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Air contamination measurements for the evaluation of internal dose to workers in nuclear medicine departments
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Air contamination measurements for the evaluation of internal dose to workers in nuclear medicine departments B. De Massimia, D. Bianchinib, A. Sarnellib, V. D’Erricob, F. Marcoccib, E. Mezzengab, ⁎ D. Mostaccia, a b
Department of Industrial Engineering, Alma Mater Studiorum - University of Bologna, Italy Medical Physics Department, Istituto Scientifico Romagnolo per lo Studio e la Cura dei Tumori (IRST) IRCCS, Meldola (FC), Italy
A R T I C L E I N F O
A BS T RAC T
Keywords: Internal doses Nuclear medicine Airborne radioactivity Air contamination monitoring
Radionuclides handled in nuclear medicine departments are often characterized by high volatility and short half-life. It is generally difficult to monitor directly the intake of these short-lived radionuclides in hospital staff: this makes measuring air contamination of utmost interest. The aim of the present work is to provide a method for the evaluation of internal doses to workers in nuclear medicine, by means of an air activity sampling detector, to ensure that the limits prescribed by the relevant legislation are respected. A continuous air sampling system measures isotope concentration with a Nal(TI) detector. Energy efficiency of the system was assessed with GEANT4 and with known activities of 18F. Air is sampled in a number of areas of the nuclear medicine department of the IRST-IRCCS hospital (Meldola- Italy). To evaluate committed doses to hospital staff involved (doctors, technicians, nurses) different exposure situations (rooms, times, radionuclides etc) were considered. After estimating the intake, the committed effective dose has been evaluated, for the different radionuclides, using the dose coefficients mandated by the Italian legislation. Error propagation for the estimated intake and personal dose has been evaluated, starting from measurement statistics.
1. Introduction The use of unsealed radiation sources exposes personnel of nuclear medicine facilities to a potential risk of external and internal exposure (Valentin, 2007). Currently the most widely used radiopharmaceutical in nuclear medicine imaging is 18F -FDG. The 18F isotope is a pure β+ emitter with a half life of 110 min, and it is produced, commercialized and utilized in radiopharmacy in liquid form. The volatility of the FDG and the radiological impact of contaminated air has been investigated thoroughly, see e.g. (Calandrino et al., 2009, 2007). Contamination by airborne agents needs to be evaluated reliably, or the absence thereof established clearly, in view of obeying the limits on effective dose to exposed personnel (EURATOM 1996). Evaluation of intake activities to be used with biokinetic models (Valentin, 2007) can be conducted with direct and indirect methods (Kocher, 2000). Direct whole body measurements can determine the occupational radionuclide intake to hospital staff (Terranova et al., 2011). Indirect measurements, on the other hand, derive intake activities from measurement of the activity present in biological samples (e.g. excreta) and physical samples (e.g. air filters). In the ICRP 103 recommendations (Valentin, 2007) the DAC (Derived Air
⁎
Concentration) is defined as the activity concentration in air that given the breathing rate and yearly exposure time of a worker would result in a dose at the Annual Limit of Intake, and used as the operative quantity to estimate the internal dose due to inhalation. Measurements connected with the utilization of the DAC can be made with PAS (Personal Air Sampler) that are individual filter based air sampler worn by the exposed workers. As a general remark, PAS sampling technique is limited by the fact that the airflow through the device will not be representative of the breathing rate of a human, and the air sampled is not the same air that the worker will breathe, so 'hot particles' in the air may cause a substantial difference in dosimetry if care is not taken. But coming specifically to the present case, the short half-life of 18F, like that of many other isotopes used in nuclear medicine, and a weak correlation with biological sampling measures (Britcher and Strong, 1994), are strongly limiting factors for the evaluation of intake radioactivity through this method, rendering the PAS essentially useless for the purpose, hence the need to evaluate the concentration of airborne activity from instant environmental radioactivity concentration: measuring air contamination becomes the most practicable possibility to evaluate inhalation-related doses to personnel. The present work presents a method for the evaluation of internal doses to personnel
Correspondence to: via dei Colli 16, 40136 Bologna, Italy. E-mail address: [email protected] (D. Mostacci).
http://dx.doi.org/10.1016/j.radphyschem.2017.03.003 Received 28 September 2016; Received in revised form 1 March 2017; Accepted 2 March 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: De Massimi, B., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.03.003
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
B. De Massimi et al.
Fig. 1. Functional scheme of the air sampling system. Table 1 Washout, drain and acquisition times.
