Radiation Protection in Nuclear Medicine Gallium-67-Citrate Procedures A. Miñambres, M. L. España, F. A. Floriano, C. Mínguez, P. López-Franco Servicio de Radiofísica y Protección Radiológica, Hospital Universitario de La Princesa, Diego de León 62, E-28006, Spain. E-mail: [email protected]

Abstract. Administration of radiopharmaceuticals to nuclear medicine (NM) patients originates radiation exposure levels to relatives, technicians, nursing staff, etc. The increasing amounts of activity administered in NM procedures and the decreasing values of exposure limits (to public and staff) lead to reexamine the radiation protection issues. The aim of this communication is the estimation of exposure levels to professionals, relatives and members of the public, and the possible reassessment of the radiation protection recommendations. Firstly, the dose rate around the patients was measured. In the study were included 28 lymphoma patients undergoing gallium-67-citrate diagnostic scans, with activities ranging between 150 and 260 MBq. Dose rate were measured at patient’s surface, at 0.7 m and 1.4 m from the patient, and in four projections (anterior, posterior, left lateral and right lateral) just after the injection and, for some patients (21/28), two days later. The integrated dose was calculated using the biexponential decay law recommended by ICRP 53 in addition to certain suppositions about time spent at each distance by the different groups. The average rates measured in the anterior projection after the injection at each distance were 0.26, 0.038 and 0.010 µSv / (h MBq). The integrated doses to a nursing baby and to the couple (sleeping with the patient) were estimated to be 2.17 and 2.22 mSv respectively. The dose to nursing staff ranges between 1 and 95 µSv per patient, depending on the kind of care required. The estimations do not involve serious changes in common radioprotection recommendations. However, nursing babies deserve special attention, taking into account that dose can exceed 1 mSv per procedure, the high incidence of lymphoma among fertile women and the frequency of the procedure, sometimes higher than once a year.

1. Introduction Every Nuclear Medicine (NM) patient becomes, after injection of the radiopharmaceutical, a radiation source for people staying around him/her. Among them, NM technologists and physicians, nursing staff, patient’s relatives, work colleagues, and, broadly, every person who stays near the patient during any long time [1]. Several authors have estimated the radiation exposure from radioactive patients [2-4] and particularly the dose received by NM technologists [5,6]. Also, patients receiving radioactive iodine-131 have deserved special attention because of the high activities administered and the high energy emission of this radioisotope [7,8]. Gallium scintigraphy has an important role in the patients with lymphoma. It contributes to patient management by detecting residual disease or relapse after treatment, monitoring response during therapy, and providing prognostic information. The administered activity in a gallium scan is about 250 MBq and in SPECT, procedure with better sensibility, the activity can reach 370 MBq. The aim of this work is to obtain estimations of the effective doses received for people close to a gallium-67-citrate patient, and discuss their possible implications with regard to radiation protection policy. 2. Materials and Methods In order to estimate the dose received for the people at risk the dose-rate method was used [1]. Firstly the dose rate was measured at different distances from some patients and later, in order to know the dose received for a particular individual, the mean dose rate was integrated, taking into account the biological and physical decay of the radiopharmaceutical, and the times spent by the individual at each distance.

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This method is simple, not much troublesome for the patient and allows inferring the dose for any behaviour pattern. Alternatively, doses can be measured individually through personal or area dosimetry [1]. 2.1. Dose rate measurement Dose rate were measured in 28 lymphoma patients, receiving gallium-67-citrate for whole-body scans. The age range was wide (20-60 years) and the gender ratios were very similar. It was used an ion chamber monitor Babyline, model 81 E793 (Nardeux), calibrated in ambient dose rate equivalent (dH*(10)/dt). Dose rates were measured at 10, 70 and 140 cm from the anterior mid trunk of the patients at four different projections (anterior, posterior, right lateral, left lateral). The rate was measured right after injection in all patients, and in some patients (21 out of 28) the rate was also measured the day of the scan (two days later). The administered activities were registered as well, ranging between 150 and 260 MBq. 2.2.Effective dose estimation The ambient dose rate equivalent is a good approximation of the effective dose (E) since the conversion factor H*(10)/E is close to unity in the gallium-67 emission energies [9]. Anyway, this factor is always slightly less than unity, so that there are neither possible underestimations nor excessive overestimations of the limiting magnitude. The ambient dose rate equivalent was integrated in time supposing the biexponential temporal decay of the gallium-67-citrate recommended by ICRP [10]. The two periods are 25.5 days (83%) and 30 hours (17%), which, allowing for the physical decay become 69.4 hours and 21.7 hours respectively. 3. Results 3.1. Dose rate measurement The measured values of ambient dose rate equivalent per administerd activity are shown in Table I for two times, three distances and four projections. The measurement distribution is described by the mean rate, the maximum rate, the minimum rate, the standard deviation and the 95th percentile. These values lie on the same order of magnitude reported in similar works [11]. Table I. Measured ambient dose rate equivalent (µSv / h per MBq administered activity) Time from Distance Projection injection 15 min 10 cm Anterior Posterior Right Lateral Left Lateral 70 cm Anterior Posterior Right Lateral Left Lateral 140 cm Anterior Posterior Right Lateral Left Lateral 50.6 h 10 cm Anterior Posterior Right Lateral Left Lateral 70 cm Anterior Posterior Right Lateral

