Radiation Protection Dosimetry Advance Access published October 31, 2013 Radiation Protection Dosimetry (2013), pp. 1–6
doi:10.1093/rpd/nct269
COUNTING EFFICIENCY OF WHOLE-BODY MONITORING SYSTEM USING BOMAB AND ANSI/IAEA THYROID PHANTOM DUE TO INTERNAL CONTAMINATION OF 131I V. P. Ghare*, H. K. Patni, D. K. Akar and D. D. Rao Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400085, India *Corresponding author: [email protected]
In this study, the effect of Indian reference BOttle MAnnikin aBsorber (BOMAB) neck with axial cavity and American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom using pencil sources of 133 Ba (131I simulant) on counting efficiency (CE) is seen experimentally in static geometry for whole-body monitoring system comprising 10.16-cm diameter and 7.62-cm-thick NaI(Tl) detector. The CE estimated using the neck part of BOMAB phantom is 50.2 % lower in comparison with ANSI phantom. In rest of the studies FLUKA code is used for Monte Carlo simulations using ANSI/IAEA thyroid phantom. The simulation results are validated in static geometries with experimental CE and the differences are within 1.3 %. It is observed that CE for pencil source distribution is 3.97 % higher for 133Ba in comparison with CE of 131I source. Simulated CE for pencil source distribution is 4.7 % lower in comparison with uniform source distribution in the volume of thyroid for 131I. Since the radiation workers are of different physique; overlying tissue thickness (OTT) and neck-todetector distance play an important role in the calculation of activity in thyroid. The CE decreases with increase in OTT and is found to be 5.5 % lower if OTT is changed from 1.1 to 2 cm. Finally, the simulations are carried out to estimate the variation in CE due to variation in the neck-to-detector distance. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. In conclusion, the calibration of whole-body monitoring system for 131I should be carried out with ANSI/IAEA thyroid phantom, the neck-to-detector distance controlled or the CE corrected for this, and the CE should be corrected for OTT.
INTRODUCTION At Bhabha Atomic Research Centre (BARC), a whole-body monitoring system (WBMS) is used for assessment of internal contamination due to highenergy photon (HEP) emitters ( photon energies .200 keV)(1, 2). One of the widely used HEP emitter in nuclear medicine is 131I(3). Being highly volatile there is a potential of internal contamination of 131I in the radiation workers handling it (4). The biokinetic behaviour of 131I is such that significant portion of activity is accumulated in the thyroid of the internally contaminated person(4). Therefore, thyroid monitoring is the most preferred method of direct monitoring of 131I. To estimate 131I retained in the thyroid of a contaminated worker using WBMS, known activity is distributed in the neck part of Indian reference BOMAB phantom(5). In rest of the discussion the Indian reference BOMAB phantom is referred as BOMAB phantom and the neck part of the Indian reference BOMAB phantom is referred as BOMAB neck. Another phantom used for calibration of WBMS is ANSI/IAEA thyroid phantom(6) which is a more realistic representation of human thyroid(7, 8). Earlier studies using experimental methods(9, 10) and Monte Carlo techniques for the studied
measurement systems suggest that counting efficiency (CE) for 131I depends on the neck-to-detector distance and overlying tissue thickness (OTT)(9). However, variation in size and shape of the thyroid does not have large effect on CE at larger neck-todetector distances(11). Since the radiation workers monitored are of different physiques and have different neck diameters, hence there is need to obtain CE which will correct the error occurring due to differences in these factors for the monitoring system of this study. Also, there is a requirement of comparing the CEs of WBMS due to ANSI/IAEA thyroid phantom and BOMAB phantom with axial cavity. The effect of pencil source distribution and source uniformly distributed in the volume of thyroid is not reported earlier. As 131 I is a short-lived (T1/2 ¼ 8.02 days)(12) radionuclide, experimental calibration is carried out using 133Ba source (T1/2 ¼ 10.551 yrs). Hence there is a need to estimate the correction factor in CE derived using 133Ba as 131I simulant for the monitoring system. Since, similar type of monitoring systems and BOMAB phantoms are used throughout India for whole-body monitoring, a study was proposed to evaluate all these aspects. In this study, the effect of BOMAB neck and ANSI/ IAEA thyroid phantom on CE has been observed
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Belgorod State University on December 29, 2013
Received 8 August 2013; revised 8 October 2013; accepted 9 October 2013
V. P. GHARE ET AL.
MATERIALS AND METHODS Whole-body monitoring system The WBMS used in the current study is a partially shielded system (20-cm-thick mild steel shielding) incorporating 10.16-cm diameter and 7.62-cm thick NaI (Tl) detector as shown in Figure 1a. This detector is housed in a 0.5-mm stainless steel casing which is kept
8 mm inside the shield surface for the purpose of collimation. The WBMS also contains a bed which is a 2.5cm thick mattress placed on a 1.2-cm thick polymethyl methacrylate (PMMA) sheet. The detector surface to bed surface distance is fixed at 34.1 cm. This system can be operated in static as well as in scanning geometry. BOMAB phantom The system is calibrated using BOMAB phantom (67 kg/168 cm) as a human simulator. The BOMAB phantom comprises 10 elliptical containers: head, neck, thorax, pelvis, arms (2), thighs (2) and legs (2). Dimension of various parts of this BOMAB phantom can be found in authors’ other work(13). Neck is an ellipsoid with semi-major axis, semi-minor axis and length of 14, 13 and 9.4 cm, respectively. Each section of the phantom has a central axial cavity of diameter 1.2 cm to place the sources and constructed with polyvinyl chloride cylinders of wall thickness of 0.3 cm. Figure 1b shows the cross-sectional view of neck part of BOMAB phantom with axial cavity. Water is filled in all parts of the phantom to simulate interactions in human body.