2. Materials and methods
Area
Washout [s]
Drain [s]
Acquisition [s]
2.1. Radioactivity measurements
Hot lab Hall Hot waiting room 18 F cell Radiopharmacy
30 30 30 30 30
10 15 25 10 60
600 600 600 600 600
A continuous air sampling system (MecMurphil® MP-AIR) with a flow of 10 m3/h was used, equipped with a series of valves operated automatically through a dedicated computer; the system is connected, through these valves and an appropriate system of air ducts, to a number of locations of interest. Through the valve system, the computer selects which locations to sample: in fact, all locations are sampled in sequence, and the process is repeated continuously, 24 h/d. Fig. 1 presents a scheme outlining the operation of the device. The air sampled is run through a 0.00312 m3 Marinelli beaker positioned on a 2”×2” Nal(TI) detector (DI-65/50 by Gamma Technical Corporation, 50 diameter by 50 mm length), and the assembly is contained in a low background shielding (lead, 5 cm around, 6 cm top and bottom, all with 3 mm copper coating); concentration of isotopes in the air flushed is assessed measuring the gamma rays from annihilation of the β+ positrons emitted in the decay. Energy efficiency of the detector is assessed filling the Marinelli beaker with water containing known concentrations of 18F: some adjustment is needed to account for the fact that the measurements of interest are conducted on air, whereas calibration uses water. For instance, the positrons emitted in the decay of 18F have a range in air such that annihilation with air electrons is rare, and the positrons emitted in the volume of the beaker have a large probability of reaching the beaker walls and annihilate there (Sarnelli et al., 2015). Hence, in most cases the gamma ray pairs produced in the annihilation will be generated within the walls of the Marinelli beaker. On the contrary, when the beaker is filled with 18F loaded water, annihilations happen essentially at the point of emission, giving rise to a uniform volume source of gamma rays. As a result, when the filling is 18F in air (as is the case in actual measurements), gamma emission takes place farther, on average, from the detector. On the other hand self absorption in air is practically negligible, but this is not so in water. To correct for these opposing measurement phenomena, a calibration factor was calculated using GEANT-4 (Sarnelli et al., 2015). As mentioned above, radionuclide concentration is determined flushing the possibly contaminated air through the Marinelli beaker and counting it with the NaI detector. For the measurement to be representative the air flushed through the counting system needs to have the same radionuclide concentration as that in the room sampled. Deposition in the piping
Table 2 Ventilation rates (ICRP 66). Activity
Ventilation rates [m3 h-1]
Sleep Rest, Sitting Light exercise Heavy exercise
0.45 0.54 1.5 3.0
in the nuclear medicine department of the IRST-IRCCS Hospital (Meldola – Italy) based on a continuous air sampling system and on individual workload evaluation. The method is of more general applicability, however, to demonstrate it, in what follows its specific application to 18F will be presented.
