Paper MONTE CARLO SIMULATION OF AN ANTHROPOMETRIC PHANTOM USED FOR CALIBRATING IN VIVO K-XRF SPECTROSCOPY MEASUREMENTS OF STABLE LEAD IN BONE Camille J. Lodwick* and Henry B. Spitz † Abstract—An anthropometric surrogate (phantom) of the hu- man leg was defined in the constructs of the Monte Carlo N Particle (MCNP) code to predict the response when used in calibrating K x-ray fluorescence (K-XRF) spectrometry mea- surements of stable lead in bone. The predicted response compared favorably with measurements using the anthropo- metric phantom containing a tibia with increasing stable lead content. These benchmark measurements confirmed the valid- ity of a modified MCNP code to accurately simulate K-XRF spectrometry measurements of stable lead in bone. A second, cylindrical leg phantom was simulated to determine whether the shape of the calibration phantom is a significant factor in evaluating K-XRF performance. Simulations of the cylindrical and anthropometric calibration phantoms suggest that a cy- lindrical calibration standard overestimates lead content of a human leg up to 4%. A two-way analysis of variance deter- mined that phantom shape is a statistically significant factor in predicting the K-XRF response. These results suggest that an anthropometric phantom provides a more accurate calibration standard compared to the conventional cylindrical shape, and that a cylindrical shape introduces a 4% positive bias in measured lead values. Health Phys. 95(6):744 –753; 2008 Key words: bones, human; Monte Carlo; physics, medical; spectroscopy, gamma INTRODUCTION THE SKELETON of the human body is the major storage location for calcium. Lead, as a congener of calcium, is also stored in the skeleton. Persons who have been chronically exposed to lead are at risk of serious health effects. The risk is directly related to the amount of exposure to lead and is especially high for young children (Hu et al. 1998; Rabinowitz 1990). Although the con- ventional method to detect exposure is to measure lead in a sample of blood, direct measurement of stable lead in bone is more indicative of cumulative, long-term expo- sure than lead in blood (Ahlgren et al. 1976; Hu et al. 1989). In vivo x-ray fluorescence spectroscopy is a non-invasive method for measuring stable lead in bone that uses 88.034 keV photons emitted by a 109 Cd source to excite the K shell electrons of stable lead atoms deposited in bone. The intensity of the characteristic x rays emitted as the electrons return to lower energy states is directly related to the quantity of stable lead present in bone (Todd and Chettle 1994; Somervaille et al. 1985). Many studies have validated the accuracy of this method by comparing in vivo K-XRF spectroscopy with chemi- cal measurements of bones from cadavers of occupation- ally exposed workers, revealing systematic bias was of the order of 1–2 g Pb (g bone mineral) -1 (Aro et al. 2000; Somervaille et al. 1986) or up to 5– 8 gg -1 (Todd et al. 2002). Being that the major interest in this diag- nostic procedure remains in its reliability at concentra- tions of lead less than 25 g Pb g -1 and in vivo XRF measurements are currently being used in epidemiologi- cal studies of both workers and children, it is vital that the method provides statistically meaningful results at low concentrations. Although the normalization technique does lessen the variation observed between K-XRF measurements, uncertainty exists. It has been reported that measurement precision varies from person-to-person (inter-subject variation) depending upon individual’s tissue thickness and bone mineral mass despite the normalization process (Aro et al. 1994). Additionally, uncertainty can be correlated to variations in source-detector positioning both during and between measurements (Spitz et al. 2000) as well as simple signal-to-noise counting statis- tics. The objective of this work is to utilize simulations to evaluate the effect of using an anthropometric calibration phantom compared to the conventional cylindrical shape. An advantage of Monte Carlo analysis is its ability to * Oregon State University, Department of Nuclear Engineering and Radiation Health Physics, 116 Radiation Center, Corvallis, OR 97331-5902; † University of Cincinnati, Department of Mechanical Industrial and Nuclear Engineering, Cincinnati, OH 45221. For correspondence contact: Camille Lodwick, Nuclear Engineer- ing and Radiation Health Physics, 116 Radiation Center, Corvallis, OR 97331-5902, or email at camille.lodwick@oregonstate.edu. (Manuscript accepted 21 May 2008) 0017-9078/08/0 Copyright © 2008 Health Physics Society 744