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Mean

Std Dev

Max

Min

0.26 0.27 0.22 0.22 0.038 0.037 0.026 0.026 0.010 0.010 0.0076 0.0075 0.11 0.137 0.111 0.090 0.017 0.015 0.013

0.07 0.07 0.05 0.06 0.011 0.009 0.007 0.010 0.004 0.004 0.029 0.0026 0.04 0.029 0.027 0.024 0.008 0.004 0.004

0.44 0.42 0.33 0.36 0.069 0.060 0.041 0.058 0.020 0.020 0.014 0.012 0.18 0.198 0.154 0.124 0.040 0.021 0.020

0.14 0.12 0.11 0.10 0.017 0.020 0.015 0.007 0.005 0.005 0.005 0.005 0.05 0.079 0.055 0.045 0.009 0.010 0.005

95th Percentile 0.36 0.37 0.28 0.30 0.051 0.050 0.039 0.035 0.017 0.017 0.012 0.011 0.17 0.175 0.154 0.119 0.035 0.020 0.017

140 cm

Left Lateral Anterior Posterior Right Lateral Left Lateral

0.010 0.0055 0.0055 0.0055 0.0052

0.004 0.0014 0.0014 0.0014 0.0006

0.015 0.0099 0.0099 0.0099 0.069

0.005 0.0050 0.0050 0.0050 0.0050

0.015 0.0080 0.0080 0.0080 0.0062

It has not been observed significant differences among the four projections. The dose rate decreasing between 70 and 140 cm is slower than predicted by inverse square distance law, which is not surprising at all since a patient is not a punctual radiation source. The rates measured in the second day were compared, for each patient, with the rate decay predicted by the literature [10]. It was found wide differences (between –17% and 36%), perhaps due to the small value of the rates measured in the second day. Therefore, the rates were integrated consistently for all patients through the decay curve proposed by ICRP. 3.2.Effective dose estimation The effective dose is determined by integrating the ambient dose rate equivalent measured in the anterior projection the day of injection, according to the biexponential curve proposed by ICRP [10]. The calculation assumes certain patterns respecting the time spent by an individual at each distance from the patient. The administered activity is supposed to be 260 MBq. The results for some critical groups are given in Table II. Table II. Effective dose (supposing 260 MBq administered activity) Behaviour pattern 1 minute at 10 cm, 5 minutes at 70 cm and 20 minutes at 140 cm

Effective dose NM technologist 2.79 µSv per procedure Partner sleeping 8 hours at 70 cm (afternoon) and 8 hours at 10 cm (night) every day, 2.22 mSv with the patient during 20 days Relative not sleeping 8 hours at 70 cm (afternoon) every day, during 20 days 0.31 mSv with the patient Breast-fed infant Close contact during feeding times. Every day, 35 minutes at the start 2.17 mSv of each hour for the first 8 hours (afternoon), 35 minutes at the start of fourth hour for the next 12 hours (night) and 35 minutes at the start of each hour for the remaining 4 hours (morning). Exposition during 20 days. Nursing staff The time spent near the patient depends on the attention needed by the 1-95 µSv per patient, from totally helpless to totally ambulant [4] patient and day

4. Discussion First of all, we must bear in mind that these estimated effective doses are the result of a simple method but with a number of sources of uncertainty. The monitor is calibrated in the quality of diagnostic Xrays, with energies lower than those emitted by gallium-67. An inaccurate positioning of the monitor may influence the rate measurements at longer distances (70 and 140 cm). The smaller rate values may be not much reliable because of the lacking sensibility of the monitor. The dose calibrator introduces an uncertainty in the values of administered activity. With regard to the bioexponential decay law utilized for calculating the dose, although it is the most reliable one, the particular biokinetics of the radiopharmaceutical will vary from patient to patient. The assumptions about the behaviour of the people at risk may not be representative and if it were, the variability in the actual behaviour may be very wide [12]. In conclusion, the effective doses shown in this paper must be regarded only as orientating values. This uncertainty, however, cannot lead to ignore or despise the final results.

