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Determination of Proper Measurement Plane for 2D IMRT QA

J Panetta*, I Veltchev , R Price , C Ma , Fox Chase Cancer Center, Philadelphia, PA


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: IMRT treatment delivery is a complex process that requires quality assurance (QA) to verify its accuracy prior to patient treatment. A 2D dose distribution in a single plane is commonly measured and compared to the treatment planning system (TPS) generated distribution. The purpose of this work is to study the relationship between gamma passing rate as a function of dose gradient, which affects the measurement accuracy of individual detectors, to allow for proper selection of the plane of measurement.

Methods: Several VMAT plans (n=10), treating various disease sites, were selected and QA was performed using a 2D ion chamber array on a centrally located coronal measurement plane (as per standard 2D IMRT QA), followed by gamma analysis (3%/3mm). Additionally, for a subset of these plans, reference plans were calculated and QA performed for a series of parallel coronal measurement planes, spanning 5 cm in 0.5 cm increments. The relationship between the passing rate and dose gradient was evaluated in the various plans and measurement planes.

Results: Among the various plans investigated, gamma passing rate was found to decrease with the number of array pixels (and fraction of included pixels) that have relatively high dose gradients, using transverse gradient as one measure of dose gradient (R²=0.30 for linear fit). Similar results were found for the vertical gradient. Gamma passing rate similarly decreased with number of high gradient pixels for a given plan measured at different coronal planes (R²=0.58 for linear fit).

Conclusion: The study indicates that gamma passing rate is dependent upon the dose gradient of the measurement plane and generally increases with decreasing dose gradient, allowing for an estimation of the optimal plane in which to deliver QA for the treatment plan.


Not Applicable / None Entered.


TH- External beam- photons: Quality Assurance - IMRT

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