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Experimental Verification of a Monte Carlo Linear Accelerator Model Using a Radiochromic Film Stack Dosimeter

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T McCaw

T McCaw*, W Culberson , L DeWerd , University of Wisconsin Medical Radiation Research Center, Madison, WI


SU-E-T-384 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To experimentally verify a Monte Carlo (MC) linear accelerator model for the simulation of intensity-modulated radiation therapy (IMRT) treatments of moving targets.

Methods: A Varian Clinac™ 21EX linear accelerator was modeled using the EGSnrc user code BEAMnrc. The mean energy, radial-intensity distribution, and divergence of the electron beam incident on the bremsstrahlung target were adjusted to achieve agreement between simulated and measured percentage-depth-dose and transverse field profiles for a 6 MV beam. A seven-field step-and-shoot IMRT lung procedure was prepared using Varian Eclipse™ treatment planning software. The plan was delivered using a Clinac™ 21EX linear accelerator and measured with a Gafchromic™ EBT2 film stack dosimeter (FSD) in two separate static geometries: within a cylindrical water-equivalent-plastic phantom and within an anthropomorphic chest phantom. Two measurements were completed in each setup. The dose distribution for each geometry was simulated using the EGSnrc user code DOSXYZnrc. MC geometries of the treatment couch, cylindrical phantom, and chest phantom were developed by thresholding CT data sets using MATLAB™. The FSD was modeled as water. The measured and simulated dose distributions were normalized to the median dose within the FSD.

Results: Using an electron beam with a mean energy of 6.05 MeV, a Gaussian radial-intensity distribution with a full width at half maximum of 1.5 mm, and a divergence of 0°, the measured and simulated dose profiles agree within 1.75% and 1 mm. Measured and simulated dose distributions within both the cylindrical and chest phantoms agree within 3% over 94% of the FSD volume. The overall uncertainty in the FSD measurements is 3.1% (k=1).

Conclusion: MC simulations agree with FSD measurements within measurement uncertainty, thereby verifying the accuracy of the linear accelerator model for the simulation of IMRT treatments of static geometries. The experimental verification will be extended to treatments of targets undergoing three-dimensional motion.

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