Liquid discharges from patients undergoing 131I treatments

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Journal of Environmental Radioactivity 99 (2008) 1530e1534 www.elsevier.com/locate/jenvrad

Liquid discharges from patients undergoing

131

I treatments

R. Barquero a,*, F. Basurto b, C. Nu~ nez c, R. Esteban d a

Servicio de Radiofı´sica y Proteccio´n Radiolo´gica, Hospital Universitario Rı´o Hortega, E-47010 Valladolid, Spain b Departamento de Fı´sica Teo´rica, Ato´mica y Optica, Universidad de Valladolid, E-47010 Valladolid, Spain c Servicio de Radiofı´sica y Proteccio´n Radiolo´gica, Fundacio´n Jimenez Dı´az, FJD, E-82001 Madrid, Spain d Servicio de Radiologı´a, Hospital Clı´nico Universitario, E-47005 Valladolid, Spain Available online 20 February 2008

Abstract This work discusses the production and management of liquid radioactive wastes as excretas from patients undergoing therapy procedures with 131I radiopharmaceuticals in Spain. The activity in the sewage has been estimated with and without waste radioactive decay tanks. Two common therapy procedures have been considered, the thyroid cancer (4.14 GBq administered per treatment), and the hyperthyroidism (414 MBq administered per treatment). The calculations were based on measurements of external exposure around the 244 hyperthyroidism patients and 23 thyroid cancer patients. The estimated direct activity discharged to the sewage for two thyroid carcinomas and three hyperthyroidisms was 14.57 GBq and 1.27 GBq, respectively, per week; the annual doses received by the most exposed individual (sewage worker) were 164 mSv and 13 mSv, respectively. General equations to calculate the activity as a function of the number of patient treated each week were also obtained. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Thyroid cancer;

131

I; Radioactive discharge; Radioactive waste treatment system

1. Introduction Within man-made radioactive environment impacts, the nuclear medicine (NM) techniques in which liquid radioactive waste is produced play a significant role. These techniques include both diagnostic and therapeutic procedures. This paper focuses on therapy procedures and other ‘‘twin’’ work in this issue (Barquero et al., 2008) is specifically dedicated to the diagnostic discharges. The use of 131I in the treatment of hyperthyroidism and thyroid disease has increased significantly and today represents about 90% of all therapies in nuclear medicine (UNSCEAR, 2000; ICRP 94, 2004). The success of the therapy depends on the uptake and retention of radioactive iodide in the thyroid, remainder tissue or metastases (Willegaignon et al., 2006). The activity which is not retained in the lesion

* Corresponding author. Tel.: þ34 983478862, þ34 630752261 (mobile); fax: þ34 98325 7511. E-mail address: [email protected] (R. Barquero). 0265-931X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2007.12.010

is excreted through the urinary system to the sewage as radioactive urine effluent. The total amount discharged during each therapeutic procedure may vary significantly according to the protocol used for the treatment, which can be on an inpatient or an outpatient basis. Although there is an international consensus for treating the hyperthyroidism patients as outpatients, two procedures are followed in thyroid cancer therapy: In Spain (as in many other countries), following the European recommendations (IAEA, 2002; ICRP 60, 1991), it is common for the thyroid cancer therapy to be performed on an inpatient basis to minimize the risk of radiation to family and other inhabitants. This is, primarily, because the urine of these patients contains significant levels of activity and could have a serious environmental impact. Thus, in these countries the patients undergoing cancer therapy with 131I remain several days (4e7) in special rooms where urine excretes are collected in dedicated decay storage tanks. In another countries, as the United States, the new regulations (NRC RG 8.39, 1997) allow the radioiodine therapy patients to be treated as outpatients, discharging directly to the sewage collector the liquid effluents that they generate.

