Monte Carlo Simulations of Photon Emissions from the Lungs of NORMAN Voxel Phantom Help to Certify Calibration Phantom Lungs
R.J. Traub; J.C. McDonald; and T.P. Lynch (Pacific Northwest National Laboratory, Richland, Washington)
An ongoing difficulty associated with the manufacture of phantom lungs for chest counting systems is that the counting efficiency of in vivo detectors for the photons emitted from the lungs is dependent on the formulation of the lungs. Consequently, it is necessary to know the photon emission rate from the lungs in addition to the activity contained within the lungs. As a first step toward developing lung specific individualized calibration factors, the NORMAN voxel phantom was obtained from the National Radiological Protection Board in Britain. The phantom was implemented in the radiation transport code MCNPX 2.5e. Virtual detection efficiencies for monoenergetic photons were obtained using a combination of the NORMAN phantom and a virtual four-detector lung counting system comprised of four 38 cm2 by 30 mm thick germanium detectors. Photons originating in the phantom lungs were transported through the phantom to the sensitive volume of the detectors and detection was simulated in MCNPX with Gaussian energy broadening. Virtual counting efficiencies for nine different phantom lung materials were calculated. The influence of density on the counting efficiency was calculated and the density of artificial lung materials that would result in counting efficiencies comparable with the counting efficiency when the lungs were human lungs at a density of 0.26 g/cm3 was also computed. Phantom lungs can shrink after they have been removed from their molds and the influence of this shrinkage was calculated. The counting efficiency for photons emitted from shrunken lungs is less than that from normal phantom lungs and the reduction can be as great as 5% for 17 keV photons. Monte Carlo simulations, such as those described above, can be used to quantify the variations in photon emission from phantoms lungs of varying formulations and can be used to compute virtual calibration factors for individual phantom lungs.