http://dx.doi.org/10.7742/jksr.2015.9.7.459 Simulation of Energy Absorption Distribution using of Lead Shielding in the PET/CT Dong-Gun Jang *, Changsoo Kim **, Junghoon Kim ** Department of Nuclear Medicine, Dongnam Institute of Radiological & Medical Sciences Cancer center *, Department of Radiological Science, College of Health Sciences, Catholic University of Pusan, Korea ** PET/CT 검사에서납차폐체사용에따른에너지흡수분포에 관한모의실험 *, **, ** * ** Abstract Energy absorption distribution according to lead shielding for 511 kev ray was evaluated using a Monte Carlo simulation in PET/CT. Experimental method was performed about the depth of skin surface(0.07), lens(3) and the depth (10) was conducted by using ICRU Slab phantom. Difference of energy absorption distribution according to lead thickness and effect of air gap according to distance of lead and phantom. As a result, study showed that using a lead shielding makes high energy distribution by backscatter electron. As a distance between lead and phantom increased, energy absorption distribution gradually decreased. 9 cm or more air gap should exist to prevent effect of backscatter electron which reaches skin surface, when 0.25 mmpb shielding is used. Also 1 cm or more air gap was needed to prevent the effect in 0.5 mmpb. If air gap was not concerned, 0.75 mm or more lead thickness was necessary to prevent effect of backscatter electron. Keyword : backscatter electron, Shielded 요약 PET/CT 에서사용되는 511 kev 선의납차폐체사용유ㆍ무에따른에너지흡수분포를몬테카를로모의모사를통해평가하였다. 실험은 ICRU Slab 팬텀을이용하여깊이에따라피부표면 (0.07), 수정체 (3), 심부 (10) 에대해실험을진행하였으며, 납두께에따른에너지흡수분포차이와납과팬텀의거리에따른공기층의영향에대해분석하였다. 그결과납차폐체사용시산란전자선에의해피부표면에에너지흡수분포가높게나타났다. 산란전자선선은납과팬텀사이의거리가증가함에따라점차제거되었으며, 0.25 mm 납차폐체사용시 9 cm 이상의공기층이있어야피부표면의도달하는산란전자선의영향을방지할수있었다. 또한 0.5 mm 의납차폐체사용시 1 cm 이상의공기층이있어야피부표면에도달하는산란전자선의영향을방지할수있었으며, 공기층을고려하지않을경우 0.75 mm 이상의납두께를사용하여야피부표면의산란전자선의영향을방지할수있다. 중심단어 : 산란전자선, 차폐체 459