R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1530e1534

This study shows the differences between 131I direct and I after-storage discharges, and it evaluates the activities released in each case and its radiological impact. As starting point, we will use the general discharged activity evaluation made in Spain within a national task group formed by the Spanish National Scientific Societies (radiation protection and medical physics), and the Regulatory Organism (SEFM, 2002). Modifications and general estimations must be done later in each hospital because their discharge procedures may be different from the more conservative ones. We will estimate the activities discharged from the University Hospital Clı´nico of Valladolid (HCUV) and Fundacio´n Jimenenz Diaz (FJD) of Madrid using external exposure rates acquired around the corresponding patients.

1531

where Aadm(t) is the administered activity A0 decayed to time t

131

2. Materials and methods The data from 244 hyperthyroidism therapy treatments, done during 1993e2000, in the FJD Hospital with a mean value of three treatments per week were retrospectively included in this work. The Na131I activities administered to patients ranged from 296 MBq to 555 MBq with a mean value of 414 MBq. Patients treated during 2005 with radioiodine therapy in the HCUV Hospital were also retrospectively included in this study, which is based on 23 cases of thyroid cancer (two treatments per week). The Na131I activities administered to patients ranged from 2.96 GBq to 5.55 GBq with a mean value of 4.14 GBq. The 131I radioactive physical decay has been always considered. It is found that with more or less the same administered activity, the decay assumption produced a lower and more accurate discharged activity than a general nondecay assumption, as used in SEFM (2002). Two models were studied, direct discharges and discharges after storage in the decay tanks. The direct discharge must be considered in the ambulant hyperthyroidism treatments. For urine excreted in carcinoma cases, both, the activity discharged with previous storage tank and as direct discharge, have been estimated.

Aadm ðtÞ ¼ A0 ðtÞelt

ð2Þ

and AThyr(t) is the activity existent in t in the enlarged thyroid of the patient calculated using AThyr ðtÞ ¼

DRThyr ðtÞ 6360

ð3Þ

The equivalent dose rate factor 6360 mSv h1 GBq1 was obtained with the Monte Carlo code (MCNPX, 2002) used to model the measuring room and the patient. The body burden simulation used a water-solution-filling Bottle Manikin Absorption Phantom (BOMAB), which is formed by 10 cylinders of circular and elliptic sections adapted to the average patient dimensions of a 62 kg female (females undergo 85% of total treatments). During simulation, the 131I decay photons were transported to the room from a point 1-cm deep under surface (that simulated the skin) of the cylinder that simulated the patient’s neck. The source term that contains the following photons is weighted by the decay probability: 0.080 MeV (2.62%), 0.284 MeV (6.14%), 0.365 MeV (81.70 %), 0.637 MeV (7.17%), 0.722 MeV (1.77%). In the simulation, the ionization chamber was modeled as a 2870 cm3 sphere, with 8.82 cm-radius. Here each photon flux was estimated using the tally for track length (MCNPX, 2002). The corresponding dose equivalent values were calculated with the ICRP-74 conversion factor introduced as input in each run. The DRThyr(t) in Eq. (3) is the external exposure in units of dose equivalent rate measured at contact thyroid of the sitting patient in mSv h1. As ambient dosimeter, throughout the years a pressurized ionization chamber VICTOREEN 450-PI n/s 1020, with bi-annual calibration by NIST, was used to determine the patient’s external exposure. The exposure rate was measured at contact of each individualized sitting patient, daily during the 6 days after the 131I administration. Two measurements more are available in some cases for the 14th and the 20th day after the administration. 2.2.1. Activity discharged in hyperthyroidism (outpatients) When all the patients were considered, with three treatments per week, after 1 week with three patients the total excreted activity was

2.1. Iodine dynamics inside the patients

Ad;1st patient þ Ad;2nd patient þ Ad;3rd patient

According to the International Commission on Radiological Protection recommendations (ICRP, 1987), related to iodine kinetics, the 131I is uptaken from 5% to 55% in the thyroid after 8 h and it is eliminated with a biological mean life of 80 days. The remaining body immediately uptakes 70% and followed by elimination, only through urinary system, with a biological mean life of 8 h. Also this publication includes immediate 30% uptake values in stomach and intestines, and posterior elimination with a biological mean life of 8 h. Although hyperthyroidism and thyroid cancer are not specifically studied in this publication, several authors suggested the applicability of this model in a similar clinical situation. In defining an overall uptake factor for the burden body, as a sum of remaining tissue, stomach and small intestine, the evolution of total patient activity can be expressed as the sum of two components’ sources with activity in the burden body and the thyroid. The two compartment model hence followed, although simplistic, allows to estimate the excreted activity from patients with an uncertainty below 20%.