Figure 1. (a) BOMAB phantom in a shadow shield scanning bed whole-body monitoring system. (b) Cross-sectional view of neck part of BOMAB phantom with the axial cavity.
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experimentally in static geometry. Monte Carlo simulations have been validated for ANSI/IAEA thyroid phantom with experimental CE. Simulations have been carried out in static geometry using BOMAB and ANSI/IAEA thyroid phantom for uniform source distribution. The CEs estimated using 133Ba and 131I for ANSI/IAEA thyroid phantom have been used to estimate the correction factor for the system calibrated using 133Ba source. Simulated CEs due to pencil source distribution and source distributed uniformly in the volume of thyroid are also reported. The simulations have been carried out to estimate the variation in CE due to different neck-to-detector distances and different OTTs. Such study will be useful for other laboratories involved in the direct monitoring of 131I.
CE USING BOMAB AND ANSI/IAEA THYROID PHANTOM
neck surface-to-detector surface distance fixed at 21.4 cm and pencil source of length 5.8 cm. The detector centre has been positioned over the centre of source placed in BOMAB neck and ANSI/IAEA thyroid phantom. The measurements have been carried out in static geometry. The region of interest (ROI) used for the estimation of CE has been taken from 260 to 460 keV. The counts in ROIs has been maintained .104, so as to keep the statistical error in experimental CE ,1 %. For the geometry discussed in this paper (large detector size and neck-to-detector distance) variation in CE due to error in horizontal positioning will be negligible.
Figure 2. (a) American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom with thyroid. (b) Cross-sectional view of ANSI/IAEA thyroid phantom with the thyroid cavity.
ANSI/IAEA thyroid phantom Figure 2a shows a picture of ANSI/IAEA thyroid phantom(6) with a small cylinder (shown on side of the phantom) used to simulate the thickness of tissue over the thyroid gland. This phantom is a PMMA cylinder of 127 mm diameter`127 mm length having a cavity of 30 mm diameter. Figure 2b shows the cross-sectional view of ANSI/IAEA thyroid phantom with thyroid cavity and its dimensions. The distance between phantom edge and cavity is 5 mm. The smaller cylinder of 29 mm diameter`90 mm length is having a hollow cavity of 17 mm diameter`58 mm length representing human thyroid. The smaller cylinder is inserted in the cavity of larger PMMA cylinder for calibration of WBMS using ANSI/IAEA thyroid phantom. The effective OTT for this thyroid phantom is 1.1 cm. Experimental calibration For experimental calibration, water has been filled in BOMAB phantom and 133Ba (131I simulant) pencil source is inserted in the neck part of the phantom. The pencil source used in experimental calibration is of 9.4-cm length with a source diameter 0.2 cm covered by a 0.4-cm polypropylene casing. The BOMAB phantom neck surface-to-detector distance has been kept at 21.1 cm during the experiment. The experiment has been repeated by replacing BOMAB neck with ANSI/IAEA thyroid phantom keeping the
Monte Carlo code ‘FLUKA’(14, 15) has been used for simulations. The experimental geometry has been incorporated into the input file of FLUKA. For creation of input file, Flair(16) an advanced user interface for FLUKA has been used. For describing source in thyroid cavity, user written source program has been used with FLUKA code. Source points have been randomly sampled in elliptical cylinders. The direction cosines have been used to propagate the photons isotropically from randomly selected source points. The detailed description of axial cavity activity distribution and uniform activity distribution can be found in authors’ other work(13). For pencil source distribution experimental setup has been simulated for both BOMAB and ANSI/IAEA thyroid phantom. For uniform distribution, source has been distributed uniformly in the neck part of the BOMAB phantom and thyroid cavity of ANSI/IAEA thyroid phantom. The simulations have been carried out in static geometry. Since in the experimental setup 133Ba source is used hence, in simulations also it has been used. In ideal situation the system should be calibrated using the 131 I source, hence simulations were also carried out using this radionuclide. ROI in both the cases has been taken from 260 to 460 keV. The photon energies and their respective yields used in the simulations are provided in Tables 1 and 2(12). Total photon yield for 133 Ba is 96.36 % and for 131I it is 97.399 %. Although the total yield for 133Ba and 131I is very similar but their energy distribution is much different. In the source program the yields have been normalised in the corresponding peak regions so as to maintain consistency with experimental setup for determining CEs. The Monte Carlo simulated response of NaI (Tl) gives line spectrum, which has been converted into the observed spectra using a user written smearing program, incorporating the detector resolution in the energy ROI. Uniform distribution of source points in ANSI/IAEA thyroid cavity was used to study the variation in the CE due to change in OTT. The original OTT of 1.1 cm, has been changed to 2.9, 2.6, 2.3, 2.0, 1.7, 1.4, 0.8 and 0.5 cm by keeping
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Simulations with BOMAB and ANSI/IAEA thyroid phantom in FLUKA in static geometry
V. P. GHARE ET AL. Table 1. Energy and yield of 133Ba used in simulation. Energy (keV) 276.4 302.8 356 383.8 Total
Yield (%) 7.16 18.3 62 8.9 96.36
Table 2. Energy and yield of 131I used in simulation.
