Verification of TG-61 dose for synchrotron-produced monochromatic x-ray beams using fluence-normalized MCNP5 calculations

摘要

Ion chamber dosimetry is being used to calibrate dose for cell irradiationsdesigned to investigate photoactivated Auger electron therapy at the LouisianaState University CAMD synchrotron facility. This study performed a dosimetryintercomparison for synchrotron-produced monochromatic x-ray beams at 25 and 35keV. Ion chamber depth-dose measurements in a PMMA phantom were compared withthe product of MCNP5 Monte Carlo calculations of dose per fluence and measuredincident fluence. Monochromatic beams of 25 and 35 keV were generated on thetomography beamline at CAMD. A cylindrical, air-equivalent ion chamber was usedto measure the ionization created in a 10x10x10-cm3 PMMA phantom for depthsfrom 0.6 to 7.7 cm. The American Association of Physicists in Medicine TG-61protocol was applied to convert measured ionization into dose. Photon fluencewas determined using a NaI detector to make scattering measurements of the beamfrom a thin polyethylene target at angles 30 degrees to 60 degrees.Differential Compton and Rayleigh scattering cross sections obtained fromxraylib, an ANSI C library for x-ray-matter interactions, were applied toderive the incident fluence. MCNP5 simulations of the irradiation geometryprovided the dose deposition per photon fluence as a function of depth in thephantom. At 25 keV the fluence-normalized MCNP5 dose overestimated theion-chamber measured dose by an average of 7.2+/-3.0% to 2.1+/-3.0% for PMMAdepths from 0.6 to 7.7 cm, respectively. At 35 keV the fluence-normalized MCNP5dose underestimated the ion-chamber measured dose by an average of 1.0+/-3.4%to 2.5+/-3.4%, respectively. These results showed that TG-61 ion chamberdosimetry, used to calibrate dose output for cell irradiations, agreed withfluence-normalized MCNP5 calculations to within approximately 7% and 3% at 25and 35 keV, respectively.