0.20± 0.03 sec, 0.14± 0.03 sec, and 0.17± 0.02 sec, which is 3.3%, 2.3%, 3.6% of the corresponding shot time, for the cones of 4mm, 8mm, and 16mm, respectively. The measured dwell times are 5.81±0.02 sec, 5.89±0.02 sec, and 8.54±0.02 sec, respectively. A simple calculation showed that the nominal shot times are the summation of the transient time and the dwell time.
Conclusion: We conclude that for GK ICON the timer errors are significant and they are compensated.