E Inam*, L Chen, Y Gonzalez, D Parsons, A Godley, S Jiang, W Lu, R Reynolds, B Cai, M Lin, University of Texas Southwestern Medical Center, Dallas, TX

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  E Inam

PO-GePV-M-111 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Ethos CBCT-based adaptive therapy (ART) system features automatic online adaptive plan generation based on identical planning goals and priorities utilized in the preplan. This study aims to analyze the pre-plan and ART-plan quality assurance (QA) results to establish the action levels for the online ART patient-specific QA

Methods: We performed measurement-based (ArcCheck) and calculation-based (Mobius) QA for the pair of pre-plan and on-couch ART-plans of 100 ART cases treated in our center. The criteria for gamma analysis was 2%,2mm for ArcCheck and 3%,2mm for Mobius 3%/2mm with 10% dose threshold. We analyzed the correlation of the modulation factors and the gamma passing rates between the pre-plan and the ART-plan. We calculated the action limits based on TG-218 recommendations for the two methods, respectively. The action levels were derived based on 30 randomly selected ART-plans and evaluated with the remaining 70 ART-plans.

Results: Although the same optimization goals are used for pre-plan and ART-plan, the modulation factor difference between pre-plan and ART-plan is 12.2%±0.6% (Range: 0.03-53%). Correlation is observed between pre-plan and ART-plan modulation-factor. The treatment sites with daily anatomy variation, such as abdomen and pelvis, are more prone to plan complexity variation. The calculated action limits are 90.6% (measurement-based) and 95.6% (calculation-based). Based on the calculated action limit, 6 ART-plans failed Mobius QA. However, these 6 ART-plans passed measurement-based QA. A strong correlation of the gamma-passing-rate between pre-plan and ART-plan is observed for the calculation-based QA and can serve as the predictor of ART-Plan QA failure.

Conclusion: ART can drastically change the modulation ratio but, calculation-based QA is robust and can flag failures in the pre-plan phase. Zero failure of measurement-based QA demonstrated the robustness of the beam model and supports calculation-based QA is sufficient for ART-Plan QA and in case of failures, measurement-based QA can be employed.

Keywords

Quality Assurance

Taxonomy

TH- External Beam- Photons: adaptive therapy

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