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A Systematic Study On the Angular Dependence of 2D Ion Chamber Array for Accurate IMRT Plan Quality Assurance

J Xu*, J Liu, I Veltchev, C Ma, Fox Chase Cancer Center, Philadelphia, PA


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

Room: AAPM ePoster Library

To clinically implement the AAPM Task Group 218 recommendations on measurement-based IMRT plan QA, a systematic study on the angular dependence of IBA MatriXX????????n 2D ion chamber array was performed and ? dose distribution comparison metrics based on homogeneous vs. heterogeneous geometry was evaluated.

The IBA MatriXX????????n system was set up in the horizontal position, and dose maps were recorded for gantry angles of 0°-180° at different field sizes ranged from 3×3 to 28×28 cm². Angular correction factors (CF) were derived as the ratio of the chamber readings to the TPS-calculated doses as a function of gantry angle for both homogeneous “water” (HU = 0) and inhomogeneous MatriXX geometry. Following recommendations of AAPM TG-218, IMRT plan QA measurements were performed using the true composition delivery method and the results were analyzed using a tight ? test criterion of 3%/2mm with a 10% threshold.

The main features of an angle-resolved dose profile are dominated by the phantom attenuation, with a sharp fine structure around 90°, which is attributed to the attenuation from the internal electronic circuit plate underneath the ion chambers. The angular correction factors showed a strong field size dependence (maximum CF difference ~4% at 6MV and ~2.5% at 10MV), and large chamber-location variation (maximum ~7% from central chambers). In general, there was a smaller variation of CF over field size and a more uniform CF cross the chamber array using homogeneous “water” than inhomogeneous geometry. This is further confirmed by the passing rate improvement in ? tests of real patient IMRT plans using homogeneous vs inhomogeneous phantoms.

A systematic study was conducted on the angular dependence of IBA MatriXX????????n 2D ion chamber array system. IMRT plan verification with homogeneous “water” MatriXX geometry has been developed for the clinical implementation of AAPM TG-218.


Quality Assurance


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

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