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

PURPOSE Ion chamber dosimetry is being used to calibrate dose for cell irradiations designed to investigate photoactivated Auger electron therapy at the Louisiana State University Center for Advanced Microstructures and Devices (CAMD) synchrotron facility. This study performed a dosimetry intercomparison for synchrotron-produced monochromatic x-ray beams at 25 and 35 keV. Ion chamber depth-dose measurements in a polymethylmethacrylate (PMMA) phantom were compared with the product of MCNP5 Monte Carlo calculations of dose per fluence and measured incident fluence. METHODS Monochromatic beams of 25 and 35 keV were generated on the tomography beamline at CAMD. A cylindrical, air-equivalent ion chamber was used to measure the ionization created in a 10 × 10 × 10-cm(3) PMMA phantom for depths from 0.6 to 7.7 cm. The American Association of Physicists in Medicine TG-61 protocol was applied to convert measured ionization into dose. Photon fluence was determined using a NaI detector to make scattering measurements of the beam from a thin polyethylene target at angles 30°-60°. Differential Compton and Rayleigh scattering cross sections obtained from xraylib, an ANSI C library for x-ray-matter interactions, were applied to derive the incident fluence. MCNP5 simulations of the irradiation geometry provided the dose deposition per photon fluence as a function of depth in the phantom. RESULTS At 25 keV the fluence-normalized MCNP5 dose overestimated the ion-chamber measured dose by an average of 7.2 ± 3.0%-2.1 ± 3.0% for PMMA depths from 0.6 to 7.7 cm, respectively. At 35 keV the fluence-normalized MCNP5 dose underestimated the ion-chamber measured dose by an average of 1.0 ± 3.4%-2.5 ± 3.4%, respectively. CONCLUSIONS These results showed that TG-61 ion chamber dosimetry, used to calibrate dose output for cell irradiations, agreed with fluence-normalized MCNP5 calculations to within approximately 7% and 3% at 25 and 35 keV, respectively.