284.305 325.789 364.489 503 636.989 642.719 722.911 Total
Yield (%) 6.12 0.273 81.5 0.359 7.16 0.217 1.77 97.399
thyroid gland surface-to-detector distance fixed (22.5 cm). Persons can be of different physique such that the thyroid-to-detector distance may change from individual to individual depending on the design of the measurement system. Hence, simulations were performed using the previously described static counting system and changing the neck-to-detector distance with the source distributed uniformly in the thyroid cavity and 1.1-cm OTT. The neck-to-detector distance was varied from 15.4 to 21.4 cm with the step of 0.5 cm. Calculation of relative error For static geometry relative error in CE is given by rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Pn xi yi 2 i¼1 Pn 100 100 Relative error ¼ i¼1 xi
Validation of simulated CE with experiment The Monte Carlo simulation for ANSI/IAEA thyroid phantom has been validated with the experimental results. Figure 3 depicts the simulated WBMS with ANSI/IAEA thyroid phantom. The simulated CE for 133 Ba pencil source distribution using ANSI/IAEA thyroid phantom is 7.41` 1023 c p21. The difference between the simulated and experimental CE is 1.3 %. Comparison of CE due to uniform activity distribution in BOMAB neck and thyroid of ANSI/IAEA thyroid phantom for 133Ba The CE for 133Ba activity uniformly distributed in whole volume of BOMAB neck (as used in the international standard BOMAB phantom) is 4.54`1023 c p21. It is observed that the experimental CE for uniform distribution of activity in BOMAB neck is 39.5 % lower in comparison with the estimated CE using ANSI/IAEA thyroid phantom. This difference
where xi is the counts per photon (c p21) in ith channel, yi the per cent relative error associated with it in that channel and sum is taken over the channels in the ROI. The number of histories (107) for Monte Carlo simulations has been selected in such a way that relative errors in the estimated CEs are ,1 %. RESULTS AND DISCUSSION Experimental comparison of CE of WBMS using BOMAB and ANSI/IAEA thyroid phantom The experimentally estimated CE for the 133Ba pencil source is 3.74` 1023 c p21 using BOMAB neck phantom, whereas using ANSI/IAEA thyroid
Figure 3. Simulated whole-body monitoring system with ANSI/IAEA thyroid phantom.
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Energy (keV)
phantom, the CE is 7.51` 1023 c p21. The CE estimated using the BOMAB neck phantom is 50.2 % lower in comparison with ANSI/IAEA thyroid phantom. This difference is due to two reasons. First, the source is nearer to detector in the case of ANSI/ IAEA thyroid phantom. The second reason for higher CE in the case of ANSI/IAEA thyroid phantom is the less attenuation media for photons emitted in this phantom due to lower OTT (1.1 cm in the case of ANSI/IAEA thyroid phantom and 5.9 cm for BOMAB neck cavity). The ANSI/IAEA thyroid phantom is more consistent with the known physiology of the thyroid and would provide a more correct human thyroid measurement than the Indian BOMAB. The above results demonstrate that the use of the Indian BOMAB phantom leads to overestimation of thyroid activity by a factor of 2.01.
CE USING BOMAB AND ANSI/IAEA THYROID PHANTOM
in CE is due to the difference in the effective source-todetector distance and the higher attenuation of photons emitted from 133Ba in BOMAB neck. Hence, the use of standard BOMAB neck phantom will also lead to overestimation of thyroid activity by a factor of 1.65.
Comparison of CE due to pencil source distribution of 131 I and 133Ba in ANSI/IAEA thyroid phantom
Figure 4. Simulated 131I spectra for uniform distribution in ANSI/IAEA thyroid phantom.
Comparison of CE for uniform and pencil source distribution of activity The computed CE for uniform distribution of 131I activity in the thyroid cavity of ANSI/IAEA thyroid phantom is 7.41` 1023 c p21. Figure 4 shows simulated 131I spectra for uniform distribution using ANSI/IAEA neck thyroid phantom for OTT of 1.1 cm with neck surface-to-detector surface distance of 21.4 cm in WBM system. The simulated photopeaks of 131I at 284 keV, 364 keV and 636 keV are clearly visible in Figure 4. It is observed that CE for pencil source distribution is 4.7 % lower than CE estimated using uniform distribution of activity. The reason for difference between CE due to uniform and pencil source distribution is given in authors’ previous work(13). Hence, correction factor should be considered if pencil source is used for calibration.
