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Portal Dosimetry Comparison for Three Beam-Matched Varian TrueBeam LINACs

K Yang1*, W Jiang2, R Patel2,1, S Sharma1, A Ju1, (1) East Carolina University, Greenville, NC, (2) 21st Century Oncology, Greenville, NC

Presentations

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

Room: AAPM ePoster Library

Purpose:
Linear accelerators with beam-matched capability can increase the flexibility in patient treatment and reduce the economic/social effects caused by machine-down time. EPIDs are extensively used for obtaining dosimetric information of pre-treatment field verification. In this study, we compare the portal dose calculation(PDC) algorithm parameters and the impact on patient-specific IMRT QA analysis.
Methods:
Three linacs were installed with dosimetric properties (photon PDD within 0.5%, and symmetry within 1%). Each PDC algorithm was configured at its machine commissioning time. The later-installed linacs used the same MLC parameters, including DLG and transmission factor. The same set of data, including intensity profile and output factors, were used in PDC configuration. The configuration of the PDC algorithm requires a set of dynamic pyramid-shaped test images to deriving the single pencil-beam kernel, with weighted sum of 9 Gaussian functions with predefined widths. The difference in kernel curves was compared. IMRT QAs (6X, 10X) were delivered using the same plan and predicted image. IMRT/VMAT QA from 100 patients, total 149 plans and 409 fields, were delivered to three linacs.
Results:
Relative weights varied slightly between linacs, while these differences were larger for flattening-filter-free beams. The kernel curves (6X, 10X) showed that the photon-scattering within a-Si material is similar in current aS1200 panels. The systematic IMRT QA results showed that 1135/1189 fields passed with 2%/2mm criteria, and 54/1189 passed 3%/3mm. The cross-validation passing rate with 2%/2mm among linacs indicated comparable passing rate distribution(e.g. Linac#3 plan 98.9±0.97%, 99.04±0.99%, 99.23±0.87%); therefore, IMRT QA on the planned linac has passing rate compared to QA at another linac.
Conclusion:
For three beam-matched linacs with the common beam and PDC parameters, the optimized PDC algorithms have a good agreement among linacs. The IMRT QA passing rate statistical analysis enables the possibility to move patient treatment on another linac without additional QA.

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Keywords

Electronic Portal Imaging, Linear Accelerator, Radiation Dosimetry

Taxonomy

TH- External Beam- Photons: portal dosimetry, in-vivo dosimetry and dose reconstruction

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