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Is Portal Dosimetry a Truly Absolute QA Tool for VMAT Pre-Treatment Verification?

H Ai*, K Huang , X Lu , Indiana University- School of Medicine, Indianapolis, IN


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: to evaluate portal dosimetry(PD) absolute mode in VMAT pre-treatment verification

Methods: a head and neck VMAT test plan and its portal dose prediction plan(PDPP) are generated in Eclipse treatment planning system(TPS) version 13.7. Wrong dose-leaf gap(DLG)(0 for 6MV) and transmission factor(5% for 6MV) are intentionally assigned in TPS to test the detectability of PD in MLC dosimetric parameters. Test plans and their PDPPs are recalculated with new MLC dosimetric parameters but same MU. To test the detectability of beam output accuracy in TPS beam configuration, beam output in TPS is intentionally tuned 10% higher than machine output. Test plan and its PDPP are also recalculated with wrong beam output factor in TPS but same MU. Gamma passing rates(GPR) between PDPPs and EPID measured portal dose images are calculated using PD. A third party QA device ArcCheck is used for the VMAT pre-treatment QA and GPRs are compared with portal dosimetry results.

Results: for the original plan, GPRs for 3% and 3 mm criteria are 97.2% using PD and 98.3% using ArcCheck. For DLG=0, GPRs are 87.8% for portal dosimetry and 88.7% for ArcCheck. For transmission factor=5%, GPRs are 83.1% for PD and 77.6% for ArcCheck. For the plan with 10% higher beam output in TPS, GPR using PD is 97.2%, same as original plan. However, GPR using ArcCheck is 89.8%. It indicates that PD cannot detect the error in beam configuration output factor in TPS.

Conclusion: PD in absolute mode as a VMAT pre-treatment verification tool can detect errors in MLC DLG and transmission factor. However, PD cannot detect the error in beam output setup in beam configuration of TPS system. Cautious should be taken when using portal dosimetry as an alternative QA tool over a third party IMRT QA tool based on diode matrix or ion chamber matrix.


Portal Imaging, Absolute Dosimetry, Quality Assurance


TH- External beam- photons: portal dosimetry

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