Submitted to: Journal of Radioanalytical and Nuclear Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2009
Publication Date: 7/16/2009
Citation: Shypailo, R.J., Ellis, K.J. 2009. Monte Carlo efficiency calibration of a neutron generator-based total-body irradiator. Journal of Radioanalytical and Nuclear Chemistry. 282:247-253. Interpretive Summary: The incidence of obesity world-wide is increasing, as is the need for accurate body composition measurements, especially of large subjects. Prompt-gamma neutron activation (PGNA) analysis is used to measure the total amount of nitrogen in the human body, which is related to the amount of lean tissue. Many PGNA systems are calibrated using a single human-shaped phantom (model). However, the performance of PGNA systems can be affected by variations in body proportions and sizes. Thus, calibrating a PGNA system against a single-sized phantom can lead to inaccurate results. To gauge our system's response to different body sizes, we used a Monte Carlo modeling program that uses statistical sampling to find answers to mathematical problems. We were able to model our PGNA system and its response to various body proportions. We developed calibration curves and correction factors based on a wide range of body sizes. These correction factors provide, for the first time, customized adjustments needed to account for differences in body weight and size of obese adults. The Monte Carlo modeling approach provided a cost-effective way of evaluating many body sizes very efficiently. The calibration curves we developed will allow us to better estimate true changes in lean body mass when studying body composition during weight loss intervention programs.
Technical Abstract: The increasing prevalence of obesity world-wide has focused attention on the need for accurate body composition assessments, especially of large subjects. However, many body composition measurement systems are calibrated against a single-sized phantom, often based on the standard Reference Man model (ICRP-23). Prompt-gamma neutron activation (PGNA) provides the only direct measure of total body nitrogen (TBN), an index of the body's lean tissue mass. The performance of PGNA analysis systems is affected by body habitus – differences in body size will influence neutron flux attenuation, induced gamma signal distribution, and counting efficiency. Thus calibration based on a single phantom could result in inaccurate TBN values. We have used Monte Carlo simulations (MCNP-5; Los Alamos National Laboratory) in order to more efficiently investigate system response to the range of body weights (100-250 kg) and body fat distributions (25%-65%) in obese humans. MCNP codes were constructed to map the system's response to a range of body proportions. Calibration curves were then constructed to derive body-size correction factors relative to a reference anthropomorphic phantom. These correction factors provide, for the first time, the customized adjustments needed to account for differences in body weight and size of obese adults. MCNP simulations allowed for the evaluation of a greater range of body dimensions than would be practical via standard phantom construction and measurement. Additionally, the use of MCNP-generated calibration curves will allow for a better estimate of the true changes in lean tissue mass during intervention programs focused only on weight loss.