Effect of OTT Figure 5 shows the effect of OTT on CE for uniform distribution of 131I in ANSI/IAEA thyroid phantom. The response of detector is higher when thyroid is close to skin surface. It is observed that CE is 5.5 % lower if OTT is changed from 1.1 to 2 cm and it goes on decreasing up to 12.5 % for OTT of 2.9 cm. It is also observed that if OTT is decreased from 1.1 to 0.5 cm the CE increases by 3.2 %. As the OTT increases, the attenuation also increases, giving rise to lower CE. Hence, precise knowledge of OTT will lead to accurate estimation of activity by using OTT-dependent CE.
Figure 5. Effect of OTT on CE for uniform distribution of 131 I in ANSI/IAEA thyroid phantom.
Effect of neck surface-to-detector distance Figure 6 shows the effect of the neck surface-to-detector distance on CE for uniform distribution of 131I in ANSI/IAEA thyroid phantom. It is observed that CE increases with the decrease in the neck surface-todetector distance. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. As the distance increases, the solid angle between detector and source decreases, this eventually leads to decease in CE. Hence, care should be taken when measuring the neck-to-detector distance. CONCLUSION The calibration of WBMS for 131I should be carried out with ANSI/IAEA thyroid or by Monte Carlo
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The simulated CE for ANSI/IAEA thyroid phantom using 133Ba pencil source distribution is 7.34` 1023 c p21 and for 131I it is 7.06` 1023 c p21. It is observed that CE is 3.97 % higher for 133Ba in comparison with CE of 131I source. The difference in yields and emitted energies of 131I and 133Ba in the ROI result in the difference in the detector response and CEs. Hence, it is suggested to use correction factor of 0.96 if 133Ba source is used for the estimation of calibration factor (counts per second per Bq). Thus, the rest of the simulation results have been arrived at using 131I source distributed in ANSI/ IAEA thyroid phantom.
V. P. GHARE ET AL.
methods to obtain a more realistic representation of the human thyroid. The correction factor of 0.96 should to be used if calibration is carried out using 133 Ba source. As CE for pencil source distribution is 4.7 % lower than CE estimated using uniform distribution of activity in the cavity of thyroid, hence correction should be done if pencil source has been used for calibration. The neck-to-detector distance as well as OTT plays an important role in the estimation of activity in thyroid in fixed geometry measurement systems such as that used at BARC. The CE is found to be 5.5 % lower if OTT is changed from 1.1 to 2 cm. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. The effect of the neck-to-detector distances and OTT values on CE will be useful in scenario where thyroidal activity is very high giving committed effective dose greater than the recording level. ACKNOWLEDGEMENT The authors are thankful to Smt. Sharda Bhati, Ex. Head, Internal Dosimetry Section, Health Physics Division, BARC for her kind encouragement and mentoring. REFERENCES 1. Bhati, S., Patni, H. K., Ghare, V. P., Singh, I. S. and Nadar, M. Y. Monte Carlo calculations for efficiency calibration of a whole-body monitor using bomab phantoms of different sizes. Radiat. Prot. Dosim. 148(4), 414– 419 (2012).
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Figure 6. Effect of the neck-to-detector distance on CE for uniform distribution of 131I in ANSI/IAEA thyroid phantom.
2. Garg, S. P., Bhati, S., Haridasan, T. K., Jaiswal, D. D., Sawant, P. D., Singh, I. S. and Pendharkar, K. A. An overview of internal dosimetry programmes in India. In: Proceedings of 16th Annual Conference of Indian Nuclear society (INSAC-2005), Mumbai, India, T2_CP11 (2005). 3. Alirezazadeh, N., Garshasbi, H. and Karimi Diba, J. Internal exposure monitoring of 131I-radiopharmaceutical workers in Iran. Radiat. Prot. Dosim. 104(2), 173–176 (2003). 4. International Commission on Radiological Protection. Individual monitoring for internal exposure of workers. ICRP Publication 78. Ann. ICRP 27(3–4). Elsevier (1997). 5. Mehta, D. J., Singh, I. S. and Sharma, R. C. A family of phantoms representative of male Indian population for calibration of whole body counters. Radiat. Prot. Environ. 26(1– 2), 327– 332 (2003). 6. International Atomic Energy Agency Safety series. Direct methods for measuring radionuclides in the human body. IAEA No. 114. IAEA (1996). 7. International Atomic Energy Agency. Report of consultants meeting on the calibration and standardization of thyroid radioiodine uptake measurements. Br. J. Radiol. 35, 205–210 (1962). 8. American National Standards Institute. American National Standard Thyroid Radioiodine Uptake Measurements Using a Neck phantom. ANSI N44.3. ANSI (1973). 9. Lahham, A., Fulop, M. and Ragan, P. Thyroidal 131I monitoring system using whole body counter capabilities. Radiat. Prot. Dosim. 51(2), 131–136 (1994). 10. Kramer, G.H., Gamarnik, K., Noel, L., Burns, L. and Meyerhof, D. The BRMD thyroid-neck phantom: design and construction. Health Phys. 71(2), 211– 214 (1996). 11. Kramer, G. H. and Paul, Crowley. The assessment of the effect of thyroid size and shape on the activity estimate using Monte Carlo simulation. Health Phys. 78(6), 727– 738 (2000). 12. Nudat2.6, National nuclear data center. Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/. 