Estimation of Conversion Factors for CTDIw Calculation in MVCBCT Acquisitions
M Myronakis1*, M Jacobson2,3, T Harris3, I Valencia Lozano2,3, C Williams3, M Lehmann4, R Fueglistaller4, P Huber4, D Morf4, R Berbeco2,3, (1) ,Heraklion, Crete, GR, (2) Dana Farber Cancer Institute, Natick, MA, (3) Brigham & Women's Hospital, Brookline, MA, (4) Varian Medical Systems, Baden, CH
Presentations
PO-GePV-M-126 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: MVCBCT dosimetric performance using standardized dose indices can be used to optimize acquisition techniques in radiation therapy imaging. We propose a method to estimate CTDIw for MVCBCT acquisitions directly from the delivered MU. This parameter will enable comparison with other x-ray based volumetric imaging (e.g. kVCBCT).
Methods: An extended PMMA body phantom was modeled with a diameter and length of 32 cm and 30 cm, respectively. A 360-degree MVCBCT acquisition was simulated using the Varian MV imaging beam (2.5 MV) and field size equal to 26×26 cm2 at isocenter. Correction factors for the large field size relative to the CTDI ion chamber length were calculated. Additional correction factors to account for dose distribution differences between MV and kV beams were calculated. The CTDIw was calculated from the simulated dose distribution using the standard formula. A conversion coefficient to estimate CTDIw from MU was derived.
Results: Three-dimensional dose distributions were generated. Axial and radial dose profiles were produced to facilitate derivation of appropriate correction factors for center and peripheral CTDI locations. The correction factor for the wide field size was 2.45 for center and periphery. Correction factors for peripheral CTDI locations that fall outside the irradiated field ranged from 1.14 to 1.16. The conversion coefficient to estimate CTDIw directly from delivered MU was equal to 0.212 cGy/MU
Conclusion: A method to directly estimate CTDIw from delivered MU after an MVCBCT acquisition was proposed. The method relies on Monte Carlo simulations to produce correction factors for large field size and ion chamber location relative to the field size. The correction factors facilitate the derivation of conversion coefficients that allow direct calculation of CTDIw from MU. The proposed method can be generalized for various field sizes and angles to provide a fast and reliable procedure for dose optimization of MVCBCT imaging techniques
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