NUMERICAL ASSESSMENT OF 131I DEPOSITED IN THYROID FOR NON-STANDARD SITUATIONS

MCNP SIMULATIONS WITH A PERSONALISED VOXEL PHANTOM TO VERIFY 131I CONTENT IN THYROID ESTIMATED BASED ON MEASUREMENTS WITH AN NaI(Tl) SPECTROMETER

One of the authors (O.K.) stayed in the area located ~110 km south from the Fukushima Daiichi Nuclear Power Plant during the arrival of radioactive plumes released into the environment due to the accident in March 2011 in Japan. A previous study determined his 131I thyroid content using an NaI(Tl) spectrometer. The one remaining issue was to investigate the measurement error due to inevitable differences in the configuration (e.g. the thyroid shape and volume) between the physical phantom employed for calibration of the spectrometer and the real subject. In the present study, Monte Carlo simulations for the thyroid measurements were performed using the Monte Carlo N-Particle (MNCP) code to investigate discrepancies in peak efficiencies of the spectrometer between the personalised voxel phantom created from O.K.'s magnetic resonance images and the several typical/reference phantoms that exist. As a result, the peak efficiencies for the Oak Ridge Institute of Nuclear Studies (ORINS) phantom were found to be comparable to those for the reference voxel phantoms reproducing realistic human anatomy (the Adult Male phantom and the Japanese Male phantom). The peak efficiency for the personalised phantom, on the other hand, was fairly close to that of the other physical phantom (the Transfer phantom) actually used for the calibration of the spectrometer, suggesting that the 131I thyroid content determined for him in the previous study was sufficiently accurate. The discrepancies of peak efficiencies at the primal photon energy of 131I (365 keV) among the different phantoms were improved by extending the distance between the neck and the spectrometer; however, the appropriate measurement geometry in an actual situation will depend on the primary purpose of the measurements and should be determined accordingly.

Counting Efficiencies Determined by Monte Carlo Methods for In Vivo Measurement of 131I Activity in Thyroid

The counting efficiencies obtained using a physical neck phantom are typically used in the measurement of I activity in the thyroid. It is well known, however, that the geometrical discrepancies between the physical neck phantom and the anatomy of the subject can significantly influence the counting efficiencies. Thus, it is necessary to consider the anatomical characteristics of individuals if we need to accurately determine the activity of I in the thyroid. This study aims to produce individualized counting efficiencies for thyroid measurement, considering the age, sex, and overlying tissue thickness of the subject being measured by Monte Carlo simulation. Simulations were performed using a series of computational human phantoms of different ages and sexes. The difference in counting efficiencies, depending on the age and sex of the phantom, were found to range from -26 to 3% for the phantoms and monitoring systems considered in the present study. The overlying tissue thickness of the computational phantoms was also modified to find the relationship between the counting ratio of the 80.2 and 364 keV gammas from I and the overlying tissue thickness. The equations for estimating the overlying tissue thickness of a subject were then derived from the relationships between counting ratios and overlying tissue thickness. Finally, in the present study, a set of equations representing the variation in counting efficiencies for the 364 keV peak as a function of the overlying tissue thickness were derived, which can be used to determine individualized counting efficiencies for the subject being measured. These individualized counting efficiencies considering the overlying tissue thickness given a subject's age and sex can provide accurate estimates of I activity for internal dosimetry.

A systematic experimental study of parameters influencing 131-iodine in vivo spectroscopic measurements using age-specific thyroid phantoms

In case of nuclear accident, the internal exposure monitoring of the population will preferably focus on the detection of ¹³¹I in the thyroid by in vivo monitoring. In most cases, the calibration of in vivo monitoring is performed with an adult thyroid phantom, which raises doubts regarding the relevance of child exposure assessment. In this study, the influence on the calibration of the thyroid volume, the counting distance and the positioning variations are studied experimentally in a systematic way. A NaI and a germanium detector along with a realistic age-specific set of four thyroid phantoms were used to carry out this study. The thyroid phantom volumes correspond to the following ages: 5, 10, 15 and adult. It was found that the counting efficiency varies linearly with the thyroid volumes for both detectors and whatever the phantom-detector distance is. The variation in counting distance strongly influences the measurement. Whatever the thyroid volume, a 30% difference in efficiency was found between the measurement at the contact and 1 cm for the NaI detector. A mathematical model giving the variation of the counting efficiency as a function of phantom-detector distance is provided. The study of positioning uncertainty has shown that the lateral/vertical displacements induce negligible efficiency variations and that it is relatively independent of the thyroid volume. The counting distance is a major parameter, which must be considered to assess the uncertainty of the subjects' measurements. The data reported here might serve to extract useful orders of magnitude when similar detectors are used. For other detectors, a similar trend might be expected and the information provided here could reduce the amount of experimental work needed to obtain it.