DOSIMETRIC ASPECTS OF OPTIMIZATION OF PROTECTION IN IRAN INDUSTRIAL RADIOGRAPHY PRACTICE

DOSIMETRIC ASPECTS OF OPTIMIZATION OF PROTECTION IN IRAN INDUSTRIAL RADIOGRAPHY PRACTICE Abstract According to the worldwide reports, industrial radiography is one of the highest risk radiation practices due to handling high activity sources. Therefore, an optimization of protection along with appropriate investigation level and/or dose constraint is required to achieve appropriate radiological safety. This research, presents some statistical aspects of this optimization using frequency distributions and percentiles of individual recorded dose of industrial radiographers from the years 2002 to 2016 in Iran. The results show that, considering an investigation level of 4 mSv for bimonthly monitoring periods, 3–7% of population of workers has been investigated during these years. Moreover, an overall reduction on average annual and collective doses is observed, despite of the relative increasing in the number of workers. Since the frequency of periodical recorded dose at lower dose range (e.g. <4 mSv) causes greater impacts on the reduction of collective dose than the higher ranges, a retrospective average dose of non-investigated workers can also be substitute with a constant value as a dose constraint. It can be concluded that all the past measures of regulatory body and attempts of the employers have been effective improving the radiological protection in this practice in Iran. However, establishing a dose constraint seems to be essential to continue and improve this optimization of protection. INTRODUCTION Among all of the medical and industrial radiation practices, radiography non-destructive tests is one of the highest risk due to handling high activity sources. According to the UNSCEAR report(1), the worldwide average of annual collective effective dose of industrial radiographers over 5-year period (1990–95) has been estimated to be ~170 man Sv. Furthermore, the corresponding annual effective dose of measurably workers calculated to be ~3.17 mSv. These dose values are greater than those of most other medical or industrial radiation practices. One of the effective factors on any optimization of protection and reduction of averaged dose is triggering of some interventional actions for the workers whose doses values are greater than a reference level, and/or establishing a dose constraint for a particular radiation practice to evaluate the work conditions. The related actions may be varied from a simple alarm announce, to a temporary or permanent work suspension for the workers. Therefore, the quality of optimization, which itself is affected by the number of investigated workers, is crucial in any policies. Therefore, the first step of this optimization of protection is establishing a reference level (i.e. an investigation level) and/or setting a dose constraint.(2) Investigation level is a reference level which first was recommended by the International commission on Radiological Protection (ICRP 12) and revised in the later publications.(3) The ICRP 26(4) defined the Investigation level as ‘values of dose equivalent or intake above which the results are considered sufficiently important to justify further investigations’. ICRP also gives general guidance on investigation levels as the 3-10th of the annual dose limits corresponding to the fraction of a year to which the individual monitoring measurement refers. The concept of reference level arises from the ALARA principle and no scientific basis has been reported for this recommended ICRP values. Despite the ICRP recommendation, different investigation level values have been used for various radiation practices by countries or regulatory bodies. The concept of dose constraint was first introduced by ICRP-60(5), and means a value (typically 10 mSv/y)(2) below which it should be planned to keep all doses. Many countries have been used this concept in their optimization of protections program, however some other not for all but for some practices.(2) In this research, first the frequency distributions of the individual and collective effective doses are derived from a 15-year recorded dose data of industrial radiographers of Iran. Then, some statistical aspects and impacts of these dose distributions on the optimization of protection are investigated. MATERIALS AND METHODS Dosimetry method Individual monitoring of industrial workers are carried out bimonthly in Iran. By 2012, this monitoring program has been provided directly by the regulatory body for averaged number of ~1500 workers per year, and continued by private service providers under supervision of the regulatory authority. At present, there are ~200 industrial radiography facilities with ~2000 workers in Iran. Individual doses have been measured using TLD-100® with hot-gas model of Harshaw-6600® card readers and recorded via a national dose registry system.(6) All dosimetry Laboratories have been accredited to ISO-17025 standard criteria in recent years.