갑상선 등가선량 평가의 불확도 감축전략 연구

Abstract

갑상선 등가선량의 평가는 체르노빌 사고 이후 방사성 요오드섭취로 인한 갑상선 암 발생율의 급격한 증가가 보고됨에 따라 의학적 후속조치와 역학 연구를 위해 평가의 중요성이 대두되고 있다. 등가선량은 직접 장기 내 선량을 측정할 수 없기 때문에 실측이 가능한 측정값과 핵종의 생리적 거동을 묘사하는 모델을 결합하여 평가된다. 그러나 이러한 평가 절차는 측정값의 불확실성과 개인의 가변성(variability), 부족한 정보 등에 의해 본질적으로 큰 불확도를 수반하게 된다. 따라서 특정 개인의 실제 선량이 선량기준을 초과할 가능성과 잠재적인 위험도를 평가하기 위해 불확도를 정량화하기 위한 노력이 필요하며 특히 사고 상황에서는 정량화와 함께 불확도를 감축하기 위한 방법이 모색되어야 한다. 따라서 본 논문에서는 사고 상황에서 원전 종사자들의 방사성 요오드 섭취를 가정하여 갑상선 등가선량 평가의 불확도를 정량화하고 IDEAS 절차서의 선량에 따른 차등적 접근을 적용하여 상대적 기여도가 높은 인자들에 대한 불확도 저감방법을 모색하고 제안하였다. 선량평가 절차에 존재하는 불확도를 정량화하기 위해 측정불확도 인자와 생리역동학 모델의 전이율, 갑상선 질량 등을 불확도 인자로 설정하였고 각 인자의 확률분포로부터 몬테칼로 샘플링 기법을 이용하여 갑상선 등가선량의 불확도를 합성하였다. 합성한 결과는 95% 신뢰구간에서 2.5th와 97.5th이 약 12배의 차이를 보였으며 실제 선량은 중앙값으로부터 3.5배의 범위 내에 존재할 것으로 판단되었다. 이는 기본값을 사용한 평가가 정해진 선량기준을 만족하더라도 실제 선량은 결정적 영향이 발현되는 문턱치를 초과할 가능성이 있음을 시사하였다. 또한 평가된 선량의 신뢰성을 확보하고 특정개인에 대한 선량의 편차를 줄이기 위한 불확도 감축 방안이 마련되어야 함이 입증되었다. 불확도 저감화의 차등적 접근을 적용하기 위해 각 인자들의 변동에 따른 선량평가 결과의 변화를 평가하는 민감도 분석을 수행되었다. 그 결과 갑상선 질량, 측정 불확도, 혈액과 갑상선간의 전이율 순으로 상대적 기여도가 높은 것으로 확인되었으며 이러한 인자들은 전체 불확도에 지배적인 영향을 미칠 것으로 판단되는 섭취시점과 함께 불확도 감축을 위한 인자로 설정되었다. 감축의 대상이 되는 인자들은 불확도의 발생 원인에 따라 적합한 방법들이 고려되었으며 가용한 정보로부터 산출된 회귀식을 이용하는 방법과 효율보정인자를 적용하는 방법이 도출되었다. 또한 전신계수기를 이용한 측정이 빈번한 국내 실정을 반영하여 체내 방사능 분포를 고려한 효율보정과 상·하부 검출기의 계수율 비를 활용하여 섭취시점을 예측하는 방법도 함께 제안하였다. 본 연구는 OIR권고안의 최신 생리 역동학 모델에 대한 불확도를 반영하지 못한 점과 전이율, 갑상선 측정효율 불확도 감축에 대한 정성적인 평가만을 수행하였다는 한계가 있다. 그러나 이전의 연구들과는 달리 측정, 섭취량 추정, 선량 평가 절차에 내포되어 있는 불확도를 종합적으로 정량화한 점과 측정 장비의 특성을 반영한 전략을 제시하였다는 점에서 의의가 있다. 결론적으로 본 연구에서 제시한 방안들은 사고 상황에서 큰 불확실성을 내포하는 인자들을 제거하고 효율적인 접근 방식을 제공함으로써 갑상선 등가선량의 불확도를 감축할 수 있을 것으로 기대된다.|Assessment of the thyroid equivalent dose is emerged as a major issue whether radio-iodine intake increased the risk of thyroid cancer since the chernobyl accident in Ukraine. Thyroid dose are deduced using biokinetic and dosimetry models combined with available measurements of activity in bioassay such as thyroid monitoring, whole body counter instead of direct measurement. But each component that enters the process of dose determination is a source of uncertainty in the estimated dose because of variability of individuals, lack of knowledge, uncertainties in measurement techniques. The quantification of uncertainty on assessed doses is necessary when dose are high, since the upper bound on the dose is likely to exceed the dose limit(or a relevant action level). In the case of an accident, the strategy of uncertainty reduction ought to be conducted with quantification. To reduce uncertainties of components, IDEAS guideline suggested approach based on proportionality that the effort applied to the evaluation should be proportionate to the dose . In this study, therefore, both quantification of uncertainty and reduction of uncertainty followed by IDEAS differentiative approach on thyroid dose assessment were carried out. Thyroid equivalent dose based on scenario that worker inhaled I-131 aerosol 50% and vapour/gases 50% (elemental iodide) released from the accident is evaluated. To quantify the uncertainty, we conducted uncertainty parameter analysis assigning probability distribution and then a probabilistic assessment of thyroid dose was generated by performing Monte Carlo method. As a result, median of distribution is 24 % higher than calculated dose using nominal value, Uncertainty factor(UF) that represent range of distribution is a factor of 2.4 with 95% confidence. It implied that the True Thyroid dose but unknown value have the manifestation possibility of deterministic effect although the nominal result satisfied the dose limit. Sensitivity analysis to select parameters that contribution is relatively high on uncertainty of thyroid dose indicated that the most important parameters are thyroid mass and measurement, rate constant in turn. In addition, Intake time was considered independently due to influence of this parameter appeared the maximum difference by a factor of 1.36. We properly took into account the strategies of uncertainty reduction according to the cause of uncertainty occurrence. The reduction methods using regression equation and applying the efficiency correction factor were drawn. Also, By reflecting domestic actual condition that the whole body counter is almost used at Nuclear Power Plant, we proposed the method that correct the efficiency about the distribution in the body and predict the intake time utilizing the ratio of count rate in upper to lower detector. After