- Email: [email protected]
Liquid discharges from the use of radionuclides in medicine (diagnosis)
Available online at www.sciencedirect.com
Journal of Environmental Radioactivity 99 (2008) 1535e1538 www.elsevier.com/locate/jenvrad
Liquid discharges from the use of radionuclides in medicine (diagnosis) R. Barquero a,*, M.M. Agulla b, A. Ruiz b a
Servicio de Radiofı´sica y Proteccio´n Radiolo´gica, Hospital Universitario Rı´o Hortega, E-47010 Valladolid, Spain b Servicio de Radiofı´sica y Proteccio´n Radiolo´gica, Hospital Clı´nico Universitario, E-47005 Valladolid, Spain Available online 20 February 2008
Abstract The production and discharge of liquid radioactive wastes as excreta from patients undergoing Nuclear Medicine Diagnostic (NMD) in a hospital were studied. Instantaneous and accumulated activity, discharged from the hospital to the sewage system, has been estimated keeping in mind radionuclide decay. This study would enable estimation of the environmental impact due to NMD procedures. Annual accumulated activities of 2.2 GBq (131I), 1.847 GBq (99mTc), 0.743 GBq (123I), 0.337 GBq (67Ga), 0.169 GBq (111In) and 0.033 GBq (201Tl) result from our model when applied to a European hospital. A comparison is made with calculations by other authors that do not consider the radionuclide decay and who overestimate by two orders of magnitude. Doses to critical people as sewage treatment workers are also significantly reduced. So, our results stress the importance of including the decay in the calculations. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Accumulated activities; Radionuclides; Diagnosis nuclear medicine
1. Introduction Within man made radioactive environment impact, nuclear medicine techniques producing liquid radioactive wastes play a significant role. These techniques include both diagnostic and therapeutic procedures. This paper focuses diagnostic whereas another ‘‘twin’’ work (Barquero et al., 2008) is specifically dedicated to waste discharges coming from therapy. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimated that, worldwide, more than 30 million diagnostic nuclear medicine procedures with radiopharmaceuticals are carried out each year (UNSCEAR, 2000). In these procedures radionuclides are administered to patients to map the radiopharmaceutical uptake in the body as the image for diagnostic. The activity, which is not retained in their organs or systems, is excreted through the patient urinary system to the sewage.
* Corresponding author. Tel.: þ34 983478862, þ34 630752261 (mobile); fax: þ34 983257511. E-mail address: [email protected] (R. Barquero). 0265-931X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2007.12.009
The Euratom Directive 96/29 (Euratom_Directive_96_29) sets, as condition for the operational protection of the population, the examination and approval of plans for the discharge of radioactive effluents. The OSPAR Commission for the Protection of Environment of the North-east Atlantic about radioactive substances sets the objectives of preventing the exposition of population to radioactive effluents and the pollution of the aquatic environment from ionising radiation through reductions of discharges and emission close to zero for artificial substances. Although the criteria of 1992 OSPAR Convention (OSPAR_Convention) include requirements of the best practicable techniques and environmental practice in order to limiting discharges emissions and waste, the accuracy of discharge evaluation receives actually little attention and better estimation techniques are needed. In fact, meeting Best Practicable Means (BPM) of minimising radioactive waste could be considered essential. This work would enable estimations, based in a set of more complex assumptions, for each one of the radionuclides used in a NMD department of a reference hospital in Europe. Our aim is to improve the coarse previous estimations and to investigate the need of storage tanks to limit the doses to critical
R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1535e1538
1536
Table 1 Data and results of annual accumulated activity for each radionuclide Radionuclide
Input activity A0 (Bq)
Half-life T1/2 (h)
Elimination factor felim
Discharged activity Ae (Bq)
Time between patients t1 (h)
Patients per day n
Accumulated activity Ayear (Bq)
67
1.11 108 5.00 108 2.96 108 5.00 107 5.55 108 2.00 108
78.24 13.20 192.96 67.92 6.02 72.96
0.7 0.7 0.8 0.3 0.3 0.04
7.634 107 3.151 108 2.351 108 1.470 107 1.323 108 7.849 106
24 7 24 3.5 0.39 24
1 2 1 3 19 1
3.37 108 7.43 108 2.20 109 1.688 108 1.847 109 3.284 107
Ga I 131 I 111 In 99m Tc 201 Tl 123
people. The main ingredient in this work is the inclusion of the radioactive decay into all models, and this would effectively reduce estimated discharges by significant amounts. Specifically, the discharged activity from our hospital for each radionuclide is estimated with a fully radioactive decay model. The results are compared with general and usual estimations made by a national task group formed by the Spanish national scientific societies (radiation protection and medical physics) and the Regulatory Organism (SEFM_2002) that do not include the radioactive decay.