(7) Optimization of protection According to the national regulations, the recording and investigation levels have been set to 0.1 and 4 mSv values respectively for industrial radiography practice on bimonthly dose assessment periods. The value of 4 mSv has been established based upon the previous experiences on the potential unusual exposures, performable number of investigations with consideration of the bimonthly official correspondence volumes, and safety culture in this practice. The value of 0.1 mSv has been used based upon the minimum measurable dose (MMD) of the dosimetry system (with 99% confidence level). Moreover, the recommended ICRP103(8) annual dose limits for occupational exposure are followed by the regulatory body. According to the previous worldwide and/or the national experiences, the most reasons for such unusual exposures are including(9): No proper use of collimators. Dose rate at the boundary of the work site not within limits set. No proper use of survey meters. No pre-operation specific equipment checks being performed. Poor operator knowledge of procedures. No proper warning system to prevent entry to the work site. Poor emergency preparedness. No proper use of alarm systems. Excess monthly duty hours. Working in elevated levels (hard to keep distance from the source during exposures). Accidents related to failure in the radiation source container (gamma projector), guide tube or remote control. Lack of or failure of survey meters or personal alarming dosimeters. Incorrect use of passive dosimeters (TLD). Placing the personal dosimeter near the source container during transportation. TLD falling near the unshielded source during radiography. Using the passive dosimeter for too long (more than 2 months). Once recording a dose above the investigation level, a procedure is triggered for the worker. A flowchart diagram of this procedure is shown in Figure 1. The first step of this interventional action is to submit a one-page questionary form to the employer. This form is divided by four parts including: General information about the employee, radiation sources and any record of direct reading dosimeter: must be filled by the associated health physicist. The reason of exposure in view point of employee: must be filled and signed by the employee. The reason of exposure in view point of employer: must be filled and signed by the health physicist (or any responsible competent liaison). The conclusion and final decision on the true recording dose: must be filled and signed by the regulatory authority. Figure 1. View largeDownload slide Diagram of the investigation procedure for the workers who receive unusual exposures in Iran. Figure 1. View largeDownload slide Diagram of the investigation procedure for the workers who receive unusual exposures in Iran. In special case of a periodical dose of more than the annual dose limit, a temporary work suspension is prescribed for the worker in addition to the questionary form. Special interviews are mandatory, and medical examinations may be required in these situations. However, the employee may continue his/her work after receiving some guidance on radiological protection and signing a written undertaking to radiation risk awareness. In critical case of periodical dose of more than 100 mSv, first a temporary work suspension (short term) is implemented for the worker in addition to the questionary form. Then, a special interviews and medical examinations (including cytogenetic dosimetry) must be carried out in these situations. If deterministic effects are indicated as the results of medical examinations and inspections, a long term (e.g. some months) or even permanent work suspensions may be implemented for the worker. Moreover, depending on the case, the employer may receive some official warnings or subjected to punitive measures by the regulatory bodies when they are culpable to the overexposure. Statistic method In this research, the bimonthly dose records of industrial radiographers from 2002 to 2016 were used as the data. Then, the numbers of dose values were counted in each 1 mSv dose intervals from 0 up to maximum recorded dose value in 1 year. The percentile value of Pn corresponds with the dose value under which the %n of population of industrial radiographers have received the dose, and calculated as follows(10): Pn=Li+(Ui−Li)(n×N100−CFiFi) (1) where N is the total number of recorded dose, Li is the lower, and Ui is the upper limits of the ith critical dose interval (e.g. in third interval, L3 = 2 mSv and U3 = 3 mSv), Fi is the frequency in the ith critical dose interval that covers n% of population, and CFi−1 is the cumulative frequency of the (i − 1)th critical dose interval. RESULTS AND DISCUSSION Individual and collective dose data Table 1 presents the averaged bimonthly and annual received doses of radiographers as well as the collective doses which have been recorded from 2002 to 2016. These dose data are associated with the routine radiography activities as well as all incidents. The average annual dose values for the first, second and third 5 years have been reduced and calculated to be 7.4, 6.8 and 4.8 mSv, respectively. The bimonthly average dose of workers also has been reduced and kept to <1 mSv in the last 5 years. The collective dose values for the first, second and third 5 years have been calculated to be 8.10, 9.22 and 8.24 man-Sv, respectively. There is a significant falling in value of dose in 2012 due to transferring dosimetry services from regulatory body to private service providers in which numbers of the workers’ dosimeters have not been delivered to the service provider for readouts. Table 1. Annual and collective doses of industrial radiographers in Iran. Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 Table 1. Annual and collective doses of industrial radiographers in Iran. Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 The p values of −0.35 and +0.59 obtained from Pearson tests show that there are a negative and positive correlation between the number of workers and the annual and collective doses, respectively. As a result, these overall reductions in doses in the last 3 years indicate that, despite the relative increase in the number of workers, all the attempts in the past years (e.g. improvement in regulations, supervisions, training, dosimetry and communications) have been effective to reduce the dose, and led to improved results in 2014 and the subsequent years. Nevertheless, in order to reach the corresponding worldwide dose values, it seems that it is essential to define a dose constraint for this radiation practice. Frequency distributions and percentiles Table 2 presents various investigation levels along with their percentile and average number of bimonthly investigated workers in each year. The reference dose values were assumed to be: 1 mSv (0.3 × 1/6 × 20 mSv)(2), 2.5 mSv (0.3 × 1/6 × 50 mSv recommended by ICRP 12)(11), 4 mSv (a value used by some countries)(12, 13) and 10 mSv (the typical value of dose constraint)(2). As it is observed, the reference level values of 1 or 2.5 mSv lead to a large number of required investigations (up to 26% of population) in each dosimetry period. By consideration of 4 mSv for investigation level value, 3–7% of population of industrial radiographers has been included for bimonthly investigations. The increasing in the entire percentile indices (p values) in the last 2 years show that the pattern of dose has been shifted from higher to lower dose frequency distributions which is a good result for a radiation practice. The calculated p values of 0.88, 0.67, 0.49 and 0.24 in Pearson tests at the investigation levels of 1, 2.5, 4 and 10 mSv, respectively, demonstrate that, there are positive correlations between the required numbers of investigated workers with those of total population size. On the other hand, In addition to the total number of workers, other more important factors such as safety culture, ability of regulatory body (e.g. designated human resources) along with type and quality of investigation process should be considered determining an optimum reference level in a particular practice. Furthermore, with the assumption of 10 mSv as a value for dose constraint, maximum 2% of workers have been subjected to evaluate their work conditions in optimization of protection program. The average number of 17 workers per 2-month corresponding to this percentile (P98) seems to be fairly reasonable for such study. Table 2. Percentiles of dose values and average number of investigated workers per 2-month with the assumption of different investigation levels. Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Table 2. Percentiles of dose values and average number of investigated workers per 2-month with the assumption of different investigation levels. Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Approaches of optimization One approach to establish the optimum values for reference level and/or dose constraint is to study their retrospective effects on the collective dose in an ideal condition. This ideal condition is defined so that no worker will receive a dose greater than the reference level/dose constraint. By this assumption, all the values of dose greater than a reference levels (or dose constraint) were replaced by (a) the corresponding reference value (or dose constraint) and (b) the average dose of other radiographers (i.e. non-investigated workers) in frequency distributions. Then the ratio of this new virtual (or ideal) collective dose to the true collective dose was used for comparisons. Figure 2 shows this relative collective dose along with the number of dose replacements (or number of investigated workers) at different reference dose values from 1 up to 10 mSv. In both approaches, there is a sharp increase in the relative doses from 1 to 4 mSv, and then the slopes are gradually decreased. In contrast, a sharp decrease is observed in the number of investigated worker by increasing the reference levels at low dose ranges. This means that the greatest impact on the collective dose reduction is related to the frequency distribution of low dose ranges regardless of any selected approaches in dose reduction. Additionally, the comparison between the results of two approaches in Figure 2 demonstrate that, a retrospective dose value based on the average doses of non-investigated