13. Akar, D. K., Patni, H. K., Nadar, M. Y., Ghare, V. P. and Rao, D. D. Monte Carlo simulation of NaI(Tl) detector in a shadow-shield scanning bed whole body monitor for uniform and axial cavity activity distribution in BOMAB phantom. Radiat. Prot. Dosim. 155(3), 292–299 (2013). 14. Fasso’, A., Ferrari, A., Ranft, J. and Sala, P. R. FLUKA: a multi-particle transport code. CERN-200510, INFN/TC_05/11, SLAC-R-773 (2005). 15. Battistoni, G., Muraro, S., Sala, P. R., Cerutti, F., Ferrari, A., Roesler, S., Fasso’, A. and Ranft, J. The FLUKA code: description and benchmarking. AIP Conf. Proc. 896, 31–49 (2007). 16. Vlachoudis, V. Flair, a powerful but user friendly graphical interface for FLUKA. In: Proceedings of the International Conference on Mathematics, Computational Methods & Reactor Physics (M&C 2009), New York, Saratoga Springs (2009).
doi:10.1093/rpd/nct269
COUNTING EFFICIENCY OF WHOLE-BODY MONITORING SYSTEM USING BOMAB AND ANSI/IAEA THYROID PHANTOM DUE TO INTERNAL CONTAMINATION OF 131I V. P. Ghare*, H. K. Patni, D. K. Akar and D. D. Rao Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400085, India *Corresponding author: [email protected]
In this study, the effect of Indian reference BOttle MAnnikin aBsorber (BOMAB) neck with axial cavity and American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom using pencil sources of 133 Ba (131I simulant) on counting efficiency (CE) is seen experimentally in static geometry for whole-body monitoring system comprising 10.16-cm diameter and 7.62-cm-thick NaI(Tl) detector. The CE estimated using the neck part of BOMAB phantom is 50.2 % lower in comparison with ANSI phantom. In rest of the studies FLUKA code is used for Monte Carlo simulations using ANSI/IAEA thyroid phantom. The simulation results are validated in static geometries with experimental CE and the differences are within 1.3 %. It is observed that CE for pencil source distribution is 3.97 % higher for 133Ba in comparison with CE of 131I source. Simulated CE for pencil source distribution is 4.7 % lower in comparison with uniform source distribution in the volume of thyroid for 131I. Since the radiation workers are of different physique; overlying tissue thickness (OTT) and neck-todetector distance play an important role in the calculation of activity in thyroid. The CE decreases with increase in OTT and is found to be 5.5 % lower if OTT is changed from 1.1 to 2 cm. Finally, the simulations are carried out to estimate the variation in CE due to variation in the neck-to-detector distance. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. In conclusion, the calibration of whole-body monitoring system for 131I should be carried out with ANSI/IAEA thyroid phantom, the neck-to-detector distance controlled or the CE corrected for this, and the CE should be corrected for OTT.
INTRODUCTION At Bhabha Atomic Research Centre (BARC), a whole-body monitoring system (WBMS) is used for assessment of internal contamination due to highenergy photon (HEP) emitters ( photon energies .200 keV)(1, 2). One of the widely used HEP emitter in nuclear medicine is 131I(3). Being highly volatile there is a potential of internal contamination of 131I in the radiation workers handling it (4). The biokinetic behaviour of 131I is such that significant portion of activity is accumulated in the thyroid of the internally contaminated person(4). Therefore, thyroid monitoring is the most preferred method of direct monitoring of 131I. To estimate 131I retained in the thyroid of a contaminated worker using WBMS, known activity is distributed in the neck part of Indian reference BOMAB phantom(5). In rest of the discussion the Indian reference BOMAB phantom is referred as BOMAB phantom and the neck part of the Indian reference BOMAB phantom is referred as BOMAB neck. Another phantom used for calibration of WBMS is ANSI/IAEA thyroid phantom(6) which is a more realistic representation of human thyroid(7, 8). Earlier studies using experimental methods(9, 10) and Monte Carlo techniques for the studied
measurement systems suggest that counting efficiency (CE) for 131I depends on the neck-to-detector distance and overlying tissue thickness (OTT)(9). However, variation in size and shape of the thyroid does not have large effect on CE at larger neck-todetector distances(11). Since the radiation workers monitored are of different physiques and have different neck diameters, hence there is need to obtain CE which will correct the error occurring due to differences in these factors for the monitoring system of this study. Also, there is a requirement of comparing the CEs of WBMS due to ANSI/IAEA thyroid phantom and BOMAB phantom with axial cavity. The effect of pencil source distribution and source uniformly distributed in the volume of thyroid is not reported earlier. As 131 I is a short-lived (T1/2 ¼ 8.02 days)(12) radionuclide, experimental calibration is carried out using 133Ba source (T1/2 ¼ 10.551 yrs). Hence there is a need to estimate the correction factor in CE derived using 133Ba as 131I simulant for the monitoring system. Since, similar type of monitoring systems and BOMAB phantoms are used throughout India for whole-body monitoring, a study was proposed to evaluate all these aspects. In this study, the effect of BOMAB neck and ANSI/ IAEA thyroid phantom on CE has been observed
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Belgorod State University on December 29, 2013