After second week with three new treatments and supposing a 131I decay factor of 0.5 per week Ad;1st  0:5 þ Ad;2nd  0:5 þ Ad;3rd  0:5 þ Ad;4th þ Ad;5th þ Ad;6th

ð5Þ

If all treatments produce an excreted activity equal to the mean value Ad 1 X

Ad  0:5n þ Ad  0:5n þ Ad  0:5n

ð6Þ

n¼0

After 3 weeks, analogously Ad;1st  0:52 þ Ad;2nd  0:52 þ Ad;3rd  0:52 þ Ad;4th  0:5 þ Ad;5th  0:5 þ Ad;6th  0:5 þ Ad;7th þ Ad;8th þ Ad;9th

ð7Þ

or 2 X

2.2. Hyperthyroidism

ð4Þ

Ad  0:5n þ Ad  0:5n þ Ad  0:5n

ð8Þ

n¼0

The dynamic of the 131I inside the patient is considered through two different compartments: the patient and the sewage as the final receptor of the activity. For hyperthyroidism patients, the main 131I uptake in tissues occurs in the enlarged thyroid of the patient. Thus, in this treatment the instantaneous discharged activity from each patient in each time t, Ad(t), is given by Ad ðtÞ ¼ Aadm ðtÞ  AThyr ðtÞ

ð1Þ

Following this behavior after several years the total discharged activity to sewage system can be estimated as Ad ¼

N X n¼0

Ad  0:5n þ

N X n¼0

Ad  0:5n þ

N X

Ad  0:5n

ð9Þ

n¼0

where n represents the number of weeks in which there are discharges, i.e. all time, because of the consecutive treatments done habitually in hospitals.

R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1530e1534

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2.3. Thyroid carcinoma In these treatments, the patient previously has been submitted to total or near total thyroidectomy. Because of this the main 131I uptake in tissues is shared in the remainder of the body. Then, Eqs. (1) and (2) were also used but now with ATB(t) in place of AThyr(t). The dynamic of the 131I inside the patient is considered through three different compartments: the patient body, the storage tank (if there is) and the sewage as the final receptor of the activity. The activity in the total body of the patient was calculated using the dose equivalent rate factor 66.4 mSv h1 GBq1 ATB ðtÞ ¼

DRTB ðtÞ 66:4

ð10Þ

This equivalent dose rate factor 66.4 mSv h1 GBq1 was obtained (as above) with the Monte Carlo code (MCNPX, 2002). Now the simulation consists in a 131I solution filling the Bottle Manikin Absorption Phantom (BOMAB) as burden body source term to estimate the equivalent dose at 1 m in front of the sitting patient, at the height of the thyroid, in unites of mSv h1. The theoretic dose equivalent rate at 1 m per unit activity of 131I for point source of 131 I in air calculated with tables of ICRP-74 for the photon emissions done above is 63.76 mSv h1 GBq1. The conversion factor obtained here for 1 m, 66.4 mSv h1 GBq1, is very similar, as it was expected, to the former value. As ambient dosimeter, throughout the years a pressurized ionization chamber VICTOREEN 450-PI n/s 2050, with bi-annual calibration by NIST was used to measure the ambient dose equivalent around each patient. The exposure rate was measured daily at 1 m of each individualized sitting patient over 3 days after the 131I administration. In some cases there is one additional measurement the following week after the administration. 2.3.1. Activity discharged in thyroid cancer treatments (outpatients) When all patients were considered, giving two treatments per week and with the procedure explained above, the total direct discharged activity to sewage system was estimated as Ad ¼

