Characterization of a gated fiber-optic-coupled detector for application in clinical electron beam dosimetry.

PURPOSE: Assessment of the fundamental dosimetric characteristics of a novel gated fiber-optic-coupled dosimetry system for clinical electron beam irradiation. METHODS: The response of fiber-optic-coupled dosimetry system to clinical electron beam, with nominal energy range of 6-20 MeV, was evaluated for reproducibility, linearity, and output dependence on dose rate, dose per pulse, energy, and field size. The validity of the detector system's response was assessed in correspondence with a reference ionization chamber. RESULTS: The fiber-optic-coupled dosimetry system showed little dependence to dose rate variations (coefficient of variation +/- 0.37%) and dose per pulse changes (with 0.54% of reference chamber measurements). The reproducibility of the system was +/- 0.55% for dose fractions of approximately 100 cGy. Energy dependence was within +/- 1.67% relative to the reference ionization chamber for the 6-20 MeV nominal electron beam energy range. The system exhibited excellent linear response (R2 = 1.000) compared to reference ionization chamber in the dose range of 1-1000 cGy. The output factors were within +/- 0.54% of the corresponding reference ionization chamber measurements. CONCLUSIONS: The dosimetric properties of the gated fiber-optic-coupled dosimetry system compare favorably to the corresponding reference ionization chamber measurements and show considerable potential for applications in clinical electron beam radiotherapy.

Modification to the Monte Carlo N-particle code for simulating direct, in vivo measurement of stable lead in bone.

Monte Carlo N-Particle version 4C (MCNP4C) was used to simulate photon interactions associated with in vivo x-ray fluorescence (XRF) measurement of stable lead in bone. Experimental measurements, performed using a cylindrical anthropometric phantom (i.e., surrogate) of the human leg made from tissue substitutes for muscle and bone, revealed a significant difference between the intensity of the observed and predicted coherent backscatter peak. The observed difference was due to the failure of MCNP4C to simulate photon scatter associated with greater than six inverse angstroms of momentum transfer. The MCNP4C source code, photon directory, and photon library were modified to incorporate atomic form factors up to 7.1 inverse angstroms for the high Z elements defined in the K XRF simulation. The intensity of the predicted coherent photon backscatter peak at 88 keV using the modified code increased from 3.50 x 10(-9) to 8.59 x 10(-7) (roughly two orders of magnitude) and compares favorably with the experimental measurements.

The influence of bone-density on in vivo K x-ray fluorescence bone-lead measurements.

Mathematical simulations and benchmark measurements were performed to assess the impact that normal variations in human calcium content have on in vivo K x-ray fluorescence measurements of lead in bone. Four sets of cortical bone tissue simulants were fabricated containing from 20.8% to 23.8% calcium (by weight) for measurement in a surrogate (phantom) of the human leg. The net counts detected in the coherent backscatter peak at 88.034 keV using a Cd source indicate a positive trend, with a variability of up to 17% over the range of assessed calcium content. Mathematical simulations confirm this trend and also demonstrate that the contribution of 87.3 keV Pb Kß2 counts, which are unresolved in measurements, do not contribute significantly to the coherent peak at low levels of bone-lead content. Both measurements and simulations confirm that calcium is a statistically significant parameter in predicting the K-XRF response and suggest that lead levels may be over-predicted for individuals having low bone density compared to the calibration matrix. Simulations identify a 4.5% negative bias in measured lead values for each 1% increase in calcium weight percent in the bone matrix as compared to the calibration matrix. It is therefore important to accommodate this uncertainty when performing epidemiological studies of populations having a wide range of bone densities.

Monte Carlo simulation of an anthropometric phantom used for calibrating in vivo K-XRF spectroscopy measurements of stable lead in bone.

An anthropometric surrogate (phantom) of the human 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 measurements of stable lead in bone. The predicted response compared favorably with measurements using the anthropometric phantom containing a tibia with increasing stable lead content. These benchmark measurements confirmed the validity 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 cylindrical calibration standard overestimates lead content of a human leg up to 4%. A two-way analysis of variance determined 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.