Table 3 Mean concentration 1-OCT to 31-DEC, 2015 [Bq/m3]. Area
Hot waiting room (C3)
Hot cell 18 F (D4/ 1)
Hall (B2)
Hot Lab. (A1)
Radiopharmacy (G7)
Mean concentration
137
132
141
129
194
2
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
B. De Massimi et al.
Table 4 Percentage of time shift by area hospital staff categories. 18
Hot waiting room (C3)
Hot cell 0
Technologist/Radiographer
5% (21 min) 0
Physician
0
20% (1 h 24 min) 0
Radiopharmacist
0
0
Nurse
F (D4/1)
certainly takes place, however if air is drawn for some time an equilibrium will be reached between deposition and resuspension: so if air is drawn for some extent of time before measurement it can be assumed that once the measurement takes place the air is essentially in this equilibrium situation and is therefore representative of the air in the room sampled. The actual measurement procedure is as follows: The controlling computer selects an area to be sampled, opening the corresponding valve, then the measurement starts, comprised of the following three steps:
• •
Nurse Technologist/ Radiographer Physician Radiopharmacist
Light exercise [h]
Heavy exercice [h]
Mean rate [m3 h-1]
7 7
2 4
5 3
0 0
1.2 0.95
8 7
5 5
3 2
0 0
0.9 0.8
Radiopharmacy (G7)
60% (4 h 12 min) 80% (5 h 36 min) 10% (48 min) 0
0
0
0
0
0
0
∼0
100% (7 h)
18
F for the several hospital staff categories. Annual dose [mSv]
Nurse Technologist/Radiographer Physician Radiopharmacist
2.541 2.074 2.400 2.880
E−02 E−02 E−03 E−02
different areas are shown in Table 1.
•
MEASURE: Air from the area selected is flushed through the Marinelli beaker for 600 s and counted to determine radionuclide concentration.
Once the sampling process has concluded, the next location is selected, air is sampled and counted. Once all the rooms have been sampled, the sampling begins again at the first room. This sampling and measurement process is continuous and occurs 24 h a day, seven days a week. The entire process was monitored for 3 months, specifically from October 1st to December 31st, 2015. Average values for the entire period were determined. Due to the characteristics of nuclear medicine daily routine, essentially organized on a weekly schedule basis, the 3month average can be taken as fully representative of the entire working year.
Table 5 Average ventilation rates for the several hospital staff categories. Sitting [h]
Hot Lab. (A1)
Table 8 Annual dose from
WASHOUT: External, clean air is flushed through the beaker for 30 s to remove all previous contaminants. Preliminary investigation has proven that 30 s is sufficient time for a thorough sweeping of the beaker, and there is no need check decontamination of the marinelli after washout is completed. DRAIN: Air from the air duct connected to the location selected is drawn for 15–60 s, depending on pipe length and other considerations, until the air sitting in the ducts is removed and fresh air from the location under exam reaches the Marinelli. Timing for the
Shift [h]
Hall (B2)
2.2. Inhalation assessment Table 6 Dose coefficients for inhalation. T½
18
F
Absorption
Table 7 Annual dose due to
18
11
F M S
1.83 h
h (g)1 μm 3,0 10 5,7 10–11 6,0 10–11
To evaluate the internal doses received by the different hospital staff categories (doctors, technicians, nurses, radiopharmacists) all the different possible exposure situations were considered: rooms visited, occupancy times, type of work effected and so forth. Inhalation was then evaluated - the breathing rates of hospital staff were determined from the exercise rates for occupational activities: the different possible activities were analyzed and categorized on the basis of the type of effort required, to determine the ventilation rate following ICRP 66, se Table 2 (ICRP, 1994).
h (g)5 μm 5,4 10–11 8,9 10–11 9,3 10–11
F, breakdown by room and hospital staff categories [mSv].
Nurse Technologist/Radiographer Physician Radiopharmacist
18
Hot waiting room (C3)
Hot cell
1.29E−03 0 0 0
0 3.94E−03 0 0
F (D4/1)
3
Hall (B2)
Hot Lab (A1)
Radiopharmacy (G7)
1.59E−02 1.68E−02 2.40E−03 0
0 0 0 0
0 0 0 2.88E−02
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
B. De Massimi et al.
the AMAD (Activity Median Aerodynamic Diameter: 1 µm or 5 µm) see Table 6. For the compounds utilized the absorption type was Medium; in the absence of a dedicated investigation the AMAD could not be specified, although from usual experience it is quite fine in these environments. The value for 5 µm was taken as representative, both to follow legislation (Legislative Decrees number 187 and 241 of 2000, unavailable in English) and because the associated value of the conversion coefficient was the higher of the two. From average concentration, occupancy times and ventilation rates, introduction by inhalation was calculated and with the conversion coefficient committed dose was assessed for all hospital staff categories. The results are presented Tables 7, 8. It can be seen that committed effective dose to all personnel proved negligible.
