(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: TrueBeam Linear accelerator (Varian, Palo Alto, CA) has 2.5 MV photon beam for portal imaging. Since the beam of the energy produces better image quality than other treatment energies, it has been used for the portal imaging. Despite of the importance of the geometrical accuracy of the 2.5 MV beam, necessity of routine QA of this beam has not been emphasized properly. This study is to demonstrate our finding of the deviation of the beam size of the 2.5 MV and to discuss the necessity of the routine QA.
Methods: 10x10 cm² and 25x25 cm² field sizes were chosen. The portal images were taken with 2.5 MV, 6MV and 15 MV photons for variable cases of phantom thickness; 1) without couch, 2) 5 cm, 3) 10 cm, and 4) 15 cm solid water. The RIT software (Colorado Springs, CO) was used to analyze. The image shift and penumbra were then evaluated for 2.5 MV, 6 MV and 15 MV images. The difference of FWHM was normalized to 6 MV.
Results: Little difference was observed for the cross plane direction. Difference of 0.6 mm and 1.12 mm variation found for the inplane between 2.5 MV and 6 MV at surface and 10 cm depth, respectively. While the differences were only 0.005 mm and 0.071 mm between 6MV and 15MV.
Conclusion: Different energy beam may generate different field boarder or penumbra unless the machine quality is maintained properly. The source of the difference is still under investigation; however, this finding justifies the necessity of routine QA of 2.5 MV if this energy has been using clinically for portal imaging.
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