A set of equations has been obtained to estimate the instantaneous and accumulated (along a day, a week and a year) activity discharged to the sewage system immediately after each patient micturition. These calculations were carried out for 67Ga, 123I, 131I, 111In, 99mTc and 201Tl, which are commonly utilized in NMD. It has been considered that a certain activity is administered to each patient. This activity is the same for each patient but it is different for each radionuclide. It was also considered that the number of patients per year is similar to that utilized by SEFM (2002) which means that the elimination factors are the same. These input values have been proposed by SEFM (2002) as typical values for Spanish hospitals. It has also been kept in mind that each patient eliminates the activity 2 h after administration, as one could see in the mentioned reference. To calculate the activities the radionuclides decay properties have been included. The estimated number of weeks is 52 and the number of working days per week is 5. Therefore, the estimated number of patients per year is 260n, where n is the number of patients per day and per each radionuclide. The time elapsed between consecutive patients along day in the NMD department was assumed to be constant. Accumulated activity along a day, a week and a year is calculated at the end of the day (w17:00 pm), at the end of the week (Friday, w17:00 pm) and at the end of the year (52nd week, Friday, w17:00 pm).
2.1. Activities calculations The activity discharged (or eliminated activity Ae in the equations) by the patient can be estimated using Eq. (1). ð1Þ
where A0 is the input activity injected to each patient, felim is the elimination factor of injected activity, l is the decay constant (in hours), t0 ¼ 2 h is an estimation of the time between patient injection and patient micturition. Accumulated activity at the end of the day (w17:00 pm) can be estimated by means of Eq. (2). Aday ¼
n1 X
Ae eilt1
Aweek ¼
4 X
Aday ejlt2
ð3Þ
j¼0
where t2 ¼ 24 h, being the time between a day and the next one. Finally, accumulated activity at the end of the year (52nd week, Friday, w17:00 pm) can be estimated using Eq. (4). Ayear ¼
m1 X
Aweek eklt3
ð4Þ
k¼0
where m is the estimated number of weeks per year and t3 ¼ 168 h is the time between a Friday and the next one. The activity accumulated along the year is calculated with Eq. (5).
2. Materials and methods
Ae ¼ A0 felim elt0
Accumulated activity at the end of the week (Friday, w17:00 pm) can be estimated using Eq. (3).
ð2Þ
P Ayear ¼ Ae
n1 ilt1 i¼0 e
P
4 jl$24 j¼0 e
1 el$168
ð5Þ
where the number of weeks m is considered very large (m / N). Any calculation software can be used to solve Eq. (5).
3. Results and discussion Data for each radionuclide and results of annual accumulated activity can be seen in Table 1, and the discharged activity along a year for each radionuclide is shown in Fig. 1. Each point represents the accumulated activity at the end of each week (Friday, 17:00 pm).1 This activity grows in the first weeks but rapidly reaches an asymptotic value. We can see that this accumulated activity along a year reaches the maximum in 3 or 4 weeks for all radionuclides, except for 131I that takes from 8 or 9 weeks, due to its longer half-life. The accumulated activity at the end of the day along a year for 131 I is shown in Fig. 2, being similar to the evolution of the rest of radionuclides. It can be noticed that at the end of the week, the activity decreases because no diagnostic procedure with radionuclides is made. So, although the accumulated activity is not constant every day, it reaches, in a few weeks, an asymptotic and maximum value at the end of the week. The largest discharged activity is due to 131I because its longer half-life and for 99mTc because is the most used radionuclide in a NMD department. To compare the results here obtained with those published by SEFM (2002), the last ones have been modified in order to
i¼0
where n is the number of patients per day (and per each radionuclide) and t1 is the time between a patient and the next one.