"Simulation of Energy Absorption Distribution using of Lead Shielding in the PET/CT" Ⅰ. INTRODUCTION PET/CT (Positron Emission Tomography/Computed T omography) 는개봉된방사성핵종을신체의해부학적또는생리학적상태를진단, 평가하는검사이다. 방사성핵종을환자에게투여하면환자는하나의방사선원으로방사선을방출하게되며, 이러한특징이방사선종사자및환자주변인에대한높은방사선피폭위험성을가지게된다 [1],[2]. 이에국제방사선방어위원회 (ICRP) 에서는방사선노출을합리적으로달성가능한낮게유지할수있도록최적의방사선방호계획을수립하고이행하도록권고하고있다 [3]. 국내또한국가표준인증종합센터에서 진료용 X선방호앞치마 KS P 6023:2007" 규정 [4] 에따라전자파방사선인 X선을의료용으로사용할경우최소 0.25 mm 이상의납당량을가진납치마를사용하도록규정하고있다. 진단용 X 선의평균광 (Photon) 에너지는약 50 kev 이며 [5],[6], P ET/CT에서사용되고있는방사선인 선은 511 kev의광에너지를사용하고있다 [7],[8]. 이처럼진단용 X선에비해서 PET/CT 에사용되는 선의에너지가매우높으며, 이로인해발생되는방사선피폭또한매우높음을알수있다 [9],[10]. 하지만 선차폐에대한연구가매우부족한실정이며, 연구자에따라 511 kev의 선에대한납치마의차폐효율이있다는의견과차폐효과가없고오히려피폭을증가시키는역효과가발생된다는서로상반된주장들이제기되고있다 [5],[11],[12]. 이에본연구는몬테카를로모의모사를이용하여 PE T/CT 검사시납차폐체에사용유ㆍ무에따른에너지흡수분포를평가하고자한다. Ⅱ. MATERIAL AND METHOD 1. 몬테카를로모의모사 ver.2.5.0, USA) 를사용하였다. 2. ICRU Slab 팬텀 인체의흡수선량을위해본연구에서는국제방사선 단위위원회 (ICRU) 의 ICRU Slab 팬텀을사용하였다. IC RU 47 보고서 [14] 에따르면 Slab 팬텀은개인선량당량 Hp(d) 는바깥표면으로부터각각피부표면 (0.07 mm), 수정체 (3 mm), 심부 (10 mm) 에대해정의하고있다. 3. 실험방법 3.1 납차폐체두께에따른에너지분포측정 실험은 Fig. 1 과같이납차폐체뒤에 ICRU Slab 팬 텀을위치시켜 Hp(0.07), Hp(3), Hp(10) 의단위질량당 흡수된에너지 (MeV/g) 를 MCNPX 의 Tally 6 을사용하 여획득하였으며, 계산은통계적인오차를 2% 이하로 줄이기위해 1 10 5 번모의추정하였다. 단위질량당 흡수에너지는광자 (Photon) 와전자 (Electron) 로구분하 여측정하였으며, 납두께는 0.05 mm ~ 1.0 mm 까지 0. 05 mm 단위로모사하였다. 납차폐체와 ICRU Slab 팬 텀의구성성분은 Table 1 과같은조직등가물질을사용 하였다. (a) Fig. 1. Lead thickness experimental using ICRU Slab phantom : a: Experimental models using MCNPX Visual Editor, b: Sectional view of the ICRP Slab phantom. (b) 몬테카를로모의모사는통계적난수 (random numbe r) 를사용한무작위적인표본추출을이용하여해결하는방법으로, 물질들의다양한 3차원구조에서입자의거동을모사할수있다 [13]. 본연구에서는로스알라모스국립연구소에서개발된 Monte Carlo N-Particle Tra nsport Code (MCNPX, Los Alamos National Laboratory, 460
Table 1. Components of the experimental mm 에서차폐체가없는경우와유사한에너지흡수분포를나타냈다 [Fig. 5]. 3.2 납차폐체와팬텀의거리에따른공기층의산란전자선제거율납차폐체의두께를납치마에서주로사용되는두께 0.25 mm, 0.5 mm 로고정한후납과 ICRU Slab 팬텀과의거리를 20 cm까지 1cm 단위로조정하여납차폐체와팬텀사이의공기층에대한영향을차폐율을통해알아보고자하였다 [Fig. 2]. Fig. 3. Photon energy absorption of the Hp(0.07). Fig. 4. Electron energy absorption of the Hp(0.07). Fig. 2. Air gap thickness experimental using ICRU Slab phantom. : a: Experimental models using MCNPX Visual Editor, b: Sectional view of the ICRP Slab phantom. Ⅲ. RESULT 1. 납차폐체두께에따른에너지분포측정 본연구에서는 PET/CT 검사에서사용되는 511 kev 의 선에대해납차폐체사용에따른에너지흡수분포를분석하였다. 