Choonsik Lee, Ph.D.
Radiation Epidemiology Branch
9609 Medical Center Drive
Rockville, MD 20850
Computational human phantoms
There are two broad approaches to determine organ doses to portions of the human body exposed to ionizing radiation: measurement and calculation. Although measurement can provide dose estimates in actual radiation exposure settings, it is not practical to perform a large number of measurements for all study subjects in epidemiologic studies because the process is usually time-consuming, expensive, and inflexible. Alternatively, organ doses can be calculated using the computer simulation of the human body and radiation exposure scenarios coupled with Monte Carlo radiation transport techniques. A number of studies have validated that the simulation approach adequately represents the physical interaction of radiation in the real human body. Calculation is more appropriate for the dosimetry in epidemiological studies because it is more efficient, less expensive, and more flexible compared to measurements. One of the key components in the calculation approach is a realistic and flexible computational human phantom mathematically representing the anatomy of the human body.
Dr. Lee has developed a series of computational human phantoms which are crucial for realistic individualized dose reconstruction. His pediatric phantom series now serves as the international reference model by the International Commission on Radiological Protection (ICRP). Dr. Lee expanded the computational phantoms with reference body size to include non-reference body sizes utilizing a library of body dimensions of the U.S. population in collaboration with the University of Florida. The resulting library of computational human phantoms is being used to increase accuracy in individualized organ dosimetry for patients undergoing a variety of medical radiation practices.
Dosimetry for patients undergoing diagnostic radiation procedures
The medical use of radiation has increased dramatically over the past few decades. This increase reflects a rapid surge in exposures from all medical sources, which made up 15% of the per capita effective dose in 1982 and 48% in 2006. About half of the per capita effective dose from medical exposures in 2006 was due to CT examinations. The annual number of CT examinations increased from 3.6 million in 1980 to 70 million in 2007. Approximately 10% of all CT examinations in the US are performed in pediatric patients who are at higher risk of radiation-related cancer than adults. In order to make informed decisions, a better understanding of the nature and magnitude of radiation dose and health risk resulting from CT examinations is needed. Although there are several CT dosimetry methods available, most of them are based on simplified computational phantoms and designed only for dose estimation of individual patient not for a large-scale epidemiological cohort.
Dr. Lee has developed the National Cancer Institute dosimetry system for Computed Tomography (NCICT) program, based on a comprehensive organ dose database calculated from Monte Carlo simulation coupled with a series of computational human phantoms with improved anatomical accuracy. Organ dose calculated from NCICT was validated by comparing with the data measured from experiments using physical anthropomorphic phantoms and radiation dosimeters. The software has been utilized for multiple epidemiological studies of CT risk. Dr. Lee is also developing dosimetry methods and tools for other diagnostic radiation procedures including radiography, interventional fluoroscopically-guided procedures, and nuclear medicine examinations.
Dosimetry for patients undergoing therapeutic radiation procedures
Although radiation therapy has been successfully used to treat many types of cancers since the 1920s, there has been an increasing concern about the long-term risk of radiation-induced second cancer among cancer survivors as their life span has increased. The objective of radiation therapy is to provide the highest radiation dose to tumor volume, while delivering the lowest possible dose to normal tissue. However, with external beam radiation therapy, it is practically impossible to irradiate the tumor volume without some radiation penetrating through the overlying normal tissues. Furthermore, organs and tissues adjacent to the tumor site receive scattered radiation from the machine head as well as from the patient body. Numerous epidemiologic studies have demonstrated increased risks of late effects including second cancers in childhood and adult patients who received radiation treatment for cancer. Quantitative estimation of the risk of developing second cancers related to radiotherapy replies on the quality of the dose estimates to normal tissues. However, there are substantial difficulties in reconstructing organ doses for a large-scale patient cohorts treated years ago.
Dr. Lee is developing new dose reconstruction methods by using Monte Carlo simulation of radiotherapy machines coupled with his computational human phantoms, named National Cancer Institute dosimetry system for Radiation Treatment (NCIRT). He is applying the method to multiple epidemiological studies of second cancer in radiotherapy patients in collaboration with the branch epidemiologists as well as extramural clinical medical physicists and oncologists. Dr. Lee is also extending NCIRT to include dosimetry methods for proton therapy patients.
Dr. Lee received his master's in health physics in 1997 and doctorate in health physics in 2002 from Hanyang University in South Korea, where he was trained in computational human phantoms and advanced dosimetry applications. He subsequently joined the Innovative Technology Center for Radiation Safety in South Korea as a postdoctoral fellow. While in the post-doctoral period, he was actively involved in national research projects investigating computational dosimetry for the Korean population. He next completed extensive training in computational medical physics as a postdoctoral researcher at the University of Florida. Dr. Lee was appointed as a tenure-track investigator in the Radiation Epidemiology Branch (REB) in 2009, and was awarded scientific tenure and promoted to senior investigator in September 2016. In October 2016, he was appointed as Head of the Dosimetry Unit of REB, which provides dose estimation in support of Branch epidemiologic studies and develops exposure assessment methods to improve the science of dosimetry.
Dr. Lee has made considerable contributions to the development of computational human phantoms and to the improvement of the dosimetry calculations for a variety of radiation exposure scenarios covering medical, occupational, and environmental exposures. He is a full member of the International Commission on Radiological Protection (ICRP). He is also a member of editorial board of Radiation Protection Dosimetry. He regularly reviews manuscripts for numerous journals including Radiology, Medical Physics, Health Physics, Radiation Protection Dosimetry, and Physics in Medicine and Biology, and is actively involved in the American Association for Physicists in Medicine and Health Physics Society.
Lee C, Kim KP, Long DJ, Bolch WE. Organ doses for reference pediatric and adolescent patients undergoing computed tomography estimated by Monte Carlo simulation. Med Phys. 2012;39(4):2129-46.
Kim KP, Berrington de González A, Pearce MS, Salotti JA, Parker L, McHugh K, Craft AW, Lee C. Development of a database of organ doses for paediatric and young adult CT scans in the United Kingdom. Radiat Prot Dosimetry. 2012;150(4):415-26.
Lamart S, Imran R, Simon SL, Doi K, Morton LM, Curtis RE, Lee C, Drozdovitch V, Maass-Moreno R, Chen CC, Whatley M, Miller DL, Pacak K, Lee C. Prediction of the location and size of the stomach using patient characteristics for retrospective radiation dose estimation following radiotherapy. Phys Med Biol. 2013;58(24):8739-53.
Lee C, Lamart S, Moroz BE. Computational lymphatic node models in pediatric and adult hybrid phantoms for radiation dosimetry. Phys Med Biol. 2013;58(5):N59-82.
Lamart S, Bouville A, Simon SL, Eckerman KF, Melo D, Lee C. Comparison of internal dosimetry factors for three classes of adult computational phantoms with emphasis on I-131 in the thyroid. Phys Med Biol. 2011;56(22):7317-35.
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This page was last updated on November 10th, 2016