1. INTRODUCTION:

Many types of radioactive isotopes are used in medical examinations, and their scope of energy is very wide ranging from low-energy to high-energy γ rays. The latter are radiated in a 4π direction rather than toward one direction. Also, unlike X rays, they possess a strong permeability which requires closer attention to the shield. Shields used in nuclear medicine include the L-block, protective gloves, lead glasses, syringe holders, syringe shields, and aprons¹. The latter are most essential protective gear to minimize radiation exposure to workers in medical fields involving radiation².

For safety assurance and excellent performance related to radiation exposure situations, the Korean Ministry of Food and Drug Safety³developed and revised a national standard for aprons to fit an international one (IEC 61331-3:2003)⁴ which. however, is based on diagnostic X rays, so the question remains whether aprons with high-energy γ rays in nuclear medicine are really efficient. It is widely known that using lead with high atomic-number lead effectively shields against radiation exposure⁵, and it is widely used due to economic aspects, superior processability, and its shielding ability against radiation in medicine⁶. However, due to the harmfulness of lead and its inconveniently high weight, aprons are reluctantly used. Therefore, a GEANT 4 Gate simulation experiment was conducted to find a lighter shielding material with similar effects as lead.

2. MATERIALS AND METHODS:

2.1. Research equipment:

Gate simulations were used with an exclusive code for nuclear medical equipment, GEANT4 version 9.4p02 and Gate version 6.1. The shielding materials information input as the gate codes were antimony, tungsten, lead, bismuth, and barium sulfate (Table 1). The radioactive isotopes used in the mock experiment were ⁹⁹^mTc, ¹²³I, ²⁰¹Tl, ¹³¹I, and ¹⁸F which are most common as described in Table 2.

Table 1. Material characteristics of gate simulation shield

Element	Atomic Number of Material	Density
Antimony(Sb)	51	6.68g/cm³
Tungsten(W)	74	19.25g/cm³
Lead(Pb)	82	11.34g/cm³
Bismuth(Bi)	83	9.78g/cm³
Barium Sulfate (BaSO₄)	Barium: 56, Sulfur: 16, Oxygen: 8	4.49g/cm³
Virtual Space(Air)	(Nitrogen: 78.1%, Oxygen: 20.1%, Argon: 0.9%)	0.001275g/cm³ (Normal State)

Table 2. Characteristics of radioisotopes

Type	Energy	Half life
^99mTc	140keV	6.03hr
¹²³I	159keV	13.3hr
²⁰¹Tl	167keV	72.9hr
¹³¹I	364keV	8.1day
¹⁸F	511keV	110mim

2.2. Research method:

The width and length of the simulation sheet was 20 cm each and the thickness ranged from 0.25 mm to 5.0 mm by one 0.25 mm increment. The detector had a 7 cm square shape for which the number of photons penetrating the shielding material was later measured. A virtual space was designed like in Figure 1 for a 3D simulation. The detector was designed with heights of 50 and 100 cm from the origin of radiation and 2 cm away from it the five types of shielding materials were positioned in relation to the different radioisotope of calculated transmissions with values taken after 7.40E+06 photon emissions.

Figure 1. 3D Simulation

3. RESULTS AND DISCUSSION:

3.1. Transmission values and shielding rates of antimony shields:

With a thickness of 0.25 mm and 5.0 mm transmission values and shielding rates of ^99mTc resulted in 2.29E+06 (6.99% shielding rate), and 2.63E+05 (96.44% shielding rate), ¹²³I exhibited 2.15E+06 (70.86% shielding rate), and 4.53E+05 (93.87% shielding rate) while the numbers for ²⁰¹Tl were 2.08E+06 (71.84% shielding rate) and 5.25E+05 (92.89% shielding rate), respectively, as displayed in Table 3 and Figure 2. For the 0.25 mm thickness, ¹³¹I and ¹⁸F showed 9.77E+05 (86.78% shielding rate) and 7.66E+05 (89.64% shielding rate) while for 5.0 mm 7.84E+05 (89.40% shielding rate) and 6.87E+05 (90.71% shielding rate) were exhibited which confirmed that an increase in thickness did not significantly influence transmission value changes.

Table 3. Transmission values of antimony shield

(unit: number of photon)

	^99mTc	¹⁸F	¹³¹I	²⁰¹Tl	¹²³I
0.25	2294734	766014	977673	2084328	2155881
0.5	2058353	763407	969348	1947875	1998093
0.75	1842755	762144	958439	1816836	1846246
1.0	1646703	758636	951647	1692868	1704486
1.25	1475547	756149	940778	1577591	1571513
1.5	1318705	752415	932504	1469167	1450711
1.75	1174414	748958	923101	1367709	1338914
2.0	1051294	746247	914406	1270980	1231648
2.25	937007	741687	903205	1185741	1135603
2.5	834808	737083	891417	1100811	1044511
2.75	744745	725848	878741	1023289	963902
3.0	664115	724247	869810	952017	886333
3.25	592231	722199	859627	881600	816482
3.5	526697	719706	849210	822063	750528
3.75	469625	714405	836583	762118	689760
4.0	418800	709276	826213	707969	634948
4.25	372351	703939	815465	656376	585042
4.5	332220	698621	804934	610591	535167
4.75	303219	692555	795723	565269	492361
5.0	263333	687230	784057	525509	453139