첫번째실험에서는납두께에따른 ICRU Slab 팬텀의 Hp(0.07), Hp(3), Hp(10) 의부위별에너지분포를측정하였다. Hp(0.07) 에너지흡수분포에서광자는납두께가증가함에따라점점감소하는그래프가나타났으며, 전자는차폐를실시하였을때오히려에너지흡수분포가높게나타났다 [Fig. 3],[Fig. 4]. 광자와전자의에너지흡수분포를합한결과약 0.7 Fig. 5. Total energy absorption of the Hp(0.07). Hp(3) 의에너지흡수분포에서광자는두께에따라감소하였고, 전자는차폐체를사용하지않았을때보다에너지흡수분포가높게나타났으며, 납두께약 0. 8 mm 에서차폐체가없는경우와에너지흡수분포가유사하게나타났다. 광자와전자의에너지흡수분포를합한결과약 0.45 mm 부터차폐효과가발생됨을알수있었다 [Fig. 6],[Fig. 7],[Fig. 8]. 461
"Simulation of Energy Absorption Distribution using of Lead Shielding in the PET/CT" Fig. 6. Photon energy absorption of the Hp(3). Fig. 9. Photon energy absorption of the Hp(10). Fig. 7. Electron energy absorption of the Hp(3). Fig. 10. Electron energy absorption of the Hp(10). Fig. 8. Total energy absorption of the Hp(3). Fig. 11. Total energy absorption of the Hp(10). Hp(10) 의에너지흡수분포에서광자는납두께에따라감소하였으며, 전자는약 0.55 mm 전후로차폐효과의유무가나타났다. 광자와전자의에너지흡수분포를합한결과약 0.3 mm 전후로차폐효과가나타났다 [Fig. 9],[Fig. 10],[Fig. 11]. 462 2. 납차폐체와팬텀의거리에따른공기층의산란전자선제거율두번째실험에서납차폐체 (0.25 mm, 0.5 mm) 사용하여 ICRU Slab 팬텀과의거리에따라공기층의산란전자선제거의영향을차폐율을통해비교하였다. 0.2 5 mm 의납차폐체를사용하였을때 Hp(0.07) 에서공기층약 9 cm 이상일때차폐효과가있는것으로나타났으며, Hp(3) 은약 3 cm 이상부터차폐효과가있는것으로나타났다. 그리고 Hp(10) 은경우약 0.5 cm 이
상부터차폐효과가있는것으로나타났다 [Fig. 12]. 0.5 mm 의납차폐체사용하였을때 Hp(0.07) 에서공 기층약 1 cm 이상일때차폐효과가있는것으로나타 났으며, 나머지부위는공기층과관계없이차폐효과가 있는것으로나타났다 [Fig. 13]. Fig. 12. Shielding rate by the thickness of the air gap with a 0.25 mmpb. Fig. 13. Shielding rate by the thickness of the air gap with a 0.5 mmpb. Ⅳ. DISCUSSION 본연구에서는 PET/CT 검사에서납차폐체사용시 발생되는산란전자선의영향과이러한산란전자선을 제거하기위한공기층의두께를알아보고자실험을진행하였다. Myeong-Hwan Park [5], Seoung-Wook Lee [11] 에따르면 511 kev의 선을 0.25 mm, 0.5 mm 의납치마로차폐 463
"Simulation of Energy Absorption Distribution using of Lead Shielding in the PET/CT" 하였을때차폐효과가있는것으로보고하였고, Soo- Kyung Na [12] 주장에따르면 511 kev의 선을 0.25 mm, 0.5 mm 납치마로차폐하였을때오히려피폭이증가된다는연구가제기되고있다. 이처럼서로상반된주장들이제기되고있어본논문에서는몬테카를로모의모사를통하여광자, 전자에대한에너지의흡수분포를측정하여 PET/CT 검사에이용되는 511 kev 선의차폐효과에대해알아보고자하였다. 그결과첫째, 511 kev의광자에대해납두께에따른에너지흡수분포는차폐효과가있었으며, 두께에따라증가함에따라차폐효과또한증가하였다. 하지만, 전자의에너지흡수분포는두께에따라차폐효과가증가하지만, 일정두께이하에서는오히려차폐체가없을때보다에너지흡수분포가높게나타났다. 이는 2차적으로발생된전자의비정이짧고에너지가낮아모두흡수되어투과의성질을가진광자와는다른양상을나타낸것으로추론된다. 