Room: ePoster Forums
Purpose: To create a simple pre-treatment QA program for SBRT by finding a gantry and couch combination which gives a worst isocenter shift and check position-repositioning of kV and MV images and to prepare an easily-accessible method to check the system for accurate dose delivery for SBRT.
Methods: Winston-Lutz (W-L) tests were performed with 6 mm tungsten ball using 2 cm x 2 cm fields with varying couch and gantry positions. Images were captured using an EPID and analyzed using PipsPro software (Ver. 5.6, Standard Imaging, Inc.). Gantry and couch angle combination was selected based on maximum deviation of isocenter. For the initial QA, an off-centered marker match phantom was used for kV and MV 2D-2D registrations. Finally, alignment of the phantom with the worst gantry-couch combination was checked using MV 2D-2D match. For daily pre-treatment QA, repositioning using MV orthogonal 2D-2D match was used. In case of failure, a further investigation was performed using MV 2D-2D match with the worst gantry-couch combination.
Results: Based on W-L tests, G45C315 was selected for the maximum isocenter deviation as G45C270 is rarely used. For daily QA, average deviation for 2D-2D matches was found to be 0.55 mm and 0.43 mm for kV and MV, respectively, in couch vertical direction. In case of failure of daily QA, MV 2D-2D match using the gantry-couch combination was performed to investigate failing result. The average deviation of isocenter was (Vrt, Lat, Lng) = (0.13, 0.31, 0.34 mm).
Conclusion: The identification of systematic weakness of LINAC is very important in the pre-treatment process of SBRT. Daily imaging QA is adapted from TG142, however, for the SBRT procedure an additional pre-treatment QA procedure is implemented. This initial and daily pre-treatment QA program is crucial to check mechanical stability of LINAC and guarantee accurate dose delivery.