1 We consider that in radionuclide diagnostic procedure with 1 patient/day, the activity A0 is injected at 15:00 pm, so activity Ae is discharged at 17:00 pm.
R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1535e1538
1537
Table 2 Comparison of annual accumulated activities for each radionuclide Radionuclide
Accumulated activity (Bq/year)
Accumulated activity SEFM (2002) (Bq/year)
Factor
67
Ga I 131 I 111 In 99m Tc 201 Tl
3.37 108 7.43 108 2.20 109 1.688 108 1.847 109 3.284 107
1.98 1010 1.64 1011 6.11 1010 1.15 1010 6.53 1011 2.04 109
58.75 220.73 27.77 68.13 353.55 62.12
Total
5.329 109
9.12 1011
171.14
123
Fig. 1. Evolution of discharged activity along a year for each radionuclide.
account of patients per year and per radionuclide to match those used in this study. This comparison is shown in Table 2. It can be noticed that annual accumulated activities are 28- to 354-times less (depending on each radionuclide) than those calculated in SEFM (2002). The differences with the conventional calculations is due to SEFM did not included the radionuclide decay. This overestimates the total accumulated activity along year by 171-fold. As the half-life of radionuclide tends to be shorter, the difference tends to increase. Knowledge of the sewage activity is useful in establishing the radiological consequences of discharges. The IAEA-TECDOC-1000 (IAEA_1000) and SEFM (2002) set radiation doses assessment guidelines for liquid radioactive releases. The main exposure pathway for liquid discharges of the radionuclides used in a NMD department is the external exposure of sewage system worker. The effective doses of the sewage worker as critical individual have been calculated using the MICROSHIELD code (MICROSHIELD) with simplified
assumptions: a worker remains 1500 h per year at a distance of 2 m of the sludge treatment tank in which the discharges has been accumulated. In terms of effective dose, 131I and 67Ga have the larger contribution mainly due to their half-life and emission energies, whereas 99mTc has the smallest contribution to effective dose. The comparison between accumulated doses along a year to critical people (sewage treatment worker) here obtained with that published by SEFM (2002) is shown in Table 3. 4. Conclusions The benefits of including radioactive decay in waste modelling are illustrated with the results obtained: the total accumulated activity throughout the year obtained here is only 0.58% of the SEFM calculations. The doses to critical people (sewage treatment worker) are 29 mSv, which is less than the established effluent limit of 100 mSv (SEFM_2002), whereas in SEFM these doses are 1068 mSv (Table 3). This study clearly shows that in our hospital there is no need to have storage tanks for these discharges because the more exposed individual receives very low doses and the time radionuclides take for the excreta of patients to be processed and returned to the ecosystem are longer than its half lives (ICRP_94). In all cases, considering the effective dose of sewage worker and accumulated activity throughout a year, 131I is the more restrictive radionuclide due to its longer half-life and its larger dose factor related to its emission energies. This decay model could be used in a ‘‘worst case’’ situation to submit to authorities for approval for liquid waste disposal, and then used retrospectively for accurate calculations of Table 3 Accumulated dose along a year to critical people (sewage treatment worker) Radionuclide
Dose factor (MICROSHIELD) (mSv/Bq)
67
1.31 108 6.07 1010 1.05 108 4.38 109 5.94 1013 7.44 109
Ga I 131 I 111 In 99m Tc 201 Tl 123
Fig. 2. Evolution of accumulated activity at the end of the day along a year for 131I.