둘째, 광자와전자의에너지흡수분포를합한결과전자의에너지흡수분포처럼차폐효과가역효과를나타내는구간이나타났다. 이는방사선피폭의주원인이투과력이강한광자가아닌 2차적으로발생된산란선이피폭의주원인이지때문인것으로판단되며, Dae Moo Shim [15] 등의연구결과와유사한경향을나타냈다. ICRU 팬텀내깊이가증가할수록비정이짧은산란전자선의영향보다투과력이강한광자의영향으로차폐효과가발생되는납두께가감소되는것을확인할수있었으며, 산란전자선에의한영향이피부표면에집중된다는것을확인할수있었다. 이처럼산란전자선은에너지가작고비정이짧아피부표면의선량을증가시키지만, 깊이가증가함에따라산란전자선에의한피부선량의증가보다광자에대한차폐효과가커져전체적인선량은감소하는것으로사료된다. 셋째, ICRU Slab 팬텀을이용하여 0.25 mm 의납차폐체를사용하였을때피부표면 Hp(0.07) 의산란전자선영향을제거하기위해선공기층 9 cm 이상이필요한것으로나타났으며, 심부로들어갈수록산란전자선의영향이약해져 Hp(10) 에서는약 0.5 cm 정도의공기층만으로도차폐효과를얻을수있다. 0.5 mm 의납차폐체를사용하였을때는피부표면 (0.07) 에서약 1 cm 이상의공기층으로차폐효과가발생되는것으로나타 났으며, 나머지 Hp(3), Hp(10) 에서는공기층과무관하게모두차폐효과가발생되는것으로나타났다. 기존 Wang-Hui Lee [16] 의연구에따르면 140 kev의 선을조사하였을때차폐체사이의공기층으로인하여피폭이감소된다는의견과유사한결과를나타냈다. Ⅴ. CONCLUSION 방사선작업종자자의피폭방어를위해다양한형태의차폐방어가필요하다. 그러나본연구결과를토대로할때, 감마선으로부터피폭방어를위한납차폐체가오히려종자자의피부표면선량을증가시킬수있다는것을알수있었다. 보다더효율적인방사선방어를위해향후방사선과차폐체그리고차폐체와종자자와의거리등을종합적으로평가하여산란전자선에의한종사자의피폭평가가지속적으로이루어져야할것으로판단된다. Reference [1] Benjamin Guillet, Pierre Quentin, Serge Waultier, P, Technologist Radiation Exposure in Routine Clinical Practice with 18F-FDG PET, Journal of Nuclear Medicine Technology, Vol. 33, No. 3, pp.175-179, 2005. [2] Fiona O. Roberts, Dishan H. Gunawardana, Kunthi Pathmaraj, et al, "Radiation Dose to PET Technologists and Strategies to lower Occupational Exposure", Journal of Nuclear Medicine Technology, Vol. 33, No. 1, pp.44-47, 2005. [3] ICRP, "Recommendations of the International Commission on Radiological Protection", Ann ICRP, Vol. 21, pp.1-201, 1991. [4] Korea Standards & certifications, "Medical X-ray Protective Aprons", KS P 6023, 2007. [5] Myeong-Hwan Park, Deok-Moon Kwon, "Measurement of Apron Shielding rate for X-ray and Gamma ray", Korean Society Radiological Science, Vol. 30, No. 3, pp.245-250, 2007. [6] Andrew J Reilly, "Report 78 Spectrum Processor", IPEM, 1997. [7] Yong-Gil Kang, "Textbook of nuclear Medicine", pp.81-86, 2013. [8] Hongmoon Jung, June ho Cho, Jaeeun Jung, et al, "Evaluation of the Radiation Dosage Flowing out of the Hot Cell During Synthesis of 18 FDG", Journal of the Korean Society of Radiology, Vol. 7, No. 5, pp365-369, 2013. 464
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