applying this strategy, the uncertainty factor is reduced by 55 percent(%). we confirmed that the use of this method is effective. But it is acknowledged that there are some limitation in the analysis, the uncertainty reduction of whole body counter is evaluated quantitatively. Furthermore, the assignment of a distribution to a parameter is assumed based on experience in the literature, sometimes this is subjective when the choice of distribution is difficult. Nevertheless, this study has significance in that Comprehensive quantification of uncertainties is conducted and uncertainty reduction is drawn from suggested method effectively. The strategy looks forward to be useful to reduce uncertainties including measurement, intake prediction, dose assessment and to assess the reliability of thyroid equivalent dose.; Assessment of the thyroid equivalent dose is emerged as a major issue whether radio-iodine intake increased the risk of thyroid cancer since the chernobyl accident in Ukraine. Thyroid dose are deduced using biokinetic and dosimetry models combined with available measurements of activity in bioassay such as thyroid monitoring, whole body counter instead of direct measurement. But each component that enters the process of dose determination is a source of uncertainty in the estimated dose because of variability of individuals, lack of knowledge, uncertainties in measurement techniques. The quantification of uncertainty on assessed doses is necessary when dose are high, since the upper bound on the dose is likely to exceed the dose limit(or a relevant action level). In the case of an accident, the strategy of uncertainty reduction ought to be conducted with quantification. To reduce uncertainties of components, IDEAS guideline suggested approach based on proportionality that the effort applied to the evaluation should be proportionate to the dose . In this study, therefore, both quantification of uncertainty and reduction of uncertainty followed by IDEAS differentiative approach on thyroid dose assessment were carried out. Thyroid equivalent dose based on scenario that worker inhaled I-131 aerosol 50% and vapour/gases 50% (elemental iodide) released from the accident is evaluated. To quantify the uncertainty, we conducted uncertainty parameter analysis assigning probability distribution and then a probabilistic assessment of thyroid dose was generated by performing Monte Carlo method. As a result, median of distribution is 24 % higher than calculated dose using nominal value, Uncertainty factor(UF) that represent range of distribution is a factor of 2.4 with 95% confidence. It implied that the True Thyroid dose but unknown value have the manifestation possibility of deterministic effect although the nominal result satisfied the dose limit. Sensitivity analysis to select parameters that contribution is relatively high on uncertainty of thyroid dose indicated that the most important parameters are thyroid mass and measurement, rate constant in turn. In addition, Intake time was considered independently due to influence of this parameter appeared the maximum difference by a factor of 1.36. We properly took into account the strategies of uncertainty reduction according to the cause of uncertainty occurrence. The reduction methods using regression equation and applying the efficiency correction factor were drawn. Also, By reflecting domestic actual condition that the whole body counter is almost used at Nuclear Power Plant, we proposed the method that correct the efficiency about the distribution in the body and predict the intake time utilizing the ratio of count rate in upper to lower detector. After applying this strategy, the uncertainty factor is reduced by 55 percent(%). we confirmed that the use of this method is effective. But it is acknowledged that there are some limitation in the analysis, the uncertainty reduction of whole body counter is evaluated quantitatively. Furthermore, the assignment of a distribution to a parameter is assumed based on experience in the literature, sometimes this is subjective when the choice of distribution is difficult. Nevertheless, this study has significance in that Comprehensive quantification of uncertainties is conducted and uncertainty reduction is drawn from suggested method effectively. The strategy looks forward to be useful to reduce uncertainties including measurement, intake prediction, dose assessment and to assess the reliability of thyroid equivalent dose.