Received 8 August 2013; revised 8 October 2013; accepted 9 October 2013
V. P. GHARE ET AL.
MATERIALS AND METHODS Whole-body monitoring system The WBMS used in the current study is a partially shielded system (20-cm-thick mild steel shielding) incorporating 10.16-cm diameter and 7.62-cm thick NaI (Tl) detector as shown in Figure 1a. This detector is housed in a 0.5-mm stainless steel casing which is kept
8 mm inside the shield surface for the purpose of collimation. The WBMS also contains a bed which is a 2.5cm thick mattress placed on a 1.2-cm thick polymethyl methacrylate (PMMA) sheet. The detector surface to bed surface distance is fixed at 34.1 cm. This system can be operated in static as well as in scanning geometry. BOMAB phantom The system is calibrated using BOMAB phantom (67 kg/168 cm) as a human simulator. The BOMAB phantom comprises 10 elliptical containers: head, neck, thorax, pelvis, arms (2), thighs (2) and legs (2). Dimension of various parts of this BOMAB phantom can be found in authors’ other work(13). Neck is an ellipsoid with semi-major axis, semi-minor axis and length of 14, 13 and 9.4 cm, respectively. Each section of the phantom has a central axial cavity of diameter 1.2 cm to place the sources and constructed with polyvinyl chloride cylinders of wall thickness of 0.3 cm. Figure 1b shows the cross-sectional view of neck part of BOMAB phantom with axial cavity. Water is filled in all parts of the phantom to simulate interactions in human body.
Figure 1. (a) BOMAB phantom in a shadow shield scanning bed whole-body monitoring system. (b) Cross-sectional view of neck part of BOMAB phantom with the axial cavity.
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experimentally in static geometry. Monte Carlo simulations have been validated for ANSI/IAEA thyroid phantom with experimental CE. Simulations have been carried out in static geometry using BOMAB and ANSI/IAEA thyroid phantom for uniform source distribution. The CEs estimated using 133Ba and 131I for ANSI/IAEA thyroid phantom have been used to estimate the correction factor for the system calibrated using 133Ba source. Simulated CEs due to pencil source distribution and source distributed uniformly in the volume of thyroid are also reported. The simulations have been carried out to estimate the variation in CE due to different neck-to-detector distances and different OTTs. Such study will be useful for other laboratories involved in the direct monitoring of 131I.
CE USING BOMAB AND ANSI/IAEA THYROID PHANTOM
neck surface-to-detector surface distance fixed at 21.4 cm and pencil source of length 5.8 cm. The detector centre has been positioned over the centre of source placed in BOMAB neck and ANSI/IAEA thyroid phantom. The measurements have been carried out in static geometry. The region of interest (ROI) used for the estimation of CE has been taken from 260 to 460 keV. The counts in ROIs has been maintained .104, so as to keep the statistical error in experimental CE ,1 %. For the geometry discussed in this paper (large detector size and neck-to-detector distance) variation in CE due to error in horizontal positioning will be negligible.
Figure 2. (a) American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom with thyroid. (b) Cross-sectional view of ANSI/IAEA thyroid phantom with the thyroid cavity.
