Room: AAPM ePoster Library
Purpose: study to implement new source sector movement QA method in Gamma Knife Radiosurgery using film dosimetry.
Methods: this study 50 shot irradiation plan simulating source sector movement (SSM) with varying x,y&z coordinate position was generated using three inbuilt collimators 4,8&16mm in the control console of Gamma Knife Perfexion unit (GKPFX). The EBT3 films (Ashland ISP Advanced Materials, NJ) of size 3.5x3.5cm2 were positioned in the spherical water phantom and mounted in the GKPFX couch at coordinate position x,y&z=100. The films were exposed to varying doses from 0 to 12Gy for calibration purpose. A film from the same batch of size 5x5cm2 was placed inside same phantom for 50 shot irradiation with treatment time of 15 minutes. The same irradiation procedure was repeated on daily basis for three consecutive days on three separate films of same batch. The exposed films were scanned using high resolution Epson Expression10000XL scanner with image resolution of 720 dpi, saved in tiff format and were further analyzed using in-house developed MATLAB software codes.
Results: 50 shot plan irradiation displayed similar dose distribution pattern in each film exposed on three consecutive days. The film exposure comparison between first and third day showed close agreement of 91% and 98% pixel pass rate at 2%/2mm and 3%/3mm gamma pass criteria respectively. The isodose overlay and line profile comparison of EBT3 films of first and third day exposure were in close agreement and showed less than 3% dose difference. The spatial dose distribution patterns generated by SSM with varying coordinate positions and collimator sizes are not verified as a periodic QA in GKPFX.
Conclusion: of SSM is essential before treatment delivery in GKPFX as subtle variations in sector movement may have clinical significance. We propose this QA test to be done on daily or weekly basis for GKPFX.