Total
Accumulated dose SEFM (2002) (mSv/year)
Accumulated dose (mSv/year)
259 99.5 644 50.2 0.39 15.2
4.398 0.451 23.189 0.740 0.001 0.244
1068.29
29.023
1538
R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1535e1538
liquid waste levels once actual patient numbers were known. The results show how the discharges increase approximately in the same factor as the number of patients, as one can see in Eq. (5). Then, the maximum workload in the NMD department must be considered in the pre-operational situation, in order to estimate with realistic assumptions the maximum effective dose of sewage worker. This maximum must be below 100 mSv, the effluent limit established by the regulatory authority. In operational time the accurate calculations can be done using Eqs. (1)e(5) anytime if required. The authorities could indicate when these calculations must be done in each NMD department, for example on a yearly basis. References Barquero, R., Basurto, F., Nu~nez, C., Esteband, R., 2008. Liquid discharges from patients undergoing I-131 treatments. Journal of Environmental Radioactivity 99, 1530e1534.
Chapel, M., Ferrer, N., Ramos, L.M., Sanchez, M., 2002. Grupo de efluentes del Foro de Proteccio´n Radiolo´gica en el medio hospitalario. SEFM. http://www.sefm.es/docs/actsefm/informefinalgrupoefluentes. pdf. Council 96/29/EURATOM laying down for the protection of the health of workers and general public against the dangers arising for ionizing radiation. May 13, 1996. Clearance of materials resulting from the use of radionuclides in medicine, industry and research: IAEA- TEC-DOC 1000, 1998. International Commission on Radiological Protection, 2004. Release of patients after therapy with unsealed radionuclides. ICRP Publication 94. MICROSHIELD: computer programme for analysing shielding and estimate exposure from gamma radiation. http://www.radiationsoftware.com/ mshield.html. 1992 OSPAR Convention Appendix 1: Criteria for the definition of practices and techniques mentioned in paragraph 3(b)(i) of article 2 of the convention. http://www.ospar.org/eng/html/convention/ospar. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2000. Sources and Effects of Ionising Radiation. 2000 Report to the General Assembly with Annexes, United Nations, Vienna.
Journal of Environmental Radioactivity 99 (2008) 1535e1538 www.elsevier.com/locate/jenvrad
Liquid discharges from the use of radionuclides in medicine (diagnosis) R. Barquero a,*, M.M. Agulla b, A. Ruiz b a
Servicio de Radiofı´sica y Proteccio´n Radiolo´gica, Hospital Universitario Rı´o Hortega, E-47010 Valladolid, Spain b Servicio de Radiofı´sica y Proteccio´n Radiolo´gica, Hospital Clı´nico Universitario, E-47005 Valladolid, Spain Available online 20 February 2008
Abstract The production and discharge of liquid radioactive wastes as excreta from patients undergoing Nuclear Medicine Diagnostic (NMD) in a hospital were studied. Instantaneous and accumulated activity, discharged from the hospital to the sewage system, has been estimated keeping in mind radionuclide decay. This study would enable estimation of the environmental impact due to NMD procedures. Annual accumulated activities of 2.2 GBq (131I), 1.847 GBq (99mTc), 0.743 GBq (123I), 0.337 GBq (67Ga), 0.169 GBq (111In) and 0.033 GBq (201Tl) result from our model when applied to a European hospital. A comparison is made with calculations by other authors that do not consider the radionuclide decay and who overestimate by two orders of magnitude. Doses to critical people as sewage treatment workers are also significantly reduced. So, our results stress the importance of including the decay in the calculations. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Accumulated activities; Radionuclides; Diagnosis nuclear medicine
1. Introduction Within man made radioactive environment impact, nuclear medicine techniques producing liquid radioactive wastes play a significant role. These techniques include both diagnostic and therapeutic procedures. This paper focuses diagnostic whereas another ‘‘twin’’ work (Barquero et al., 2008) is specifically dedicated to waste discharges coming from therapy. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimated that, worldwide, more than 30 million diagnostic nuclear medicine procedures with radiopharmaceuticals are carried out each year (UNSCEAR, 2000). In these procedures radionuclides are administered to patients to map the radiopharmaceutical uptake in the body as the image for diagnostic. The activity, which is not retained in their organs or systems, is excreted through the patient urinary system to the sewage.