3.2. Transmission values and shielding rates of bismuth shields:

As a 0.25 mm thickness, ²⁰¹Tl displayed 1.56E+06 (78.87% shielding rate), ¹²³I 1.55E+06 (78.98% shielding rate), ^99mTc 1.45E+06 (80.35% shielding rate), and ¹³¹I 9.45E+05 (87.22% shielding rate), while ¹⁸F with 7.58E+05 (89.75 % shielding rate) was the lowest in Table 4 and Figure 3. For ²⁰¹Tl, ¹²³I, and ^99mTc with increasing thickness, transmission values decreased and shielding rate increased while ¹³¹I and ¹⁸F did not show noteworthy changes.

Table 4. Transmission values of bismuth shield

(unit: number of photon)

	^99mTc	¹⁸F	¹³¹I	²⁰¹Tl	¹²³I
0.25	1453880	758270	945085	1563528	1555162
0.5	821129	744701	897954	1088141	1029099
0.75	463243	724994	851733	753756	679109
1.0	260619	708861	808313	524263	450435
1.25	146465	690579	764054	361945	296099
1.5	82519	671893	722472	250583	195526
1.75	46278	654381	683759	173668	129265
2.0	25835	636368	645610	119994	84195
2.25	14519	617431	610505	82553	55677
2.5	8269	599863	574992	57132	36565
2.75	4646	584354	542083	39679	24073
3.0	2651	566862	511453	27395	15907
3.25	1527	549725	482862	18914	10519
3.5	910	533637	453570	12983	6851
3.75	502	516890	427537	9002	4514
4.0	313	502021	402938	6233	3084
4.25	228	486963	378974	4410	2022
4.5	175	473120	356728	3004	1331
4.75	136	457366	335414	2027	866
5.0	93	443528	316009	1458	627

3.3. Transmission values and shielding rates of lead shields:

The transmission values and shielding rates of lead were similar to bismuth see Table 5 and Figure 4. As for the transmission values and shielding rates at a 0.25 mm thickness, ²⁰¹Tl displayed 1.49E+06 (79.86% shielding rate), ¹²³I 1.47E+06 (80.04% shielding rate), ^99mTc 1.34E+06 (81.77% shielding rate), ¹³¹I 9.41E+05 (87.28 % shielding rate), and ¹⁸F 7.57E+05 (89.75% shielding rate).

Table 5. Transmission values of lead shield

(unit: number of photon)

	^99mTc	¹⁸F	¹³¹I	²⁰¹Tl	¹²³I
0.25	1348644	757932	941188	1490214	1476932
0.5	706510	738826	887801	991870	927398
0.75	369388	719477	837659	654533	579742
1.0	193366	698407	786448	432048	361475
1.25	100252	679561	738954	285619	225927
1.5	52236	660701	692189	187751	140940
1.75	27158	637679	648406	123660	87792
2.0	14119	618192	608512	81190	54774
2.25	7427	600242	570300	53708	34602
2.5	3910	579826	533156	35794	21308
2.75	2022	560590	499397	23322	13281
3.0	1047	540785	467028	15238	8230
3.25	605	523533	435917	10012	5126
3.5	326	505787	408439	6585	3140
3.75	219	488906	381530	4324	2053
4.0	153	472007	356448	2822	1325
4.25	112	455494	333368	1908	827
4.5	95	439742	311253	1265	556
4.75	91	423804	291389	820	358
5.0	84	408668	271343	610	241

Table 6. Transmission values of tungsten shield

(unit: number of photon)

	^99mTc	¹⁸F	¹³¹I	²⁰¹Tl	¹²³I
0.25	1099508	751797	924135	1313113	1273926
0.5	466867	727250	855454	764057	690350
0.75	197170	700372	790766	443926	371760
1.0	83522	674092	720607	257035	199670
1.25	32834	646726	670163	148028	106944
1.5	14664	616762	614773	85350	57425
1.75	6297	593517	563307	49306	30956
2.0	2683	566078	515565	28353	16397
2.25	1738	542356	470366	16451	8713
2.5	1172	517914	429851	9427	4649
2.75	558	493025	393701	5234	2516
3.0	153	470245	359176	3182	1380
3.25	100	448093	326451	1792	791
3.5	87	426549	297968	1013	517
3.75	85	405326	271517	634	287
4.0	76	385356	246896	394	172
4.25	69	366837	225400	270	145
4.5	67	349422	204212	193	107
4.75	67	331547	186086	129	98
5.0	65	314926	169701	100	86

3.4. Transmission values and shielding rates of tungsten shields:

As for tungsten, ^99mTc had the highest transmission value (99.96% shielding rate) until 2.0 mm thickness followed by ¹²³I (99.77% shielding rate), and ²⁰¹Tl (99.61% shielding rate) (Table 6 and Figure 5). The lowest shielding rates were shown by ¹³¹I with (93.03% shielding rate ) and ¹⁸F with (92.35% shielding rate).

