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Patient Specific QA for Online Adaptive Radiotherapy On An MR-Linac System

B Maraghechi*, A Heermann , J Setianegara , J Cammin , A Price , V Rodriguez , D Yang , S Mutic , H Li , O Green , Washington University School of Medicine, St. Louis, MO


(Monday, 7/15/2019) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 2

Purpose: To validate the patient-specific quality assurance (QA) procedure for online adaptive radiotherapy performed with a 0.35-T MR-Guided Linac.

Methods: The online patient QA process for adaptive treatments consisted of two components: a vendor-provided Monte Carlo-based secondary dose calculation module and an in-house software package for plan and contour integrity and deliverability checks. The Monte Carlo (MC) module provided an independently calculated dose distribution that was compared to that calculated by the treatment planning system (TPS). The program performed a gamma analysis with a 2%/2mm criteria between the two dose distributions. For the first five adaptive patients with a total number of 42 adapted fractions, post-treatment patient-specific QA measurements were performed for each adapted fraction. The QA measurement was performed using an MRI-compatible ArcCHECK with an ionization chamber (IC) placed at its center and an in-house software to check the delivery record. The purpose was to validate that the secondary dose calculation module along with our in-house software was sufficient for online adapted plan QA.

Results: The average and standard deviation (SD) of the ratio of IC-measured to TPS-calculated dose for all the fractions of each patient was 0.991±0.021, 0.988±0.012, 0.984±0.015, 0.983±0.01, and 0.977±0.01. The average and SD of gamma passing rate for the 2%/2mm gamma criteria was 91.2±2.4%, 93.0±3.5%, 90.7±2%, 94.2±1.2%, 89.2±2.7% obtained from ArcCHECK measurements. The average and SD of gamma passing rate for the 2%/2mm gamma criteria was 97.2±0.4%, 91.5±1%, 97.6±0.4%, 98.9±0.1%, 96.9±0.3% from secondary dose calculation module.

Conclusion: A strong agreement was obtained between the measured and TPS calculated dose distributions for all adapted fractions. The secondary check also agreed well with the TPS. Therefore, the secondary dose calculation check along with our in-house plan integrity and deliverability check software were considered to be sufficient as primary QA tools for online adaptive treatments.


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