* Corresponding author. Tel.: þ34 983478862, þ34 630752261 (mobile); fax: þ34 983257511. E-mail address: [email protected] (R. Barquero). 0265-931X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2007.12.009
The Euratom Directive 96/29 (Euratom_Directive_96_29) sets, as condition for the operational protection of the population, the examination and approval of plans for the discharge of radioactive effluents. The OSPAR Commission for the Protection of Environment of the North-east Atlantic about radioactive substances sets the objectives of preventing the exposition of population to radioactive effluents and the pollution of the aquatic environment from ionising radiation through reductions of discharges and emission close to zero for artificial substances. Although the criteria of 1992 OSPAR Convention (OSPAR_Convention) include requirements of the best practicable techniques and environmental practice in order to limiting discharges emissions and waste, the accuracy of discharge evaluation receives actually little attention and better estimation techniques are needed. In fact, meeting Best Practicable Means (BPM) of minimising radioactive waste could be considered essential. This work would enable estimations, based in a set of more complex assumptions, for each one of the radionuclides used in a NMD department of a reference hospital in Europe. Our aim is to improve the coarse previous estimations and to investigate the need of storage tanks to limit the doses to critical
R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1535e1538
1536
Table 1 Data and results of annual accumulated activity for each radionuclide Radionuclide
Input activity A0 (Bq)
Half-life T1/2 (h)
Elimination factor felim
Discharged activity Ae (Bq)
Time between patients t1 (h)
Patients per day n
Accumulated activity Ayear (Bq)
67
1.11 108 5.00 108 2.96 108 5.00 107 5.55 108 2.00 108
78.24 13.20 192.96 67.92 6.02 72.96
0.7 0.7 0.8 0.3 0.3 0.04
7.634 107 3.151 108 2.351 108 1.470 107 1.323 108 7.849 106
24 7 24 3.5 0.39 24
1 2 1 3 19 1
3.37 108 7.43 108 2.20 109 1.688 108 1.847 109 3.284 107
Ga I 131 I 111 In 99m Tc 201 Tl 123
people. The main ingredient in this work is the inclusion of the radioactive decay into all models, and this would effectively reduce estimated discharges by significant amounts. Specifically, the discharged activity from our hospital for each radionuclide is estimated with a fully radioactive decay model. The results are compared with general and usual estimations made by a national task group formed by the Spanish national scientific societies (radiation protection and medical physics) and the Regulatory Organism (SEFM_2002) that do not include the radioactive decay.
A set of equations has been obtained to estimate the instantaneous and accumulated (along a day, a week and a year) activity discharged to the sewage system immediately after each patient micturition. These calculations were carried out for 67Ga, 123I, 131I, 111In, 99mTc and 201Tl, which are commonly utilized in NMD. It has been considered that a certain activity is administered to each patient. This activity is the same for each patient but it is different for each radionuclide. It was also considered that the number of patients per year is similar to that utilized by SEFM (2002) which means that the elimination factors are the same. These input values have been proposed by SEFM (2002) as typical values for Spanish hospitals. It has also been kept in mind that each patient eliminates the activity 2 h after administration, as one could see in the mentioned reference. To calculate the activities the radionuclides decay properties have been included. The estimated number of weeks is 52 and the number of working days per week is 5. Therefore, the estimated number of patients per year is 260n, where n is the number of patients per day and per each radionuclide. The time elapsed between consecutive patients along day in the NMD department was assumed to be constant. Accumulated activity along a day, a week and a year is calculated at the end of the day (w17:00 pm), at the end of the week (Friday, w17:00 pm) and at the end of the year (52nd week, Friday, w17:00 pm).
