Room: AAPM ePoster Library
Purpose: To characterize the dynamic dose variation of the Leksell Gamma Knife ICON unit by using a new scintillation detector system with one to four 1-mm diameter and 1-mm long cylindrical scintillator sensors connected to optical fibers of 0.25-mm in diameter with a length of up to 20-m.
Methods: We used a prototype system (Blue Physics LLC, FL) with one scintillator sensor for relative output factor (ROF) and dynamic dose measurements. The sensor was placed in a 3D printed cylindrical insert with the size equal to the Capintec PR05P ionization chamber. The sensor in the Gamma Knife spherical QA phantom was at the beam focus point of irradiation. We recorded the sensor output signal as a function of time. The effects of the Cherenkov lights produced in the transport fiber cable were eliminated by subtracting the signal of the second fiber next to the primary fiber cable. The integration of the signal over time gave us the relative dose delivered to the sensor. Using single shot plans, we measured the outputs of 4mm and 8mm relative to 16mm collimators. By analyzing the sensor signal vs. time for two plans made of one shot and four shots with 5 Gy to 50% dose level, we quantified transit, shutter, and flash doses.
Results: Measured ROFs were 0.771 and 0.900 for 4-mm and 8-mm collimators, respectively. The measured flash dose of a 16-mm shot was 0.5 cGy (325.2 cGy/min at the focus point). The transit doses delivered to the patient while the source moves between the parking position and the 16-mm or 8-mm collimator positions were smaller than 0.01% of the dose delivered by the shot.
Conclusion: We demonstrated excellent performance and its usefulness of the new scintillation detector system for small field dosimetry of the Gamma Knife ICON unit.