The same member of the personnel exerting different activities and hence effort - in different areas, the ventilation rates for each person varied from area to area and had to be determined separately: from the ventilation rate and the time of occupancy the total air inhalation was calculated for every location of interest. To this effect, operating schedules and modalities of all personnel were observed for the same 3-month period considered above and times measured accurately; averages were calculated for total air inhalation in all the several areas of interest and for all the healthcare professionals involved. 2.3. Committed dose assessment From air inhalation and air contamination average data, yearly radionuclide inhalation was calculated for the different hospital staff categories involved in the survey. Using the dose conversion factors prescribed by the present Italian legislation - Legislative Decrees number 187 and 241 of 2000 Legislative Decrees number 187 and 241 of 2000 (unavailable in English), adopting the Council Directive 96/29/Euratom (EURATOM, 1996) that was based on ICRP Publication 60 (ICRP, 1991) - the committed dose for every person involved was assessed and compared to the dose limits prescribed in the radiation protection legislation.
3.4. Discussion and future prospects As can be seen from the results presented, the committed dose proves to be negligible even compared to the more stringent 1 mSv/a limit prescribed for the general public, to the point that even an increase of, say, 200% in the above evaluation would still maintain the committed dose widely below the limit. The present procedure will be applied to other radionuclide of interest in nuclear medicine (e.g., 99 mTc) to prove its wider applicability.
3. Results Results for the procedure will be presented with reference to
18
4. Conclusions
F.
The method proposed permitted to assess the committed effective dose for all personnel of the nuclear medicine department, fulfilling the legal prescriptions with the proof that dose limits were not infringed – actually, the contribution of inhalation to annual dose was found to be practically null. The procedure served its purpose and, albeit it has been discussed only in reference to 18F, it can be applied to any radionuclide used in nuclear medicine as long as it can be measured effectively in the apparatus.
3.1. III.1 Radioactivity measurements As discussed in section II.1, continuous measurements were taken every day for three months for sampled work areas in the hospital. This produced 1850 measurements for every area. The measurements were averaged calculating the sample mean and variance with Eqs. (1) and (2):
zk =
σk2 =
1 1850
1850
1 1850 2
∑ xik i =1
(1)
References
1850
∑ σ x2ik i =1
Britcher, A.R., Strong, R., 1994. Personal air sampling-a technique for the assessment of chronic low level exposure? Radiat. Prot. Dosim. 53 (1–4), 59–62. Calandrino, R., del Vecchio, A., Todde, S., Fazio, F., 2007. Measurement and control of the air contamination generated in a medical cyclotron facility for PET radiopharmaceuticals. Health Phys. 92 (5), S70–S77. http://dx.doi.org/10.1097/ 01.HP.0000253941.56400.76. Calandrino, R., del Vecchio, A., Savi, A., Todde, S., Belloli, S., 2009. Intake risk and dose evaluation methods for workers in radiochemistry labs of a medical cyclotron facility. Health Phys. 97 (4), 315–321. http://dx.doi.org/10.1097/HP.0b013e3181ad8192. EURATOM 1996: Council Directive 96/29/Euratom of 13 May 1996. laying down basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. ICRP Publication 66, 1994. Human respiratory tract model for radiological protection. Ann. ICRP 24 (1–3). ICRP Publication 60, 1991. 1990 Recommendations of the International commission on radiological protection. Ann. ICRP 21 (1–3). Kocher, D.C., 2000. Assessment of occupational exposure due to intakes of radionuclides. Health Phys. 78 (5), 567. http://dx.doi.org/10.1097/00004032-200005000-00018. MecMurphil web site - 〈http://www.molimag.eu/mecmurphil.html〉 (accessed Jul 2016). Sarnelli, A., Negrini, M., D’Errico, V., Bianchini, D., Strigari, L., Mezzenga, E., Menghi, E., Marcocci, F., Benassi, M., 2015. Monte Carlo based calibration of an air monitoring system for gamma and beta+ radiation. Appl. Radiat. Isot. 105, 273–277. http://dx.doi.org/10.1016/j.apradiso.2015.08.040. Terranova, N., Testoni, R., Cicoria, G., Mostacci, D., Marengo, M., 2011. Assessment of internal contamination hazard and fast monitoring for workers involved in maintenance operations on PET cyclotrons. Radiat. Prot. Dosim. 144, 468–472. http://dx.doi.org/10.1093/rpd/ncq327. Valentin, J., 2007. The 2007 Recommendations of the International commission on radiological protection. ICRP Publication 103 (ed.)Ann. ICRP 37.
