Room: Exhibit Hall | Forum 4
Purpose: To evaluate dosimetric errors in radiation therapy produced by skewed radiation beams and evaluate capability of different commercially available QA systems to detect these errors.
Methods: The skewed 6 MV beams were produced by placing 3D-printed plastic blocks in the beam (0Â° â€œwedgeâ€?, â€œ5Â° wedgeâ€? and â€œ10Â° wedgeâ€?). Open field symmetries were measured with all 3 blocks and with PTW 729 and 1500 matrix, Sun nuclear daily QA3 and IC profiler. Results of water tank measurements were set as a reference in open field and the symmetry was determined according to IEC 60976 protocol. Two clinical IMRT plan dose distributions (8 cm and 14 cm target volume) were evaluated with PTW Octavius4D phantom with 729 matrix and 1500 matrix, ScandiDos Delta4 and IBA Multicube with MatriXX ionization chamber detector array using 3D gamma analysis with 2mm DTA, 2% DD and 3mm DTA, 3% DD settings. The measured doses were compared to the calculated dose without block (Varian Eclipse).
Results: The mean difference in open field symmetries between water tank and other QA systems was 0.3 percentage units. The mean reduction of gamma analysis result in 14 cm target volume IMRT plan was 7.1Â±3.2 percentage units when 10Â° inplane wedge and 2mm/2% settings were used. The other measurements did not show reduction of gamma analysis result even with 2mm/2% settings.
Conclusion: The symmetry errors measured by QA systems were similar to water tank measurements. Only large profile error in inplane direction was detected by QA systems of the IMRT plans. Other profile errors did not produce significant reduction in gamma analysis results. This may be because our IMRT plans averages off the profile errors in the high dose region. This study shows the value of basic linac QA tests in detecting certain error types better than end-to-end type tests.
Not Applicable / None Entered.