N N X X ðAd Þ1st patient  ð0:5Þn þ ðAd Þ2nd patient  ð0:5Þn n¼0

ð11Þ

measurement. The percentage of radioiodine excreted as a function of time is estimated using the relationship between the discharged activity Ad calculated with Eqs. (1) and (2) and the administered activity A0 decayed to time calculated with Eq. (2). Fig. 1 shows the percentage averaged for all patients of 131I activity excreted as a function of elapsed time after radioiodine hyperthyroidism therapy. The overall weighted averaged percent of activity excreted at any time was 51  9%. 3.1.1. Sewage activity (direct discharge for hyperthyroidism) When all the patients were considered, with three treatments per week n, the total discharged activity to sewage system was estimated with Eq. (9). The Taylor approximation is used in order to know the series values. Then, the reported value at first column of Table 1 was obtained for the average administered activity

1:27 GBq ¼ 0:414  0:51   0:51 

1 þ 0:414 0:5

1 1 þ 0:414  0:51  0:5 0:5

ð13Þ

where 0.414 ¼ activity administered in hyperthyroidism (mean value), 0.51 ¼ percentage of activity excreted in each hyperthyroidism treatment (mean value), 1/0.5 ¼ limit of Taylor approximation included in Eq. (9). The corresponding results from reference SEFM (2002) are also included in Table 1.

n¼0

100

where n represents the number of weeks where there are discharges, i.e. all time, because of the consecutive treatments done habitually in hospitals.

95 90

n¼0

n¼0

where n represents the number of weeks in which there are discharges, i.e. all time, because of the consecutive treatments done habitually in hospitals. There are two tanks and the period of decay in each depends on the filling time of the other. The volume of the available tank in HCUV Hospital is 3722 l, and the range for the filling time goes from a minimum of 3 months (when the excreta was mixed with a maximum of clean water, 40 l per day) and a maximum of 1 year when the clean water is minimum. In the calculation, the more conservative assumption has been taken, i.e. 3 months.

85 80 75 70

Activity excreted (%)

2.3.2. Activity discharged in thyroid carcinoma (inpatients) with storage tank for decay Eq. (11) remains valid to calculate the total stored activity by replacing the upper limit in the summations with the number of storage weeks, N. The discharged activity is calculated with the decay factor corresponding to the period from the closure of the tank (tc) to the discharge (td) ! N N X X Ad  0:5n þ Ad  0:5n  elðtd tc Þ ð12Þ Ad ¼

65 60 55 50 45 40 35 30 25 20 15 10 5 0

3. Results 3.1. Excreted activity in hyperthyroidism From the exposure rates acquired in contact with each patient and using Eq. (3), an AThyr was calculated for each

0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627282930

Time after INa131 I administration for Hyperthyroidism (d)

Fig. 1. Percentage of 131I activity excreted (averaged for all patients) in hyperthyroidism as a function of time post-therapy. The error bar means 1 standard deviation of percentages obtained for all patients. The mean of these averages, 51  9%, is considered as the mean percentage activity excreted for discharge calculations.

R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1530e1534 Table 1 Estimation of the discharged activity (in GBq) in the excreta of patients undergoing 131I therapy

Sewage activity Sewage activity (SEFM, 2002)

Hyperthyroidism (outpatients)

Thyroid carcinoma (outpatients)

Thyroid carcinoma (inpatients) 3 months decayed

1.274 48.56

15.62 592

0.007 0.091

A similar process is used to the thyroid cancer treatments. From the exposure rates acquired at 1 m of each patient and using Eq. (10), an ATB was calculated for each measurement. The percentage of radioiodine excreted as a function of time is estimated using the relationship between the discharged activity Ad calculated with Eqs. (1) and (2) and the administered activity A0 decayed to time calculated with Eq. (2). All percentages that were calculated here are averaged together because there are only 81 acquired measurements for 23 treatments. The general mean value for the percentage of total activity excreted per patient as urine during his/her cancer thyroid treatment was 88  13%. 3.2.1. Sewage activity (direct discharge in cancer thyroid treatment outpatients) The results when all patients undergoing thyroid carcinoma treatments were considered, following Eq. (11), were obtained using the Taylor approximation in a similar way as in Section 3.1.1. Then the reported value at second column of Table 1 was obtained for the average administered activity 14:57 GBq ¼ 4:14  0:88 