workers can be 42% more effective in average than a fixed dose as a dose constraint. Figure 2. View largeDownload slide Relative virtual to true collective doses at different reference dose values from two approaches: (a) a fixed value and (b) an average dose of non-investigated workers, as the reference doses. Figure 2. View largeDownload slide Relative virtual to true collective doses at different reference dose values from two approaches: (a) a fixed value and (b) an average dose of non-investigated workers, as the reference doses. CONCLUSION Statistical results of frequency distributions on recorded dose from a 15-year range in addition to the overall reductions in average annual and collective doses demonstrated that, despite the relative increase in the number of workers, all the past measures have been effective to improve the radiological protection in industrial radiography practice in Iran. Nevertheless, establishing a dose constraint seems to be necessary to continue the optimization of protection and reach the corresponding worldwide dose values in this practice. By using the percentile values of dose distribution frequencies, regulatory bodies can establish a retrospective optimum reference level and/or dose constraint in accordance with the safety culture and their current conditions in planning and implementing of an optimization of protection program. Moreover, this retrospective study on recorded doses becomes clear the critical dose range that has great impact on the collective doses. It may also provide a perspective on the future of collective dose values via definition of a virtual collective dose obtained through the replacement of recorded doses of unusual exposed workers with a fraction of optimum different reference levels or dose constraint values. ACKNOWLEDGEMENTS The authors wish to thank Mrs Shadi Nemati from INRA for providing the raw dose data. REFERENCES 1 UNSCEAR—United Nations Scientific Committee on the Effects of Atomic Radiation. Annex E, Occupational radiation exposures ( 2008 ). 2 NEA—Nuclear Energy Agency. Dose constraints—dose constraints in optimisation of occupational radiation protection and implementation of the dose constraint concept into radiation protection regulations and its use in operators’ practices, NEA/CRPPH/R ( 2011 ). 3 Johns , T. F. Investigation levels and reporting level . Radiat. Prot. Dosim. 2 ( 1 ), 2 ( 1982 ). Google Scholar CrossRef Search ADS 4 ICRP—International Commission on Radiological Protection, ICRP 26. Recommendations of the international commission on radiological protection ( 1977 ). 5 ICRP—International Commission on Radiological Protection, ICRP 60. 1990 Recommendations of the international commission on radiological protection ( 1991 ). 6 Jafarizadeh , M. et al. . Occupational dose assessment and National Dose Registry System in Iran . Radiat. Prot. Dosim. 144 ( 1–4 ), 52 – 55 ( 2010 ). 7 Hosseini Pooya , S. M. , Mianji , F. , Kardan , M. R. and Rastkhah , N. Quantifiable technical aspects of a quality management system for TL personal dosimetry services. In: Proceeding of the Third International Conference on Radiation and Application in Various Fields of Research, Budva, Montenegro, pp. 239–242 ( 2015 ). 8 ICRP—International Commission on Radiological Protection, ICRP 103. The 2007 recommendations of the international commission on radiological protection ( 2007 ). 9 Mianji , F. , Hosseini Pooya , S. M. , Zakeri , F. and Dashtipour , M. R. A root cause analysis of the high occupational doses of industrial radiographers in Iran . J. Radiol. Prot. 36 , 184 – 194 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 10 Schmidt , M. J. Understanding and Using Statistics: Basic Concepts ( Washington, DC: D.C. Health ) pp. 326 – 327 ( 1975 ). 11 ICRP—International Commission on Radiological Protection, ICRP 12. General principles of monitoring for radiation protection of workers ( 1969 ). 12 Economides , S. , Tritakis , P. , Papadomarkaki , E. , Carinou , E. , Hourdakis , C. , Kamenopoulou , V. and Dimitriou , P. Occupational exposure in Greek industrial radiography laboratories (1996–2003) . Radiat. Prot. Dosim. 118 ( 3 ), 260 – 264 ( 2006 ). Google Scholar CrossRef Search ADS 13 Code of safe practice for the use of x-rays and radioactive material in industrial radiography. CSP9, ISSN 0110–9316, Published by Ministry of Health, New Zealand ( 2010 ). © The Author(s) 2018. Published by Oxford University Press. 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DOSIMETRIC ASPECTS OF OPTIMIZATION OF PROTECTION IN IRAN INDUSTRIAL RADIOGRAPHY PRACTICE

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Abstract