ANSI/IAEA thyroid phantom Figure 2a shows a picture of ANSI/IAEA thyroid phantom(6) with a small cylinder (shown on side of the phantom) used to simulate the thickness of tissue over the thyroid gland. This phantom is a PMMA cylinder of 127 mm diameter`127 mm length having a cavity of 30 mm diameter. Figure 2b shows the cross-sectional view of ANSI/IAEA thyroid phantom with thyroid cavity and its dimensions. The distance between phantom edge and cavity is 5 mm. The smaller cylinder of 29 mm diameter`90 mm length is having a hollow cavity of 17 mm diameter`58 mm length representing human thyroid. The smaller cylinder is inserted in the cavity of larger PMMA cylinder for calibration of WBMS using ANSI/IAEA thyroid phantom. The effective OTT for this thyroid phantom is 1.1 cm. Experimental calibration For experimental calibration, water has been filled in BOMAB phantom and 133Ba (131I simulant) pencil source is inserted in the neck part of the phantom. The pencil source used in experimental calibration is of 9.4-cm length with a source diameter 0.2 cm covered by a 0.4-cm polypropylene casing. The BOMAB phantom neck surface-to-detector distance has been kept at 21.1 cm during the experiment. The experiment has been repeated by replacing BOMAB neck with ANSI/IAEA thyroid phantom keeping the
Monte Carlo code ‘FLUKA’(14, 15) has been used for simulations. The experimental geometry has been incorporated into the input file of FLUKA. For creation of input file, Flair(16) an advanced user interface for FLUKA has been used. For describing source in thyroid cavity, user written source program has been used with FLUKA code. Source points have been randomly sampled in elliptical cylinders. The direction cosines have been used to propagate the photons isotropically from randomly selected source points. The detailed description of axial cavity activity distribution and uniform activity distribution can be found in authors’ other work(13). For pencil source distribution experimental setup has been simulated for both BOMAB and ANSI/IAEA thyroid phantom. For uniform distribution, source has been distributed uniformly in the neck part of the BOMAB phantom and thyroid cavity of ANSI/IAEA thyroid phantom. The simulations have been carried out in static geometry. Since in the experimental setup 133Ba source is used hence, in simulations also it has been used. In ideal situation the system should be calibrated using the 131 I source, hence simulations were also carried out using this radionuclide. ROI in both the cases has been taken from 260 to 460 keV. The photon energies and their respective yields used in the simulations are provided in Tables 1 and 2(12). Total photon yield for 133 Ba is 96.36 % and for 131I it is 97.399 %. Although the total yield for 133Ba and 131I is very similar but their energy distribution is much different. In the source program the yields have been normalised in the corresponding peak regions so as to maintain consistency with experimental setup for determining CEs. The Monte Carlo simulated response of NaI (Tl) gives line spectrum, which has been converted into the observed spectra using a user written smearing program, incorporating the detector resolution in the energy ROI. Uniform distribution of source points in ANSI/IAEA thyroid cavity was used to study the variation in the CE due to change in OTT. The original OTT of 1.1 cm, has been changed to 2.9, 2.6, 2.3, 2.0, 1.7, 1.4, 0.8 and 0.5 cm by keeping
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Simulations with BOMAB and ANSI/IAEA thyroid phantom in FLUKA in static geometry
V. P. GHARE ET AL. Table 1. Energy and yield of 133Ba used in simulation. Energy (keV) 276.4 302.8 356 383.8 Total
Yield (%) 7.16 18.3 62 8.9 96.36
Table 2. Energy and yield of 131I used in simulation.
284.305 325.789 364.489 503 636.989 642.719 722.911 Total
Yield (%) 6.12 0.273 81.5 0.359 7.16 0.217 1.77 97.399
thyroid gland surface-to-detector distance fixed (22.5 cm). Persons can be of different physique such that the thyroid-to-detector distance may change from individual to individual depending on the design of the measurement system. Hence, simulations were performed using the previously described static counting system and changing the neck-to-detector distance with the source distributed uniformly in the thyroid cavity and 1.1-cm OTT. The neck-to-detector distance was varied from 15.4 to 21.4 cm with the step of 0.5 cm. Calculation of relative error For static geometry relative error in CE is given by rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Pn xi yi 2 i¼1 Pn 100 100 Relative error ¼ i¼1 xi
Validation of simulated CE with experiment The Monte Carlo simulation for ANSI/IAEA thyroid phantom has been validated with the experimental results. Figure 3 depicts the simulated WBMS with ANSI/IAEA thyroid phantom. The simulated CE for 133 Ba pencil source distribution using ANSI/IAEA thyroid phantom is 7.41` 1023 c p21. The difference between the simulated and experimental CE is 1.3 %. Comparison of CE due to uniform activity distribution in BOMAB neck and thyroid of ANSI/IAEA thyroid phantom for 133Ba The CE for 133Ba activity uniformly distributed in whole volume of BOMAB neck (as used in the international standard BOMAB phantom) is 4.54`1023 c p21. It is observed that the experimental CE for uniform distribution of activity in BOMAB neck is 39.5 % lower in comparison with the estimated CE using ANSI/IAEA thyroid phantom. This difference
where xi is the counts per photon (c p21) in ith channel, yi the per cent relative error associated with it in that channel and sum is taken over the channels in the ROI. The number of histories (107) for Monte Carlo simulations has been selected in such a way that relative errors in the estimated CEs are ,1 %. RESULTS AND DISCUSSION Experimental comparison of CE of WBMS using BOMAB and ANSI/IAEA thyroid phantom The experimentally estimated CE for the 133Ba pencil source is 3.74` 1023 c p21 using BOMAB neck phantom, whereas using ANSI/IAEA thyroid
Figure 3. Simulated whole-body monitoring system with ANSI/IAEA thyroid phantom.
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Energy (keV)
phantom, the CE is 7.51` 1023 c p21. The CE estimated using the BOMAB neck phantom is 50.2 % lower in comparison with ANSI/IAEA thyroid phantom. This difference is due to two reasons. First, the source is nearer to detector in the case of ANSI/ IAEA thyroid phantom. The second reason for higher CE in the case of ANSI/IAEA thyroid phantom is the less attenuation media for photons emitted in this phantom due to lower OTT (1.1 cm in the case of ANSI/IAEA thyroid phantom and 5.9 cm for BOMAB neck cavity). The ANSI/IAEA thyroid phantom is more consistent with the known physiology of the thyroid and would provide a more correct human thyroid measurement than the Indian BOMAB. The above results demonstrate that the use of the Indian BOMAB phantom leads to overestimation of thyroid activity by a factor of 2.01.