(2)
with the index k running over the different areas and the index i running over the single measurements in every area. The results, assumed to be representative of the entire year, are presented in Table 3. Propagating the error, and considering a 5% incertitude in the efficiency calibration, an overall error of ca. 7% (1 sd.) was determined for the radiopharmacy value, 9% for the hot lab. 3.2. Inhalation results Occupancy time and activity conducted were timed for hospital staff: they are reported in Table 4. From these data and the ventilation rates of Table 2 average ventilation rates were assessed, for all the hospital staff categories: results are presented in Table 5; total air inhalation was calculated, for all personnel, in every area. 3.3. Committed dose results The conversion coefficients prescribed in the Italian legislation have values that depend on the absorption type (Slow, Medium, Fast) and on
4
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Air contamination measurements for the evaluation of internal dose to workers in nuclear medicine departments B. De Massimia, D. Bianchinib, A. Sarnellib, V. D’Erricob, F. Marcoccib, E. Mezzengab, ⁎ D. Mostaccia, a b
Department of Industrial Engineering, Alma Mater Studiorum - University of Bologna, Italy Medical Physics Department, Istituto Scientifico Romagnolo per lo Studio e la Cura dei Tumori (IRST) IRCCS, Meldola (FC), Italy
A R T I C L E I N F O
A BS T RAC T
Keywords: Internal doses Nuclear medicine Airborne radioactivity Air contamination monitoring
Radionuclides handled in nuclear medicine departments are often characterized by high volatility and short half-life. It is generally difficult to monitor directly the intake of these short-lived radionuclides in hospital staff: this makes measuring air contamination of utmost interest. The aim of the present work is to provide a method for the evaluation of internal doses to workers in nuclear medicine, by means of an air activity sampling detector, to ensure that the limits prescribed by the relevant legislation are respected. A continuous air sampling system measures isotope concentration with a Nal(TI) detector. Energy efficiency of the system was assessed with GEANT4 and with known activities of 18F. Air is sampled in a number of areas of the nuclear medicine department of the IRST-IRCCS hospital (Meldola- Italy). To evaluate committed doses to hospital staff involved (doctors, technicians, nurses) different exposure situations (rooms, times, radionuclides etc) were considered. After estimating the intake, the committed effective dose has been evaluated, for the different radionuclides, using the dose coefficients mandated by the Italian legislation. Error propagation for the estimated intake and personal dose has been evaluated, starting from measurement statistics.