Abstract According to the worldwide reports, industrial radiography is one of the highest risk radiation practices due to handling high activity sources. Therefore, an optimization of protection along with appropriate investigation level and/or dose constraint is required to achieve appropriate radiological safety. This research, presents some statistical aspects of this optimization using frequency distributions and percentiles of individual recorded dose of industrial radiographers from the years 2002 to 2016 in Iran. The results show that, considering an investigation level of 4 mSv for bimonthly monitoring periods, 3–7% of population of workers has been investigated during these years. Moreover, an overall reduction on average annual and collective doses is observed, despite of the relative increasing in the number of workers. Since the frequency of periodical recorded dose at lower dose range (e.g. <4 mSv) causes greater impacts on the reduction of collective dose than the higher ranges, a retrospective average dose of non-investigated workers can also be substitute with a constant value as a dose constraint. It can be concluded that all the past measures of regulatory body and attempts of the employers have been effective improving the radiological protection in this practice in Iran. However, establishing a dose constraint seems to be essential to continue and improve this optimization of protection. INTRODUCTION Among all of the medical and industrial radiation practices, radiography non-destructive tests is one of the highest risk due to handling high activity sources. According to the UNSCEAR report(1), the worldwide average of annual collective effective dose of industrial radiographers over 5-year period (1990–95) has been estimated to be ~170 man Sv. Furthermore, the corresponding annual effective dose of measurably workers calculated to be ~3.17 mSv. These dose values are greater than those of most other medical or industrial radiation practices. One of the effective factors on any optimization of protection and reduction of averaged dose is triggering of some interventional actions for the workers whose doses values are greater than a reference level, and/or establishing a dose constraint for a particular radiation practice to evaluate the work conditions. The related actions may be varied from a simple alarm announce, to a temporary or permanent work suspension for the workers. Therefore, the quality of optimization, which itself is affected by the number of investigated workers, is crucial in any policies. Therefore, the first step of this optimization of protection is establishing a reference level (i.e. an investigation level) and/or setting a dose constraint.(2) Investigation level is a reference level which first was recommended by the International commission on Radiological Protection (ICRP 12) and revised in the later publications.(3) The ICRP 26(4) defined the Investigation level as ‘values of dose equivalent or intake above which the results are considered sufficiently important to justify further investigations’. ICRP also gives general guidance on investigation levels as the 3-10th of the annual dose limits corresponding to the fraction of a year to which the individual monitoring measurement refers. The concept of reference level arises from the ALARA principle and no scientific basis has been reported for this recommended ICRP values. Despite the ICRP recommendation, different investigation level values have been used for various radiation practices by countries or regulatory bodies. The concept of dose constraint was first introduced by ICRP-60(5), and means a value (typically 10 mSv/y)(2) below which it should be planned to keep all doses. Many countries have been used this concept in their optimization of protections program, however some other not for all but for some practices.(2) In this research, first the frequency distributions of the individual and collective effective doses are derived from a 15-year recorded dose data of industrial radiographers of Iran. Then, some statistical aspects and impacts of these dose distributions on the optimization of protection are investigated. MATERIALS AND METHODS Dosimetry method Individual monitoring of industrial workers are carried out bimonthly in Iran. By 2012, this monitoring program has been provided directly by the regulatory body for averaged number of ~1500 workers per year, and continued by private service providers under supervision of the regulatory authority. At present, there are ~200 industrial radiography facilities with ~2000 workers in Iran. Individual doses have been measured using TLD-100® with hot-gas model of Harshaw-6600® card readers and recorded via a national dose registry system.(6) All dosimetry Laboratories have been accredited to ISO-17025 standard criteria in recent years.