1 1 þ 4:14  0:88  0:5 0:5

ð14Þ

where 4.14 ¼ activity administered in thyroid cancer therapy (mean value), 0.88 ¼ percentage of activity excreted in each cancer treatment (mean value), 1/0.5 ¼ limit of Taylor approximation included in Eq. (11). The corresponding results from reference SEFM (2002) are also included in Table 1. 3.2.2. Sewage activity for inpatients with decay storage tank A similar process is used to obtain the activity discharged from the storage tank after 3 months of decay following Eq. (12), reporting the value at third column of Table 1 6:80 MBq ¼ 15:62  1000  e0:08690

Table 2 Estimation of the annual dose absorbed (in mSv) by the sewage treatment worker as a consequence of the excreta of patients undergoing 131I therapy Hyperthyroidism Thyroid Thyroid carcinoma (outpatients) carcinoma (inpatients) 3 months (outpatients) decayed Annual absorbed dose 13.38 Annual absorbed dose 509.9 (SEFM, 2002)

3.2. Thyroid carcinoma: excreted activity

ð15Þ

The corresponding results from reference SEFM (2002) are also included in Table 1. 3.3. Radiological consequences Knowledge of the sewage activity is useful in establishing the radiological consequences of the 131I discharges. These activities are important for estimating the environmental impact

1533

164.0 6216

0.070 0.956

in the ecosystem as well as to evaluate the potential effective doses received by the critical people who are found to be (SEFM, 2002) the sewage treatment workers. In this reference, an external dose factor for this worker was calculated using the MICROSHIELD code (Microshield, 2002) 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 of 131 I have been accumulated. According to the dose factor obtained, 1.05E08 mSv/Bq, the absorbed doses of Table 2 were evaluated. 4. Discussion This study allowed evaluation of the differences between I direct discharges and after-storage 131I discharges and to evaluate the activities released in each case and its radiological impact. Using daily external exposure measurements around the patients is possible to estimate the uptake and retention of radioactive iodine in the patient and the proportions of activity released in their excreta. The main error in the technique was associated with the activity-to-equivalent dose rate conversion factor in thyroid for hyperthyroidism. As the thyroid size, position and mass can change between patients (i.e. mass can vary from 20 g to 100 g), the variability of the conversion factor between patients can be large. Nevertheless, the corresponding variability to the conversion coefficient at 1 m used in thyroid cancer estimations, as mentioned above, is very little. In spite of this, the results obtained for these discharged activity 51% and 88% agree with than those from ICRP 94, 2004, 54% and 84e90% for hyperthyroidism and thyroid carcinoma, respectively, and with the other corresponding to the thyroid cancer from Willegaignon et al. (2006), 92%. The consideration of the 131I decay in the process of estimating the discharged activities improves the information given in SEFM (2002), see the last row of Tables 1 and 2. The values of this reference were estimated for regulatory purposes, with the maximum activity per treatment, and without decay calculations. The uses of such values are conservative and overestimate the discharged activity in 37 times for direct discharges and in 13 times for discharges after storage. As the time 131I takes for the excreta of patients to be processed and returned to the ecosystem is longer than 8.04 days, which is its mean life (ICRP 94, 2004), the main radiological consequence of these clinical liquid effluents is the external exposition of the sewage treatment worker who may remain 131