3.5. Transmission values and shielding rates of barium sulfate shields

Regarding barium sulfate, the thicker the shielding material the lower the transmission values for ^99mTc, ¹²³I, and ²⁰¹Tl, but ¹³¹I and ¹⁸F showed hardly any changes even with increased thickness (Table 7 and Figure 6).

Table 7. Transmission values of barium sulfate shield

(unit: number of photon)

	^99mTc	¹⁸F	¹³¹I	²⁰¹Tl	¹²³I
0.25	2350495	765855	977953	2112714	2192406
0.5	2154362	765409	971795	2004553	2063397
0.75	1975581	761272	965975	1895676	1938049
1.0	1808825	762156	957360	1795968	1821270
1.25	1660039	755415	949825	1698807	1711902
1.5	1516750	754898	941920	1608591	1608110
1.75	1389714	752490	934065	1521741	1505248
2.0	1271479	748766	927983	1434716	1412355
2.25	1162228	747187	919017	1357059	1323337
2.5	1065336	742533	912885	1282269	1243541
2.75	972507	740960	903634	1208375	1165493
3.0	888678	736413	895186	1144587	1092391
3.25	810697	732386	885712	1086576	1024011
3.5	743336	728957	877972	1019738	959631
3.75	679641	725450	870207	963291	899485
4.0	619413	722542	861805	910196	841089
4.25	565748	718870	853064	858380	788307
4.5	516852	714974	843893	811278	738497
4.75	472372	710260	835225	764747	690726
5.0	431667	706190	825907	722609	647477

4. DISCUSSION:

In this study, a mock experiment was conducted to analyze the permeability values and shielding rates of five shields with regard to radioactive isotopes using a Monte Carlo simulation. When compared to a similar investingation^7,8, the shielding rates of ^99mTc, ²⁰¹Tl, and ¹²³I decreased exponentially and increased by thickness In another study, the shielding ability of a barium compound in 2 mm using X rays⁹, showed an 80% shielding rates in ^99mTc, ²⁰¹Tl, and ¹²³I which confirmed that barium sulfate¹⁰ with its superior economic characteristics and processability can be used as a γ ray shield. In the field of medical diagnosis, X rays and γ rays are very important, but the latter are problematic since they use radioactive isotopes and if not protected properly, the permeability is very intense. Thus, next to using γ rays and diagnostic X rays it is imperative to develop suitable aprons for nuclear medicine. A gate simulation as employed in this study was not considered the detector’s response characteristics. As outcomes only indicate transmission values, there is a need to conduct additional research on the detector, as well as comparative studies with shielding materials that can actually be produced as the research on the detector, as well as comparative studies with shielding materials that can actually be produced as the result of simulation experiments.

5. CONCLUSION:

In order to examine the shielding material which could replace the radiation defense apron used in department of nuclear medicine, this study conducted GEANT4 Gate simulation by each energy characteristics of radioisotope that is most popular and conducted the analysis on transmission value and shielding rate in accordance with the component of shielding material. The shielding materials with antimony, ²⁰¹Tl and ¹²³I exhibited good shielding rates. For bismuth, lead, and tungsten, the shielding rates were high in the order of ^99mTc, ¹²³I, and ²⁰¹Tl. With respect to shielding materials using barium sulfate, the shielding rate was high in the order of ²⁰¹Tl, ¹²³I, and ^99mTc. From the results of this study, it is anticipated to be helpful for the development of aprons with a variety of shielding materials without actual production to predict the anticipated outcomes, to enhance the applicability and to minimize the errors based on the anticipated outcomes. Therefore, it is considered that the technicians in charge of nuclear medicine could prevent the wrong behavior to increase the radiation exposure with the existing aprons which have little shielding effect against X-ray, and their productivity could be enhanced because of weight reduction of the shielding materials as well as effective shielding if they use the radioprotective apron by the type of radioisotopes.

6. REFERENCES:

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3. The Korean Ministry of Food and Drug Safety, Some Revision Notice of Electronic Medical Device Standard Specification, 2011, Notice No. 2011-8, http://www.mfds.go.kr/index.do? mid=686&seq=3444&cmd=v

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5. Korea Radioisotope Association, The Theory and Practice of Radiation, 2008, pp. 72-79.

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8. Heo S Y, Kim Y H, Kim J H, Evaluation of the Effectiveness of an Apron in Handling Radiopharmaceuticals within the PET/CT Work Environment, Journal of Korean Society of Radiological Science Autumn Conference, 2015, 9(2), pp. 99-104.

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Received on 22.03.2018 Modified on 21.05.2018

Research J. Pharm. and Tech 2018; 11(12): 5631-5636.

DOI: 10.5958/0974-360X.2018.01021.1