(7) Optimization of protection According to the national regulations, the recording and investigation levels have been set to 0.1 and 4 mSv values respectively for industrial radiography practice on bimonthly dose assessment periods. The value of 4 mSv has been established based upon the previous experiences on the potential unusual exposures, performable number of investigations with consideration of the bimonthly official correspondence volumes, and safety culture in this practice. The value of 0.1 mSv has been used based upon the minimum measurable dose (MMD) of the dosimetry system (with 99% confidence level). Moreover, the recommended ICRP103(8) annual dose limits for occupational exposure are followed by the regulatory body. According to the previous worldwide and/or the national experiences, the most reasons for such unusual exposures are including(9): No proper use of collimators. Dose rate at the boundary of the work site not within limits set. No proper use of survey meters. No pre-operation specific equipment checks being performed. Poor operator knowledge of procedures. No proper warning system to prevent entry to the work site. Poor emergency preparedness. No proper use of alarm systems. Excess monthly duty hours. Working in elevated levels (hard to keep distance from the source during exposures). Accidents related to failure in the radiation source container (gamma projector), guide tube or remote control. Lack of or failure of survey meters or personal alarming dosimeters. Incorrect use of passive dosimeters (TLD). Placing the personal dosimeter near the source container during transportation. TLD falling near the unshielded source during radiography. Using the passive dosimeter for too long (more than 2 months). Once recording a dose above the investigation level, a procedure is triggered for the worker. A flowchart diagram of this procedure is shown in Figure 1. The first step of this interventional action is to submit a one-page questionary form to the employer. This form is divided by four parts including: General information about the employee, radiation sources and any record of direct reading dosimeter: must be filled by the associated health physicist. The reason of exposure in view point of employee: must be filled and signed by the employee. The reason of exposure in view point of employer: must be filled and signed by the health physicist (or any responsible competent liaison). The conclusion and final decision on the true recording dose: must be filled and signed by the regulatory authority. Figure 1. View largeDownload slide Diagram of the investigation procedure for the workers who receive unusual exposures in Iran. Figure 1. View largeDownload slide Diagram of the investigation procedure for the workers who receive unusual exposures in Iran. In special case of a periodical dose of more than the annual dose limit, a temporary work suspension is prescribed for the worker in addition to the questionary form. Special interviews are mandatory, and medical examinations may be required in these situations. However, the employee may continue his/her work after receiving some guidance on radiological protection and signing a written undertaking to radiation risk awareness. In critical case of periodical dose of more than 100 mSv, first a temporary work suspension (short term) is implemented for the worker in addition to the questionary form. Then, a special interviews and medical examinations (including cytogenetic dosimetry) must be carried out in these situations. If deterministic effects are indicated as the results of medical examinations and inspections, a long term (e.g. some months) or even permanent work suspensions may be implemented for the worker. Moreover, depending on the case, the employer may receive some official warnings or subjected to punitive measures by the regulatory bodies when they are culpable to the overexposure. Statistic method In this research, the bimonthly dose records of industrial radiographers from 2002 to 2016 were used as the data. Then, the numbers of dose values were counted in each 1 mSv dose intervals from 0 up to maximum recorded dose value in 1 year. The percentile value of Pn corresponds with the dose value under which the %n of population of industrial radiographers have received the dose, and calculated as follows(10): Pn=Li+(Ui−Li)(n×N100−CFiFi) (1) where N is the total number of recorded dose, Li is the lower, and Ui is the upper limits of the ith critical dose interval (e.g. in third interval, L3 = 2 mSv and U3 = 3 mSv), Fi is the frequency in the ith critical dose interval that covers n% of population, and CFi−1 is the cumulative frequency of the (i − 1)th critical dose interval. RESULTS AND DISCUSSION Individual and collective dose data Table 1 presents the averaged bimonthly and annual received doses of radiographers as well as the collective doses which have been recorded from 2002 to 2016. These dose data are associated with the routine radiography activities as well as all incidents. The average annual dose values for the first, second and third 5 years have been reduced and calculated to be 7.4, 6.8 and 4.8 mSv, respectively. The bimonthly average dose of workers also has been reduced and kept to <1 mSv in the last 5 years. The collective dose values for the first, second and third 5 years have been calculated to be 8.10, 9.22 and 8.24 man-Sv, respectively. There is a significant falling in value of dose in 2012 due to transferring dosimetry services from regulatory body to private service providers in which numbers of the workers’ dosimeters have not been delivered to the service provider for readouts. Table 1. Annual and collective doses of industrial radiographers in Iran. Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 Table 1. Annual and collective doses of industrial radiographers in Iran. Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 Year Average number of measurably workers Bimonthly average dose (mSv) Annual dose (mSv) Collective dose (man-Sv) 2002 541 1.02 6.14 3.3 2003 990 0.93 5.56 5.5 2004 1153 1.43 8.58 10.0 2005 1265 1.44 8.65 10.9 2006 1347 1.34 8.01 10.8 2007 1457 0.97 5.84 8.5 2008 1377 1.07 6.45 8.9 2009 1310 1.18 7.11 9.3 2010 1379 1.25 7.51 10.3 2011 1309 1.16 6.95 9.1 2012 1150 0.67 4.05 4.7 2013 1690 0.96 5.70 9.6 2014 1955 0.81 4.90 9.6 2015 1895 0.77 4.61 8.7 2016 1833 0.78 4.67 8.6 The p values of −0.35 and +0.59 obtained from Pearson tests show that there are a negative and positive correlation between the number of workers and the annual and collective doses, respectively. As a result, these overall reductions in doses in the last 3 years indicate that, despite the relative increase in the number of workers, all the attempts in the past years (e.g. improvement in regulations, supervisions, training, dosimetry and communications) have been effective to reduce the dose, and led to improved results in 2014 and the subsequent years. Nevertheless, in order to reach the corresponding worldwide dose values, it seems that it is essential to define a dose constraint for this radiation practice. Frequency distributions and percentiles Table 2 presents various investigation levels along with their percentile and average number of bimonthly investigated workers in each year. The reference dose values were assumed to be: 1 mSv (0.3 × 1/6 × 20 mSv)(2), 2.5 mSv (0.3 × 1/6 × 50 mSv recommended by ICRP 12)(11), 4 mSv (a value used by some countries)(12, 13) and 10 mSv (the typical value of dose constraint)(2). As it is observed, the reference level values of 1 or 2.5 mSv lead to a large number of required investigations (up to 26% of population) in each dosimetry period. By consideration of 4 mSv for investigation level value, 3–7% of population of industrial radiographers has been included for bimonthly investigations. The increasing in the entire percentile indices (p values) in the last 2 years show that the pattern of dose has been shifted from higher to lower dose frequency distributions which is a good result for a radiation practice. The calculated p values of 0.88, 0.67, 0.49 and 0.24 in Pearson tests at the investigation levels of 1, 2.5, 4 and 10 mSv, respectively, demonstrate that, there are positive correlations between the required numbers of investigated workers with those of total population size. On the other hand, In addition to the total number of workers, other more important factors such as safety culture, ability of regulatory body (e.g. designated human resources) along with type and quality of investigation process should be considered determining an optimum reference level in a particular practice. Furthermore, with the assumption of 10 mSv as a value for dose constraint, maximum 2% of workers have been subjected to evaluate their work conditions in optimization of protection program. The average number of 17 workers per 2-month corresponding to this percentile (P98) seems to be fairly reasonable for such study. Table 2. Percentiles of dose values and average number of investigated workers per 2-month with the assumption of different investigation levels. Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Table 2. Percentiles of dose values and average number of investigated workers per 2-month with the assumption of different investigation levels. Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Year Values of reference level 1 mSv 2.5 mSv 4 mSv 10 mSv Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations Percentile Average number of bimonthly investigations 2002 P81 81 P93 32 P96 17 P99 5 2003 P77 223 P91 89 P96 41 P99 10 2004 P75 305 P88 142 P93 71 P98 18 2005 P74 323 P89 144 P93 83 P98 24 2006 P78 303 P89 148 P94 87 P98 25 2007 P80 298 P91 128 P95 72 P99 17 2008 P80 271 P91 128 P95 74 P98 23 2009 P82 237 P91 122 P94 75 P98 27 2010 P78 302 P89 150 P93 93 P98 29 2011 P78 287 P90 123 P95 65 P99 18 2012 P83 190 P93 76 P97 40 P99 7 2013 P77 386 P91 159 P95 78 P99 20 2014 P80 392 P93 141 P97 66 P99 14 2015 P82 344 P93 127 P97 62 P99 13 2016 P82 328 P93 119 P97 57 P99 14 Approaches of optimization One approach to establish the optimum values for reference level and/or dose constraint is to study their retrospective effects on the collective dose in an ideal condition. This ideal condition is defined so that no worker will receive a dose greater than the reference level/dose constraint. By this assumption, all the values of dose greater than a reference levels (or dose constraint) were replaced by (a) the corresponding reference value (or dose constraint) and (b) the average dose of other radiographers (i.e. non-investigated workers) in frequency distributions. Then the ratio of this new virtual (or ideal) collective dose to the true collective dose was used for comparisons. Figure 2 shows this relative collective dose along with the number of dose replacements (or number of investigated workers) at different reference dose values from 1 up to 10 mSv. In both approaches, there is a sharp increase in the relative doses from 1 to 4 mSv, and then the slopes