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Using An Independent Dose Calculation System to Optimize Clinical Thresholds and Reduce the Number of Physical Dose Measurements Required for Patient-Specific Quality Assurance

K Hasse*, B Ziemer, Y Natsuaki, G Valdes, E Hirata, T Solberg, J Scholey, A Witztum, UCSF Comprehensive Cancer Center, San Francisco, CA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To assess the use of Mobius 3D (M3D) dose calculation software to reduce the number of physical ion chamber (IC) dose measurements required for patient-specific quality assurance (PSQA).

Methods: For this study, a total of 1114 inversely-planned treatments using Pinnacle or RayStation treatment planning software (TPS) were considered. These plans were delivered using Elekta Versa HD (n=208) and Varian Truebeam (n=544) and Truebeam STx (n=362) linear accelerators between June 2018 and November 2019. For each plan, an independent dose calculation was performed using M3D, and an absolute dose measurement was performed using an ion chamber inside the Mobius phantom. The point dose differences between the TPS and M3D calculation, and between TPS and IC measurement were calculated. Agreement between the TPS and IC was used to define the ground truth plan failure. ROC analysis was used to determine the M3D agreement threshold for IC plans failing at 3%, 4%, and 5%.

Results: A total of 1033 plans (92.7%) passed using both the M3D point-dose calculation and the IC reading using a 5% threshold for both. All 8 plans that failed the IC measurement also failed the M3D calculation, while 73 plans failed the M3D calculation but not the IC measurement. In order to identify plans that fail at the 4% and 3% IC threshold while keeping sensitivity at 100% a M3D threshold of 3% and 0.05% must be used, for which 280 and 980 of the 1114 plans fail M3D, respectively.

Conclusion: A pre-delivery M3D calculation can identify plans failing an IC measurement using a 5% threshold with 100% sensitivity. By only taking IC measurements on this subset, 1033 fewer physical measurements would have been taken over a period of 15 months, freeing up significant clinical resources while still detecting potentially failing plans and maintaining patient safety.

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Quality Assurance, Decision Theory, Optimization


TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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