Monte Carlo 시뮬레이션과 3D Printing Phantom을 이용한 99mTc, 18F 선원 핵의학 검사 시 내부피폭 선량평가

Abstract

Nuclear medicine is a diagnostic field of anatomical and physiological state on the human body by using radioisotope. As the number of nuclear medicine imaging have been increased, the medical exposure involved inevitably. The radiation used is difficult to defend against enters the human body and provides many harmful effects. In particular, radiation detection equipment is difficult to accurately measure the internal exposure dose. In addition, accurate measurement of internal exposure is difficult because the characteristics vary depending on the nuclide and radioactive source. Although the reduction of radiation dose was studied in the past studies, the estimation of radiation in radiation workers and caregivers was not studied. In this study, the dose distribution of the human body was evaluated through dosimetry data using a water phantom, ionization chamber, photoluminescence glass dosimeter and Monte carlo simulation for 99mTc and 18F sources, which are frequently used in nuclear medicine. To evaluate whether the dose limit was exceeded, the lens absorbed dose was calculated using a brain phantom made by a 3d printer. Absorbed dose rate(mGy/h) was respectively measured in a section of 3 ∼ 13 cm from the radioisotope(99mTc, 18F) of 10 mCi. The absorbed dose rate of 99mTc was shown in the range of 2.80 ∼ 0.35 mGy/h in the ionization chamber, 3.17 ∼ 0.36 mGy/h in the photoluminescence glass dosimeter and 3.18 ∼ 0.31 mGy/h in Monte carlo simulation. Under 18F radiation, the absorbed dose rates were gauged in the range of 24.4 ∼ 11.30 mGy/h in the ionization chamber, 25.79 ∼1.65 mGy/h in the photoluminescence glass dosimeter and 26.08 ∼ 1.42 mGy/h in Monte carlo simulation, respectively. From these results, it was confirmed that the higher radiation dose, the higher absorbed dose rate proportionally. Utilization of 18F source, which has relatively high energy, showed higher absorbed dose rate than that of 99mTc source. Also, the absorbed dose rate decreased exponentially with increase of the distance from the radioisotope. Especially, these results showed a tendency to decrease rapidly when the distance from the radioisotope increased over 5 cm. These results indicate that a large amount of dose is delivered to an organ located within 4 cm of source’s movement path when a source is an uptake in the human body. In the 3d printing brain phantom study, the average absorbed dose rates for the left and right lenses based on 10 mCi of 99mTc and 18F were 3.37 and 28.62 mGy/h, respectively. The calculated values using the Monte carlo simulation were 3.72 and 28.86 mGy/h, respectively. In comparison with actual measurement and simulation results, 99mTc showed a difference of about 10％ and 18F showed a difference of less than 1％. These differences originated from differences in measurement environment and simulation conditions such as the structure of brain phantom, radiation volume and specific radioactivity in this study. Finally, the lens absorbed dose rate measured in the brain phantom is converted to a value that the effective half-life has elapsed 10 times. The absorbed dose was calculated by referring to the brain distribution fraction of biokinetic data for makers of HM-PAO and FDG in ICRP 80 and 53. As a result, the exposure dose to the lens was 1.16 mGy in the brain perfusion SPECT using 99mTc-HM-PAO 10 mCi, and the exposure dose to the lens in the PET torso using 18F-FDG 10 mCi was 3.46 mGy. These results satisfy the annual cap on the equivalent dose to the lens of the Korean Nuclear Safety Act(150 mSv). Also, these results fulfill the condition of the annual dose limit for the public in the recently revised ICRP 118(15 mSv). The importance of the dose limit for the lens become gradually emerging, and data on the radiation sensitivity of the lens are being derived. Therefore, various types of research have been conducted to reduce lens exposure in the diagnostic radiation field. To reduce lens exposure during brain CT, using shielding material made of bismuth and tungsten filament or scanning baseline change is suggested. In nuclear medicine, this study is an important data to determine whether the amount of radioactivity is appropriate by complying with the dose standards of radiopharmaceuticals according to the Diagnostic Reference Levels (DRLs) of nuclear medicine. This study presented reliable results by comparing the actual measurement and Monte carlo simulation results to quantitatively evaluate the internal exposure dose by radioisotope. In conclusion, this study is considered to be helpful as basic data for setting the diagnostic reference level for nuclear medicine.