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Transmission Verification Using the On-Board EPID


p Jin

p Jin*, R Munbodh, A Lin, T Zhu, Univ Pennsylvania, Philadelphia, PA

SU-D-141-5 Sunday 2:05PM - 3:00PM Room: 141

Purpose:
To provide in-vivo dose and patient geometry verification for radiation therapy using an MV image acquired during treatment and a fast dose calculation method through the patient geometry.

Methods:
Dosimetric calibration of the EPID is performed using the known profile for a series of uniform phantoms with thickness between 0 to 22 cm. Phantom scattering within the EPID is corrected to minimize systemic error. The dose-response of the EPID is linear with dose after correction. A fast ray-tracing algorithm is used to calculate the total dose (separated into primary and scatter components from the patient geometry) on the EPID active imaging plane under perspective geometry. Attenuation of the primary dose as a function of radiological length is approximated by the attenuation function μ and off-axis softening coefficient η. An iterative procedure is used to find a self-consistent solution of total dose measured by the EPID and calculation through a series of phantoms with uniform thicknesses. 3D verification of the patient geometry is achieved through automatic 2D and 3D registration of a 2D radiograph kilovoltage OBI or megavoltage EPI to the 3D planning CT using one of three similarity measures: Pearson correlation coefficient, maximum likelihood measure with Gaussian noise [1] or mutual information.

Results:
The dose-response curve for the amorphous-silicon EPID was obtained. The fast ray-tracing algorithm has been developed and tested on both phantom and Rando phantom patient geometry. First results have shown that the estimated patient dose is within 5% of that calculated by the treatment planning system.

Conclusion:
A fast in vivo 3D dose verification using EPID is demonstrated. Further development is warranted with a goal of clinical implementation, given its promise as a routine patient QA procedure for IMRT dose and geometry verifications.

[1] Munbodh et al. Med. Phys. 36(10), 2009


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