Objectives:

Gamma Knife ICON has three independent collimators in diameters of 4mm, 8mm, and 16mm, and therefore three on-off timer errors. This study used two methods to measure the timer errors, aiming to establish an efficient and accurate measurement method

Methods:

The conventional method is manual with two types of detectors in a PTW Semiflex chamber and a PTW diamond detector. The detector, inside a 16 cm diameter sphere solid water phantom, was positioned at the focal point (isocenter) of a GK. The two-exposure method (Orton 1972) were used to calculate the timer errors. For 4mm and 8mm collimator, one timer error was calculated using the charges collected for shots of 0.5 min and 1 min. For 16mm collimator, total of six timer errors were calculated with charges collected for shots of 0.15, 0.5, 1, and 3 min. It takes average of 3.5 min to measure one data point.

The novel method used the diamond detector connected to an electrometer with the time-series data logger function to log the shot time of 6 sec, 6 sec, and 9 sec for the collimators of 4mm, 8mm, and 16mm, respectively. Each collimator was measured for 10 times for statistics purpose. A sigmoid function was used to fit the transit period. Triangle function was used to fit the 4mm collimator contribution in the 16mm collimator shot. The charge is the integration of current over time. The average time to collect charge for all three collimators are less than 5 min and the data processing time is less than 0.5 min for an i7 pc.

Results:

For PTW Semiflex chamber, the timer errors for 4mm and 8mm collimators are 0.054 sec and 0.095 sec, respectively. For 16mm collimator, the calculated timer errors using two pair of shots of 0.15 min - 0.5 min, 0.15 min - 1 min, 0.15 min - 3 min, 0.5 min - 1 min, 0.5 min - 3 min, and 1 min – 3 min are -0.212, -0.205, -0.189, -0.169, -0.124, -0.056 sec, respectively. The 16mm timer errors are negative and the max to min ratio is 3.8 times. The negative timer error has no physical meaning.
For PTW Diamond detector, the timer errors for 4mm and 8mm collimators are 0.052 sec and -0.048 sec, respectively. For 16mm collimator, the calculated timer errors using two pair of shots of 0.15 min - 0.5 min, 0.15 min - 1 min, 0.15 min - 3 min, 0.5 min - 1 min, 0.5 min - 3 min, and 1 min – 3 min are 0.00768, 0.0215, 0.0301, 0.0862, 0.0927, and 0.102, respectively. The max/min ratio is 13.3. The large ratio could indicate that the method is not adequate to measure timer error for GK ICON.
With the automatic method, the timer errors for 4mm, 8mm, and 16 mm are fitted as 0.26, 0.16, and 0.48 sec, respectively.

Conclusion(s):

For timer error measurement of the GK Icon, the conventional manual method seems not adequate. The novel method using time-series data logger with an electrometer can obtain the results correctly and efficiently.