1. Introduction The use of unsealed radiation sources exposes personnel of nuclear medicine facilities to a potential risk of external and internal exposure (Valentin, 2007). Currently the most widely used radiopharmaceutical in nuclear medicine imaging is 18F -FDG. The 18F isotope is a pure β+ emitter with a half life of 110 min, and it is produced, commercialized and utilized in radiopharmacy in liquid form. The volatility of the FDG and the radiological impact of contaminated air has been investigated thoroughly, see e.g. (Calandrino et al., 2009, 2007). Contamination by airborne agents needs to be evaluated reliably, or the absence thereof established clearly, in view of obeying the limits on effective dose to exposed personnel (EURATOM 1996). Evaluation of intake activities to be used with biokinetic models (Valentin, 2007) can be conducted with direct and indirect methods (Kocher, 2000). Direct whole body measurements can determine the occupational radionuclide intake to hospital staff (Terranova et al., 2011). Indirect measurements, on the other hand, derive intake activities from measurement of the activity present in biological samples (e.g. excreta) and physical samples (e.g. air filters). In the ICRP 103 recommendations (Valentin, 2007) the DAC (Derived Air
⁎
Concentration) is defined as the activity concentration in air that given the breathing rate and yearly exposure time of a worker would result in a dose at the Annual Limit of Intake, and used as the operative quantity to estimate the internal dose due to inhalation. Measurements connected with the utilization of the DAC can be made with PAS (Personal Air Sampler) that are individual filter based air sampler worn by the exposed workers. As a general remark, PAS sampling technique is limited by the fact that the airflow through the device will not be representative of the breathing rate of a human, and the air sampled is not the same air that the worker will breathe, so 'hot particles' in the air may cause a substantial difference in dosimetry if care is not taken. But coming specifically to the present case, the short half-life of 18F, like that of many other isotopes used in nuclear medicine, and a weak correlation with biological sampling measures (Britcher and Strong, 1994), are strongly limiting factors for the evaluation of intake radioactivity through this method, rendering the PAS essentially useless for the purpose, hence the need to evaluate the concentration of airborne activity from instant environmental radioactivity concentration: measuring air contamination becomes the most practicable possibility to evaluate inhalation-related doses to personnel. The present work presents a method for the evaluation of internal doses to personnel
Correspondence to: via dei Colli 16, 40136 Bologna, Italy. E-mail address: [email protected] (D. Mostacci).
http://dx.doi.org/10.1016/j.radphyschem.2017.03.003 Received 28 September 2016; Received in revised form 1 March 2017; Accepted 2 March 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: De Massimi, B., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.03.003
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
B. De Massimi et al.
Fig. 1. Functional scheme of the air sampling system. Table 1 Washout, drain and acquisition times.
2. Materials and methods
Area
Washout [s]
Drain [s]
Acquisition [s]
2.1. Radioactivity measurements
Hot lab Hall Hot waiting room 18 F cell Radiopharmacy
30 30 30 30 30
10 15 25 10 60
600 600 600 600 600
A continuous air sampling system (MecMurphil® MP-AIR) with a flow of 10 m3/h was used, equipped with a series of valves operated automatically through a dedicated computer; the system is connected, through these valves and an appropriate system of air ducts, to a number of locations of interest. Through the valve system, the computer selects which locations to sample: in fact, all locations are sampled in sequence, and the process is repeated continuously, 24 h/d. Fig. 1 presents a scheme outlining the operation of the device. The air sampled is run through a 0.00312 m3 Marinelli beaker positioned on a 2”×2” Nal(TI) detector (DI-65/50 by Gamma Technical Corporation, 50 diameter by 50 mm length), and the assembly is contained in a low background shielding (lead, 5 cm around, 6 cm top and bottom, all with 3 mm copper coating); concentration of isotopes in the air flushed is assessed measuring the gamma rays from annihilation of the β+ positrons emitted in the decay. Energy efficiency of the detector is assessed filling the Marinelli beaker with water containing known concentrations of 18F: some adjustment is needed to account for the fact that the measurements of interest are conducted on air, whereas calibration uses water. For instance, the positrons emitted in the decay of 18F have a range in air such that annihilation with air electrons is rare, and the positrons emitted in the volume of the beaker have a large probability of reaching the beaker walls and annihilate there (Sarnelli et al., 2015). Hence, in most cases the gamma ray pairs produced in the annihilation will be generated within the walls of the Marinelli beaker. On the contrary, when the beaker is filled with 18F loaded water, annihilations happen essentially at the point of emission, giving rise to a uniform volume source of gamma rays. As a result, when the filling is 18F in air (as is the case in actual measurements), gamma emission takes place farther, on average, from the detector. On the other hand self absorption in air is practically negligible, but this is not so in water. To correct for these opposing measurement phenomena, a calibration factor was calculated using GEANT-4 (Sarnelli et al., 2015). As mentioned above, radionuclide concentration is determined flushing the possibly contaminated air through the Marinelli beaker and counting it with the NaI detector. For the measurement to be representative the air flushed through the counting system needs to have the same radionuclide concentration as that in the room sampled. Deposition in the piping
Table 2 Ventilation rates (ICRP 66). Activity
Ventilation rates [m3 h-1]
Sleep Rest, Sitting Light exercise Heavy exercise
0.45 0.54 1.5 3.0
in the nuclear medicine department of the IRST-IRCCS Hospital (Meldola – Italy) based on a continuous air sampling system and on individual workload evaluation. The method is of more general applicability, however, to demonstrate it, in what follows its specific application to 18F will be presented.