CE USING BOMAB AND ANSI/IAEA THYROID PHANTOM
in CE is due to the difference in the effective source-todetector distance and the higher attenuation of photons emitted from 133Ba in BOMAB neck. Hence, the use of standard BOMAB neck phantom will also lead to overestimation of thyroid activity by a factor of 1.65.
Comparison of CE due to pencil source distribution of 131 I and 133Ba in ANSI/IAEA thyroid phantom
Figure 4. Simulated 131I spectra for uniform distribution in ANSI/IAEA thyroid phantom.
Comparison of CE for uniform and pencil source distribution of activity The computed CE for uniform distribution of 131I activity in the thyroid cavity of ANSI/IAEA thyroid phantom is 7.41` 1023 c p21. Figure 4 shows simulated 131I spectra for uniform distribution using ANSI/IAEA neck thyroid phantom for OTT of 1.1 cm with neck surface-to-detector surface distance of 21.4 cm in WBM system. The simulated photopeaks of 131I at 284 keV, 364 keV and 636 keV are clearly visible in Figure 4. It is observed that CE for pencil source distribution is 4.7 % lower than CE estimated using uniform distribution of activity. The reason for difference between CE due to uniform and pencil source distribution is given in authors’ previous work(13). Hence, correction factor should be considered if pencil source is used for calibration.
Effect of OTT Figure 5 shows the effect of OTT on CE for uniform distribution of 131I in ANSI/IAEA thyroid phantom. The response of detector is higher when thyroid is close to skin surface. It is observed that CE is 5.5 % lower if OTT is changed from 1.1 to 2 cm and it goes on decreasing up to 12.5 % for OTT of 2.9 cm. It is also observed that if OTT is decreased from 1.1 to 0.5 cm the CE increases by 3.2 %. As the OTT increases, the attenuation also increases, giving rise to lower CE. Hence, precise knowledge of OTT will lead to accurate estimation of activity by using OTT-dependent CE.
Figure 5. Effect of OTT on CE for uniform distribution of 131 I in ANSI/IAEA thyroid phantom.
Effect of neck surface-to-detector distance Figure 6 shows the effect of the neck surface-to-detector distance on CE for uniform distribution of 131I in ANSI/IAEA thyroid phantom. It is observed that CE increases with the decrease in the neck surface-todetector distance. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. As the distance increases, the solid angle between detector and source decreases, this eventually leads to decease in CE. Hence, care should be taken when measuring the neck-to-detector distance. CONCLUSION The calibration of WBMS for 131I should be carried out with ANSI/IAEA thyroid or by Monte Carlo
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The simulated CE for ANSI/IAEA thyroid phantom using 133Ba pencil source distribution is 7.34` 1023 c p21 and for 131I it is 7.06` 1023 c p21. It is observed that CE is 3.97 % higher for 133Ba in comparison with CE of 131I source. The difference in yields and emitted energies of 131I and 133Ba in the ROI result in the difference in the detector response and CEs. Hence, it is suggested to use correction factor of 0.96 if 133Ba source is used for the estimation of calibration factor (counts per second per Bq). Thus, the rest of the simulation results have been arrived at using 131I source distributed in ANSI/ IAEA thyroid phantom.
V. P. GHARE ET AL.
methods to obtain a more realistic representation of the human thyroid. The correction factor of 0.96 should to be used if calibration is carried out using 133 Ba source. As CE for pencil source distribution is 4.7 % lower than CE estimated using uniform distribution of activity in the cavity of thyroid, hence correction should be done if pencil source has been used for calibration. The neck-to-detector distance as well as OTT plays an important role in the estimation of activity in thyroid in fixed geometry measurement systems such as that used at BARC. The CE is found to be 5.5 % lower if OTT is changed from 1.1 to 2 cm. The CE is 6.2 % higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9 % if the distance is decreased to 15.4 cm. The effect of the neck-to-detector distances and OTT values on CE will be useful in scenario where thyroidal activity is very high giving committed effective dose greater than the recording level. ACKNOWLEDGEMENT The authors are thankful to Smt. Sharda Bhati, Ex. Head, Internal Dosimetry Section, Health Physics Division, BARC for her kind encouragement and mentoring. REFERENCES 1. Bhati, S., Patni, H. K., Ghare, V. P., Singh, I. S. and Nadar, M. Y. Monte Carlo calculations for efficiency calibration of a whole-body monitor using bomab phantoms of different sizes. Radiat. Prot. Dosim. 148(4), 414– 419 (2012).
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Figure 6. Effect of the neck-to-detector distance on CE for uniform distribution of 131I in ANSI/IAEA thyroid phantom.
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