Assessing skull burdens of actinides using a mathematical phantom: a Monte Carlo approach

In vivo monitoring techniques are needed to estimate the amount of an actinide in the skeleton in addition to that in the lungs and liver. Skull counting with external detectors has been a standard procedure for this purpose. Realistic skull phantoms are normally used to calibrate the counting systems. However, the fabrication of realistic phantoms is extremely difficult and expensive. Therefore, a theoretical approach based on Monte Carlo methods in conjunction with a Cristy mathematical phantom has been examined for assessing skull burdens of actinides. A computer program that generates surface sources of actinides on the skull and simulates low-energy photon transport in the heterogeneous media of the head region of the Cristy phantom was developed for this purpose. The program determines the observable pulse height spectrum of the detector and the corresponding calibration factors for different counting geometries. The computer program was used to generate the pulse height spectra and the corresponding calibration factors of 20 cm and 15 cm diameter phoswich detectors, each positioned on the left and right sides and on the top of the head region of the Cristy phantom, whose skull surfaces were assumed to have a uniform distribution of 241Am. The computed calibration factor for a counting geometry consisting of three phoswich detectors (15 cm diameter each) surrounding the phantom's skull was found to be in excellent agreement with the experimental results obtained for the same geometry using a realistic physical skull phantom. This provided a validation of the realistic design of the skull in the Cristy phantom, implying that the results reported in this paper could be used for in vivo measurements of skull burdens of 241Am for the stated counting geometry.