Table 3 Mean concentration 1-OCT to 31-DEC, 2015 [Bq/m3]. Area
Hot waiting room (C3)
Hot cell 18 F (D4/ 1)
Hall (B2)
Hot Lab. (A1)
Radiopharmacy (G7)
Mean concentration
137
132
141
129
194
2
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
B. De Massimi et al.
Table 4 Percentage of time shift by area hospital staff categories. 18
Hot waiting room (C3)
Hot cell 0
Technologist/Radiographer
5% (21 min) 0
Physician
0
20% (1 h 24 min) 0
Radiopharmacist
0
0
Nurse
F (D4/1)
certainly takes place, however if air is drawn for some time an equilibrium will be reached between deposition and resuspension: so if air is drawn for some extent of time before measurement it can be assumed that once the measurement takes place the air is essentially in this equilibrium situation and is therefore representative of the air in the room sampled. The actual measurement procedure is as follows: The controlling computer selects an area to be sampled, opening the corresponding valve, then the measurement starts, comprised of the following three steps:
• •
Nurse Technologist/ Radiographer Physician Radiopharmacist
Light exercise [h]
Heavy exercice [h]
Mean rate [m3 h-1]
7 7
2 4
5 3
0 0
1.2 0.95
8 7
5 5
3 2
0 0
0.9 0.8
Radiopharmacy (G7)
60% (4 h 12 min) 80% (5 h 36 min) 10% (48 min) 0
0
0
0
0
0
0
∼0
100% (7 h)
18
F for the several hospital staff categories. Annual dose [mSv]
Nurse Technologist/Radiographer Physician Radiopharmacist
2.541 2.074 2.400 2.880
E−02 E−02 E−03 E−02
different areas are shown in Table 1.
•
MEASURE: Air from the area selected is flushed through the Marinelli beaker for 600 s and counted to determine radionuclide concentration.
Once the sampling process has concluded, the next location is selected, air is sampled and counted. Once all the rooms have been sampled, the sampling begins again at the first room. This sampling and measurement process is continuous and occurs 24 h a day, seven days a week. The entire process was monitored for 3 months, specifically from October 1st to December 31st, 2015. Average values for the entire period were determined. Due to the characteristics of nuclear medicine daily routine, essentially organized on a weekly schedule basis, the 3month average can be taken as fully representative of the entire working year.