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Dose to the partner is likely to exceed the annual dose limit to public (1 mSv) established by ICRP [13] and the Spanish health authorities [14], though it is hard to reach a dose constraint level recommended by the EU [15] to individuals knowingly and willingly helping in the support and comfort of patients undergoing medical exposure. More attention must be paid to infants, whose doses can also exceed the annual dose limit to public, but to whom dose constraints are not applicable. Moreover, the incidence of lymphoma is high within population in reproductive age, and so, with high probability of being parents of infants. Not forgetting that a breast-fed infant may ingest radioactive substances from the breast milk [16]. The follow-up of lymphoma patients usually involves the prescription of more than a gallium procedure per year (up to four), during several years (up to five). The administered activities may vary among centres, whereas it is around 370 MBq for gallium SPECT. The annual effective doses shown above refer to one procedure per year and an administered activity typical of a body scan (260 MBq). These circumstances must be taken into account when it is evaluated, in a particular case, the risk and the possible adoption of preventive actions. For example, if a young woman under lymphoma treatment undergoes two gallium scans per year, she should be advised of the possible effects of radiation on her infants, because of the high probability that their dose will exceed 1 mSv per year This example points out the benefits of an individualized analysis, with a view to the optimisation of the radiation protection of the public and people caring for radioactive patients. The estimation of doses to NM technologists and nursing staff per year will depend, clearly, on annual workload and the distribution of tasks in the centre. In relation to technologists it is more appropriate a personal dosimetry that allows for every NM procedures.

References 1. Mountford, P. J., O’Doherty, M. J., Exposure of critical groups to nuclear medicine patients. Appl. Radiat. Isot., 50:89-111, (1999). 2. Harding, L. K., Mostafa, A. B., Roden, L., Williams, N., Dose rates from patients having nuclear medicine investigations. Nucl. Med. Comm., 6:191-194, (1995). 3. Cormack, J., Shearer, J., Calculation of radiation exposures from patients to whom radiactive materials have been administered. Phys. Med. Biol., 43:501-516, (1998). 4. Mountford, P. J., O’Doherty, M. J., Forge, N. I., Jeffries, A., Coakley, A. J., Radiation dose rates from adult patients undergoing nuclear medicine investigations. Nucl. Med. Comm., 12:767-777, (1991). 5. Greaves, C. D., Tindale, W. B., Dose rate measurements from radiopharmaceuticals: Implications for nuclear medicine staff and for children with radioactive parents. Nucl. Med. Comm., 20:179-187, (1999). 6. Chiesa, C., De Sanctis, V., Crippa, F., Schiavini, M., Fraigola, C. E., Bogni, A., Pascali, C., Decise, D., Marchesini, R., Bombardieri, E., Radiation dose to technicians per nuclear medicine procedure: comparision between technetium-99m, gallium-67, and iodine-131 radiotracers and fluorine-18 fluorodeoxyglucose. Eur. J. Nucl. Med., 24:1380-1389, (1997). 7. Barrington, S. F., Kettle, A. G., O´Doherty, M. J., Wells, C. P., Somer, E. J. R., Coakley A. J., Radiation dose rates from patients receiving iodine-131 therapy for carcinoma of the thyroid. Eur. J. Nucl. Med., 23:123-130, (1996).

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8. Grisby, P. W., Siegel, B. A., Baker, S., Eichling, J. O., Radiation exposure from outpatient radioactive iodine (131I) therapy for thyroid carcinoma. JAMA., 283:2272-2274, (2000). 9. International Commission on Radiological Protection, Conversion Coefficients for Use in Radiation Protection Against External Radiations. Publication 74. Annals of the ICRP, 26, No. 3, Pergamon Press, Oxford and New York (1996). 10. International Commission on Radiological Protection, Radiation Dose to Patients from Radiopharmaceuticals. Publication 53. Annals of the ICRP, 18, No. 1-4, Pergamon Press, Oxford and New York (1987). 11. Castronovo, F. P., Schleipman, A. R., Radiation exposure from gallium-67 citrate patients, J. Nucl. Med. Tech., 27:57-61, (1999). 12. Greaves, C. D., Tindale, W. B., Flymm, P. J., A survey if close contact regimes between patients undergoing diagnostic radioisotope procedures and children. Nucl. Med. Comm., 17:554-561, (1996). 13. International Commission on Radiological Protection, 1990 Recomendations of the International Commission on Radiological Protection. Publication 60. Annals of the ICRP, 21, No. 1-3, Pergamon Press, Oxford and New York (1990). 14. REAL DECRETO 783/2001. Reglamento sobre protección sanitaria contra radiaciones ionizantes. Boletín Oficial del Estado del 26 de julio de 2001 15. COUNCIL DIRECTIVE 97/43/EURATOM of 30 June 1997 on health protection of individuals against the dangers of ionising 16. Stabin, M. G., Breitz, H. B., Breast Milk Excretion of Radiopharmaceuticals: Mechanisms, Findings, and Radiation Dosimetry. J. Nucl. Med. 41:863-873, (2000).

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