2.1. Activities calculations The activity discharged (or eliminated activity Ae in the equations) by the patient can be estimated using Eq. (1). ð1Þ
where A0 is the input activity injected to each patient, felim is the elimination factor of injected activity, l is the decay constant (in hours), t0 ¼ 2 h is an estimation of the time between patient injection and patient micturition. Accumulated activity at the end of the day (w17:00 pm) can be estimated by means of Eq. (2). Aday ¼
n1 X
Ae eilt1
Aweek ¼
4 X
Aday ejlt2
ð3Þ
j¼0
where t2 ¼ 24 h, being the time between a day and the next one. Finally, accumulated activity at the end of the year (52nd week, Friday, w17:00 pm) can be estimated using Eq. (4). Ayear ¼
m1 X
Aweek eklt3
ð4Þ
k¼0
where m is the estimated number of weeks per year and t3 ¼ 168 h is the time between a Friday and the next one. The activity accumulated along the year is calculated with Eq. (5).
2. Materials and methods
Ae ¼ A0 felim elt0
Accumulated activity at the end of the week (Friday, w17:00 pm) can be estimated using Eq. (3).
ð2Þ
P Ayear ¼ Ae
n1 ilt1 i¼0 e
P
4 jl$24 j¼0 e
1 el$168
ð5Þ
where the number of weeks m is considered very large (m / N). Any calculation software can be used to solve Eq. (5).
3. Results and discussion Data for each radionuclide and results of annual accumulated activity can be seen in Table 1, and the discharged activity along a year for each radionuclide is shown in Fig. 1. Each point represents the accumulated activity at the end of each week (Friday, 17:00 pm).1 This activity grows in the first weeks but rapidly reaches an asymptotic value. We can see that this accumulated activity along a year reaches the maximum in 3 or 4 weeks for all radionuclides, except for 131I that takes from 8 or 9 weeks, due to its longer half-life. The accumulated activity at the end of the day along a year for 131 I is shown in Fig. 2, being similar to the evolution of the rest of radionuclides. It can be noticed that at the end of the week, the activity decreases because no diagnostic procedure with radionuclides is made. So, although the accumulated activity is not constant every day, it reaches, in a few weeks, an asymptotic and maximum value at the end of the week. The largest discharged activity is due to 131I because its longer half-life and for 99mTc because is the most used radionuclide in a NMD department. To compare the results here obtained with those published by SEFM (2002), the last ones have been modified in order to
i¼0
where n is the number of patients per day (and per each radionuclide) and t1 is the time between a patient and the next one.
1 We consider that in radionuclide diagnostic procedure with 1 patient/day, the activity A0 is injected at 15:00 pm, so activity Ae is discharged at 17:00 pm.
R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1535e1538
1537
Table 2 Comparison of annual accumulated activities for each radionuclide Radionuclide
Accumulated activity (Bq/year)
Accumulated activity SEFM (2002) (Bq/year)
Factor
67
Ga I 131 I 111 In 99m Tc 201 Tl
3.37 108 7.43 108 2.20 109 1.688 108 1.847 109 3.284 107
1.98 1010 1.64 1011 6.11 1010 1.15 1010 6.53 1011 2.04 109
58.75 220.73 27.77 68.13 353.55 62.12
Total
5.329 109
9.12 1011
171.14
123
Fig. 1. Evolution of discharged activity along a year for each radionuclide.
