Monte Carlo simulations of in vivo K-shell X-ray fluorescence bone lead measurement and implications for radiation dosimetry.

In order to improve measurement precision and decrease minimum detectable limit, recent applications of K-shell X-ray fluorescence (KXRF) bone lead measurement have used shorter source-to-sample (S-S) distances (approximately 0.5 cm) than the traditionally standard values ranging between 2.0 and 3.0 cm. This alteration will have an effect on subject radiation dose. This paper reports a comprehensive Monte Carlo study performed to investigate the radiation energy deposition values delivered to the leg of model human subjects of various ages. The simulations were run for models approximating 1-year, 5-year, and adult subjects, assuming lead concentrations of 10 microg/g in bone and tracing 500 million photons in each simulation. Trials were performed over a range of S-S distances, from 0.5 to 6.0 cm. The energy deposition due to photoelectric and Compton processes occurring in bone and soft tissue are presented. For each subject age, the Monte Carlo analysis demonstrates that the amount of energy deposited in the bone is increased as the sample is moved closer to the source (from 3.0 to 0.5 cm). The amount of energy deposited in the bone was found to increase by approximately 91% (1-year old), 66% (5-year old), and 41% (adult). The amount of energy deposited to the leg sample as a whole increased by approximately 43% (1-year old), 32% (5-year old), and 21% (adult). Results are used to estimate the changes in the amount of dose received by subjects of different ages.