Environmental Health Perspectives 105, Supplement 6, December 1997

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Considerations in Assigning Dose Based on Uncertainties from in Vivo Counting

Gary H. Kramer

National Calibration Reference Centre for In Vivo Monitoring, Human Monitoring Laboratory, Ottawa, Ontario, Canada

Abstract
Lung counting is highly geometry dependent, especially at low photon energies. Monte Carlo simulations have been used to determine the magnitude of the errors obtained if it is assumed that the deposition is homogeneous, when in fact it is not. Simulation for a germanium lung counting system consisting of four, 70 mm Multiple 30 mm diameter thick detectors have been performed for 70 deposition patterns. The detector efficiencies for 20, 40, 60, 120, 240, 660, and 1000 kiloelectron volts were calculated for a homogeneous deposition and these efficiencies were used to estimate the bias when the deposition was heterogeneous. A bias of 800% was not unusual. Whole-body counting is prone to errors that can arise from the activity distribution and/or size of the subject. The latter are geometry dependent and, for example, a bias result of 200% can be obtained when measuring children in a chair geometry using a calibration factor based on reference man. Thyroid counting is the least prone to error. If the measurement is done correctly, biases can usually be kept below 20%. However, if the measurement is made with a collimator or if the detector is in contact with the subject's neck, biases exceeding 200% can be obtained. -- Environ Health Perspect 105(Suppl 6):1393-1395 (1997)

Key words: thyroid monitoring, whole body counting, lung counting, uncertainty, internal dosimetry, body burden, phantoms, Monte Carlo

This paper is based on a presentation at the International Conference on Radiation and Health held 3-7 November 1996 in Beer Sheva, Israel. Abstracts of these papers were previously published in Public Health Reviews 24(3-4):205-431 (1996). Manuscript received at EHP 11 March 1997; accepted 7 June 1997.

Address correspondence to Dr. G.H. Kramer, National Calibration Reference Centre for In Vivo Monitoring, Human Monitoring Laboratory, Postal Locator 6302D1, Radiation Protection Bureau, 775 Brookfield Rd., Ottawa, Ontario K1A 1C1, Canada. Telephone: 613 954 6668. Fax: 613 957 1089. E-mail: gary_h_kramer@cnet.hc-sc.gc.ca

Abbreviations used: BOMAB, Bottle Mannikin Absorber; Ge, germanium; HML, Human Monitoring Laboratory; keV, kiloelectron volt; obs, observed value; ORNL, Oak Ridge National Laboratory; P4, 4-year-old phantom; PM95, 95-percentile-male phantom; U.S. DOE, U.S. Department of Energy.

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