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R. Barquero et al. / Journal of Environmental Radioactivity 99 (2008) 1530e1534

a long time near contaminated sludge. The annual doses estimated here must be compared with the Spanish regulatory limits for clinical radwastes established as 100 mSv. It can be supposed that the use of decay tanks reduces the discharges and the associated doses to trivial values. For direct discharges, the case hyperthyroidism is below this limit but the thyroid cancer treatment is slightly above this limit if more than two treatments per week are conducted. Although the estimation of this dose is conservative and the true dose was not known, precaution on management of discharges of more patients or patients who receive large amounts of radioiodine for therapy must be kept in mind. The new establishment of more liberal patient release criteria of NRC RG 8.39 (1997) and Willegaignon et al. (2006) must be carefully considered. 5. Conclusion The aim of this work was to present a short review of the clinical liquid discharges from patients undergoing therapy with 131I. Two easy approximations have been obtained in order to know this discharges as a function of the number of treatments of hyperthyroidism and thyroid cancer per week conducted in the hospital AHyperthyroidism ¼ 2  N  0:51  AAdministered

ð16Þ

AThyroid-cancer ¼ 2  M  0:88  AAdministered

ð17Þ

where AHyperthyroidism ¼ steady activity excreted to liquid environment from the hospital in which N treatments per week of hyperthyroidism with 131I were conducted; AThyroidcancer ¼ steady activity excreted to liquid environment from the hospital in which M treatments per week of thyroid cancer with 131I were conducted; 0.51, 0.88 ¼ defined in Eqs. (13)  administered ¼ average of activity administered for and (14); A hyperthyroidism (or thyroid cancer, as correspond) therapy in the hospital.

Acknowledgments The authors wish to thank N. Ferrer and M.L. Chapel for their interest in this work and to the Spanish Foro for Radiation Protection in Hospitals for their support and promotion of this work. References Barquero, R., Agulla, M.M., Ruiz, A., 2008. Liquid discharges from the use of radionuclides in medicine (diagnosis). Journal of Environmental Radioactivity 99, 1535e1538. Chapel, M., Ferrer, N., Ramos, L.M., Sanchez, M., 2002. Grupo de efluentes del Foro de Proteccio´n Radiolo´gica en el medio hospitalario. Sociedad Espa~nola de Fı´sica Me´dica, SEFM. http://www.sefm.es/docs/actsefm/ informefinalgrupoefluentes.pdf. IAEA, 2002. Radiological Protection for Medical Exposure to Ionization Radiation. IAEA Safety Guide RS-1.5. International Atomic Energy Agency, Vienna. International Commission on Radiological Protection (ICRP), 1987. Dose to Patients Alter Administration of Radiopharmaceuticals. In: ICRP Publication 53. Pergamon Press, Oxford. International Commission on Radiological Protection (ICRP), 1991. Recommendations of the International Commission on Radiological Protection. In: ICRP Publication 60. Pergamon Press, Oxford. International Commission on Radiological Protection (ICRP), 2004. Release of Patients after Therapy with Unsealed Radionuclides. In: ICRP Publication 94. Elsevier. MCNPX, 2002. In: Waters, L.S. (Ed.), MCNPX e Monte Carlo N-particle Transport Code System for Multi-particle and High Energy Applications. Los Alamos National Laboratory. LA-UR-02-2607. Microshield, 2002. MICROSHIELD e a computer programme for analysing shielding and estimate exposure from gamma radiation, http://www. radiationsoftware.com/mshield. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2000. Sources and effects of ionizing radiation. Report to the General Assembly with Scientific Annexes. Vol. I: Sources. United Nations, New York. US Nuclear Regulatory Commission Regulatory Guide 8.39, 1997. Release of Patients Administered Radioactive Materials. US Government Printing Office, Washington, DC. Willegaignon, J., Stabin, M.G., Guimaraes, I.C., Malvestiti, L.F., Sapienza, M.T., Maroni, M., Sordi G-M, A.A., 2006. Evaluation of the potential absorbed doses from patients based on whole-body 131I clearance in thyroid cancer therapy. Health Physics J 91, 123e127.