are gradually decreased. In contrast, a sharp decrease is observed in the number of investigated worker by increasing the reference levels at low dose ranges. This means that the greatest impact on the collective dose reduction is related to the frequency distribution of low dose ranges regardless of any selected approaches in dose reduction. Additionally, the comparison between the results of two approaches in Figure 2 demonstrate that, a retrospective dose value based on the average doses of non-investigated workers can be 42% more effective in average than a fixed dose as a dose constraint. Figure 2. View largeDownload slide Relative virtual to true collective doses at different reference dose values from two approaches: (a) a fixed value and (b) an average dose of non-investigated workers, as the reference doses. Figure 2. View largeDownload slide Relative virtual to true collective doses at different reference dose values from two approaches: (a) a fixed value and (b) an average dose of non-investigated workers, as the reference doses. CONCLUSION Statistical results of frequency distributions on recorded dose from a 15-year range in addition to the overall reductions in average annual and collective doses demonstrated that, despite the relative increase in the number of workers, all the past measures have been effective to improve the radiological protection in industrial radiography practice in Iran. Nevertheless, establishing a dose constraint seems to be necessary to continue the optimization of protection and reach the corresponding worldwide dose values in this practice. By using the percentile values of dose distribution frequencies, regulatory bodies can establish a retrospective optimum reference level and/or dose constraint in accordance with the safety culture and their current conditions in planning and implementing of an optimization of protection program. Moreover, this retrospective study on recorded doses becomes clear the critical dose range that has great impact on the collective doses. It may also provide a perspective on the future of collective dose values via definition of a virtual collective dose obtained through the replacement of recorded doses of unusual exposed workers with a fraction of optimum different reference levels or dose constraint values. ACKNOWLEDGEMENTS The authors wish to thank Mrs Shadi Nemati from INRA for providing the raw dose data. REFERENCES 1 UNSCEAR—United Nations Scientific Committee on the Effects of Atomic Radiation. Annex E, Occupational radiation exposures ( 2008 ). 2 NEA—Nuclear Energy Agency. Dose constraints—dose constraints in optimisation of occupational radiation protection and implementation of the dose constraint concept into radiation protection regulations and its use in operators’ practices, NEA/CRPPH/R ( 2011 ). 3 Johns , T. F. Investigation levels and reporting level . Radiat. Prot. Dosim. 2 ( 1 ), 2 ( 1982 ). Google Scholar CrossRef Search ADS 4 ICRP—International Commission on Radiological Protection, ICRP 26. Recommendations of the international commission on radiological protection ( 1977 ). 5 ICRP—International Commission on Radiological Protection, ICRP 60. 1990 Recommendations of the international commission on radiological protection ( 1991 ). 6 Jafarizadeh , M. et al. . Occupational dose assessment and National Dose Registry System in Iran . Radiat. Prot. Dosim. 144 ( 1–4 ), 52 – 55 ( 2010 ). 7 Hosseini Pooya , S. M. , Mianji , F. , Kardan , M. R. and Rastkhah , N. Quantifiable technical aspects of a quality management system for TL personal dosimetry services. In: Proceeding of the Third International Conference on Radiation and Application in Various Fields of Research, Budva, Montenegro, pp. 239–242 ( 2015 ). 8 ICRP—International Commission on Radiological Protection, ICRP 103. The 2007 recommendations of the international commission on radiological protection ( 2007 ). 9 Mianji , F. , Hosseini Pooya , S. M. , Zakeri , F. and Dashtipour , M. R. A root cause analysis of the high occupational doses of industrial radiographers in Iran . J. Radiol. Prot. 36 , 184 – 194 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 10 Schmidt , M. J. Understanding and Using Statistics: Basic Concepts ( Washington, DC: D.C. Health ) pp. 326 – 327 ( 1975 ). 11 ICRP—International Commission on Radiological Protection, ICRP 12. General principles of monitoring for radiation protection of workers ( 1969 ). 12 Economides , S. , Tritakis , P. , Papadomarkaki , E. , Carinou , E. , Hourdakis , C. , Kamenopoulou , V. and Dimitriou , P. Occupational exposure in Greek industrial radiography laboratories (1996–2003) . Radiat. Prot. Dosim. 118 ( 3 ), 260 – 264 ( 2006 ). Google Scholar CrossRef Search ADS 13 Code of safe practice for the use of x-rays and radioactive material in industrial radiography. CSP9, ISSN 0110–9316, Published by Ministry of Health, New Zealand ( 2010 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com

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Radiation Protection DosimetryOxford University Press

Published: Feb 9, 2018

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