단반감기 핵종에 의한 내부피폭선량 평가방법론 개발

Abstract

핵의학 절차에서는 진단이 완료된 후 지속되는 환자의 피폭을 피하기 위해 종종 99mTc나 18F와 같은 단반감기 핵종을 이용한다. 짧은 반감기 때문에 이러한 핵종은 사용되는 의료기관에서 발생기에서 착유하거나 사이클로트론을 이용해 생산하는 것이 보통이다. 따라서 이러한 방사성의약품을 생산하거나 취급하는 종사자는 늘 내부피폭 잠재성에 직면해 왔지만 급속한 소멸로 인해 효과적인 감시방법이 없어 이들의 내부피폭을 간과해 왔다. 단반감기 핵종에 의한 내부피폭의 감시를 돕기 위해 이 연구에서는 섭취 후 최초 소변을 분석하는 새로운 방법을 18FDG에 초점을 맞춰 개발했다. 기존의 소변 배설함수는 일일 배설량을 기준으로 구성되어 있기 때문에 이 목적에 적합한 최초 소변 배설함수를 흡입한 핵종의 생리역동학 모델로부터 도출했다. 이 계산에는 ICRP 30의 소화관 모델, ICRP 66의 호흡기 모델 및 ICRP 106에 주어진 18FDG의 생리역동학 모델을 적용했다. 새 방법의 실용성을 내보이기 위해 서로 다른 두 사이클로트론 시설에서 일하는 종사자에 대해 작업 후 최소 소변을 채취하여 감마분광분석을 통해 방사능을 계측하고 그 결과를 해석하여 예탁유효선량을 평가하였다. 비록 새 방법이 소변분석을 서둘러야 하고 시범적으로 평가된 선량이 사소한 수준으로 나타났지만 이 방법을 통해 단반감기 핵종에 의한 개인 내부피폭을 감시할 수 있음이 중요하다. 나아가 소변 생물검정의 보완으로서 공기샘플링법이 대안으로 적용 가능한지를 평가하였다. 위와 동일한 작업장에서 개인공기샘플러와 고정공기샘플러를 사용하여 종사자가 흡입한 18FDG 방사능을 측정하였다. 유리섬유 필터는 18FDG 포집에 비효율적으로 나타나 활성탄필터를 포집기로 사용했다. 그 결과 두 공기샘플러로 평가한 섭취량과 소변분석으로부터 평가된 섭취량이 좋은 일치를 보였다. 이러한 일치는 공기샘플링법이 생물검정의 대안으로 사용될 수 있음을 보이는 것은 물론 이 연구에서 개발한 새 방법이 단수명핵종 내부피폭 감시에 적절함을 지원한다. 따라서 해당 배설함수를 사용한다면 반감기가 수 시간인 다른 방사성핵종, 특히 99mTc에 의한 내부피폭 감시에도 새 방법이 유용할 것이다.| In order to avoid persisting exposure of patients after termination of a diagnostic nuclear medicine procedure, short-lived radionuclides like 99mTc, 18F are often employed. Because of their short half-lives, such radionuclides are usually produced in the medical institute where they are applied, by either milking from a generator or generating with a cyclotron. Workers in the nuclear medicine department have been faced the potential of internal exposure in the course of producing or handling such radiophamarceuticals but their internal exposure has been overlooked because no effective monitoring procedures are available due to their quick disappearance. To facilitate monitoring of internal exposures to short-lived radionuclides, specifically focused on 18FDG in this study, a new methodology was developed, which employs an indirect bioassay of the first urine sample after an intake. The urinary excretion functions specific to this purpose were prepared by using the biokinetic model of inhaled nuclides because the existing excretion functions are given in terms of daily excretion. The gastro-intestinal model of ICRP 30, the respiratory model of ICRP 66, and the 18FDG biokinetic model given in ICRP 106 were used for this calculation. To demonstrate utility of the new methodology, the first urine samples of workers producing 18FDG in two different cyclotron facilities were analyzed by gamma-spectroscopy and the results were interpreted to obtain committed effective doses. Although the new procedure requires hurry measurement of the urine samples and the resulting doses were not significant in these trial assay cases, it showed that the methodology is applicable to individual monitoring of internal exposure to short-lived radionuclides. As a complement to the urine bioassay, the air sampling approach was testified to determine if it can be applied to the monitoring program as an alternative. Two types of air samplers, a personal air sampler and a fixed air sampler, were used to monitor inhaled activity of 18FDG at the same workplaces. Since glassfiber filters showed unsatisfactory collection efficiencies for 18FDG, charcoal filters are used in the air sampling. The resulting estimates of intake activities from both of the air samplers showed good agreement with the intake estimated from the first urine assay. This agreement supports not only that the air sampling approach can be applied as an alternative but also that the new bioassay methodology is a valid approach to internal monitoring of short-lived nuclides. With a pertinent excretion function, the new methodology is applicable to a nuclide having half-life of a few hours, particularly 99mTc.; In order to avoid persisting exposure of patients after termination of a diagnostic nuclear medicine procedure, short-lived radionuclides like 99mTc, 18F are often employed. Because of their short half-lives, such radionuclides are usually produced in the medical institute where they are applied, by either milking from a generator or generating with a cyclotron. Workers in the nuclear medicine department have been faced the potential of internal exposure in the course of producing or handling such radiophamarceuticals but their internal exposure has been overlooked because no effective monitoring procedures are available due to their quick disappearance. To facilitate monitoring of internal exposures to short-lived radionuclides, specifically focused on 18FDG in this study, a new methodology was developed, which employs an indirect bioassay of the first urine sample after an intake. The urinary excretion functions specific to this purpose were prepared by using the biokinetic model of inhaled nuclides because the existing excretion functions are given in terms of daily excretion. The gastro-intestinal model of ICRP 30, the respiratory model of ICRP 66, and the 18FDG biokinetic model given in ICRP 106 were used for this calculation. To demonstrate utility of the new methodology, the first urine samples of workers producing 18FDG in two different cyclotron facilities were analyzed by gamma-spectroscopy and the results were interpreted to obtain committed effective doses. Although the new procedure requires hurry measurement of the urine samples and the resulting doses were not significant in these trial assay cases, it showed that the methodology is applicable to individual monitoring of internal exposure to short-lived radionuclides. As a complement to the urine bioassay, the air sampling approach was testified to determine if it can be applied to the monitoring program as an alternative. Two types of air samplers, a personal air sampler and a fixed air sampler, were used to monitor inhaled activity of 18FDG at the same workplaces. Since glassfiber filters showed unsatisfactory collection efficiencies for 18FDG, charcoal filters are used in the air sampling. The resulting estimates of intake activities from both of the air samplers showed good agreement with the intake estimated from the first urine assay. This agreement supports not only that the air sampling approach can be applied as an alternative but also that the new bioassay methodology is a valid approach to internal monitoring of short-lived nuclides. With a pertinent excretion function, the new methodology is applicable to a nuclide having half-life of a few hours, particularly 99mTc.