Room: ePoster Forums
Purpose: Standard commissioning procedure suggested by manufacturer (Elekta, Inc) includes evaluating a 3ABUT (three 6cmx24cm segments) MLC pattern on iViewGT (MV portal imager), and a list of QA plans using a 2D array. The spatial resolution (0.25mm at iso-centre) of iViewGT is insufficient to identify errors in MLC bank offset, and these semi-quantitative methods lack the sensitivity to determine the magnitude of MLC offset error. We propose using the DLG measurement instead to quantify the MLC bank offset, and irradiating the 3ABUT pattern on EDR films as a counter check of the leaf position deviations.
Methods: Measurements were done on a Versa HD Linac with Agility Collimator. A known MLC bank offset (0.22mm at iso-center) was applied. The DLG error was then estimated by measurements using fields of different sliding MLC gaps. The 3ABUT pattern was irradiated at source-to-film distance of 100cm and also on the iViewGT. 6 IMRT QA plans were irradiated using Delta4TM to examine the deviation in QA results with such a small MLC offset.
Results: With a 0.22mm MLC offset, the patterns revealed no observable difference in the iViewGT images, while the EDR films showed an obvious deviation in the gap signal intensity. The DLG method gave a close estimation of the MLC bank offset (+0.21mm). The IMRT plans resulted in an average decrease of 4.1% in gamma analysis (from 98.6% to 94.5%) when 0.22mm MLC offset was applied.
Conclusion: Using 3ABUT in iViewGT alone are not sensitive and accurate enough to determine small bank offset error. These errors, albeit small in magnitude, is large enough to cause a significant degradation of QA result in IMRT treatments. DLG method is a fast and sensitive way for matching and quantifying the MLC bank offset, while EDR films can be used for verification of the leaf position.
Not Applicable / None Entered.