account of patients per year and per radionuclide to match those used in this study. This comparison is shown in Table 2. It can be noticed that annual accumulated activities are 28- to 354-times less (depending on each radionuclide) than those calculated in SEFM (2002). The differences with the conventional calculations is due to SEFM did not included the radionuclide decay. This overestimates the total accumulated activity along year by 171-fold. As the half-life of radionuclide tends to be shorter, the difference tends to increase. Knowledge of the sewage activity is useful in establishing the radiological consequences of discharges. The IAEA-TECDOC-1000 (IAEA_1000) and SEFM (2002) set radiation doses assessment guidelines for liquid radioactive releases. The main exposure pathway for liquid discharges of the radionuclides used in a NMD department is the external exposure of sewage system worker. The effective doses of the sewage worker as critical individual have been calculated using the MICROSHIELD code (MICROSHIELD) with simplified
assumptions: a worker remains 1500 h per year at a distance of 2 m of the sludge treatment tank in which the discharges has been accumulated. In terms of effective dose, 131I and 67Ga have the larger contribution mainly due to their half-life and emission energies, whereas 99mTc has the smallest contribution to effective dose. The comparison between accumulated doses along a year to critical people (sewage treatment worker) here obtained with that published by SEFM (2002) is shown in Table 3. 4. Conclusions The benefits of including radioactive decay in waste modelling are illustrated with the results obtained: the total accumulated activity throughout the year obtained here is only 0.58% of the SEFM calculations. The doses to critical people (sewage treatment worker) are 29 mSv, which is less than the established effluent limit of 100 mSv (SEFM_2002), whereas in SEFM these doses are 1068 mSv (Table 3). This study clearly shows that in our hospital there is no need to have storage tanks for these discharges because the more exposed individual receives very low doses and the time radionuclides take for the excreta of patients to be processed and returned to the ecosystem are longer than its half lives (ICRP_94). In all cases, considering the effective dose of sewage worker and accumulated activity throughout a year, 131I is the more restrictive radionuclide due to its longer half-life and its larger dose factor related to its emission energies. This decay model could be used in a ‘‘worst case’’ situation to submit to authorities for approval for liquid waste disposal, and then used retrospectively for accurate calculations of Table 3 Accumulated dose along a year to critical people (sewage treatment worker) Radionuclide
Dose factor (MICROSHIELD) (mSv/Bq)
67
1.31 108 6.07 1010 1.05 108 4.38 109 5.94 1013 7.44 109
Ga I 131 I 111 In 99m Tc 201 Tl 123
Fig. 2. Evolution of accumulated activity at the end of the day along a year for 131I.
Total
Accumulated dose SEFM (2002) (mSv/year)
Accumulated dose (mSv/year)
259 99.5 644 50.2 0.39 15.2
4.398 0.451 23.189 0.740 0.001 0.244
1068.29
29.023
1538
R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1535e1538
liquid waste levels once actual patient numbers were known. The results show how the discharges increase approximately in the same factor as the number of patients, as one can see in Eq. (5). Then, the maximum workload in the NMD department must be considered in the pre-operational situation, in order to estimate with realistic assumptions the maximum effective dose of sewage worker. This maximum must be below 100 mSv, the effluent limit established by the regulatory authority. In operational time the accurate calculations can be done using Eqs. (1)e(5) anytime if required. The authorities could indicate when these calculations must be done in each NMD department, for example on a yearly basis. References Barquero, R., Basurto, F., Nu~nez, C., Esteband, R., 2008. Liquid discharges from patients undergoing I-131 treatments. Journal of Environmental Radioactivity 99, 1530e1534.
Chapel, M., Ferrer, N., Ramos, L.M., Sanchez, M., 2002. Grupo de efluentes del Foro de Proteccio´n Radiolo´gica en el medio hospitalario. SEFM. http://www.sefm.es/docs/actsefm/informefinalgrupoefluentes. pdf. Council 96/29/EURATOM laying down for the protection of the health of workers and general public against the dangers arising for ionizing radiation. May 13, 1996. Clearance of materials resulting from the use of radionuclides in medicine, industry and research: IAEA- TEC-DOC 1000, 1998. International Commission on Radiological Protection, 2004. Release of patients after therapy with unsealed radionuclides. ICRP Publication 94. MICROSHIELD: computer programme for analysing shielding and estimate exposure from gamma radiation. http://www.radiationsoftware.com/ mshield.html. 1992 OSPAR Convention Appendix 1: Criteria for the definition of practices and techniques mentioned in paragraph 3(b)(i) of article 2 of the convention. http://www.ospar.org/eng/html/convention/ospar. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2000. Sources and Effects of Ionising Radiation. 2000 Report to the General Assembly with Annexes, United Nations, Vienna.