Table 5 Average ventilation rates for the several hospital staff categories. Sitting [h]
Hot Lab. (A1)
Table 8 Annual dose from
WASHOUT: External, clean air is flushed through the beaker for 30 s to remove all previous contaminants. Preliminary investigation has proven that 30 s is sufficient time for a thorough sweeping of the beaker, and there is no need check decontamination of the marinelli after washout is completed. DRAIN: Air from the air duct connected to the location selected is drawn for 15–60 s, depending on pipe length and other considerations, until the air sitting in the ducts is removed and fresh air from the location under exam reaches the Marinelli. Timing for the
Shift [h]
Hall (B2)
2.2. Inhalation assessment Table 6 Dose coefficients for inhalation. T½
18
F
Absorption
Table 7 Annual dose due to
18
11
F M S
1.83 h
h (g)1 μm 3,0 10 5,7 10–11 6,0 10–11
To evaluate the internal doses received by the different hospital staff categories (doctors, technicians, nurses, radiopharmacists) all the different possible exposure situations were considered: rooms visited, occupancy times, type of work effected and so forth. Inhalation was then evaluated - the breathing rates of hospital staff were determined from the exercise rates for occupational activities: the different possible activities were analyzed and categorized on the basis of the type of effort required, to determine the ventilation rate following ICRP 66, se Table 2 (ICRP, 1994).
h (g)5 μm 5,4 10–11 8,9 10–11 9,3 10–11
F, breakdown by room and hospital staff categories [mSv].
Nurse Technologist/Radiographer Physician Radiopharmacist
18
Hot waiting room (C3)
Hot cell
1.29E−03 0 0 0
0 3.94E−03 0 0
F (D4/1)
3
Hall (B2)
Hot Lab (A1)
Radiopharmacy (G7)
1.59E−02 1.68E−02 2.40E−03 0
0 0 0 0
0 0 0 2.88E−02
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
B. De Massimi et al.
the AMAD (Activity Median Aerodynamic Diameter: 1 µm or 5 µm) see Table 6. For the compounds utilized the absorption type was Medium; in the absence of a dedicated investigation the AMAD could not be specified, although from usual experience it is quite fine in these environments. The value for 5 µm was taken as representative, both to follow legislation (Legislative Decrees number 187 and 241 of 2000, unavailable in English) and because the associated value of the conversion coefficient was the higher of the two. From average concentration, occupancy times and ventilation rates, introduction by inhalation was calculated and with the conversion coefficient committed dose was assessed for all hospital staff categories. The results are presented Tables 7, 8. It can be seen that committed effective dose to all personnel proved negligible.
The same member of the personnel exerting different activities and hence effort - in different areas, the ventilation rates for each person varied from area to area and had to be determined separately: from the ventilation rate and the time of occupancy the total air inhalation was calculated for every location of interest. To this effect, operating schedules and modalities of all personnel were observed for the same 3-month period considered above and times measured accurately; averages were calculated for total air inhalation in all the several areas of interest and for all the healthcare professionals involved. 2.3. Committed dose assessment From air inhalation and air contamination average data, yearly radionuclide inhalation was calculated for the different hospital staff categories involved in the survey. Using the dose conversion factors prescribed by the present Italian legislation - Legislative Decrees number 187 and 241 of 2000 Legislative Decrees number 187 and 241 of 2000 (unavailable in English), adopting the Council Directive 96/29/Euratom (EURATOM, 1996) that was based on ICRP Publication 60 (ICRP, 1991) - the committed dose for every person involved was assessed and compared to the dose limits prescribed in the radiation protection legislation.
3.4. Discussion and future prospects As can be seen from the results presented, the committed dose proves to be negligible even compared to the more stringent 1 mSv/a limit prescribed for the general public, to the point that even an increase of, say, 200% in the above evaluation would still maintain the committed dose widely below the limit. The present procedure will be applied to other radionuclide of interest in nuclear medicine (e.g., 99 mTc) to prove its wider applicability.
3. Results Results for the procedure will be presented with reference to
18
4. Conclusions
F.
The method proposed permitted to assess the committed effective dose for all personnel of the nuclear medicine department, fulfilling the legal prescriptions with the proof that dose limits were not infringed – actually, the contribution of inhalation to annual dose was found to be practically null. The procedure served its purpose and, albeit it has been discussed only in reference to 18F, it can be applied to any radionuclide used in nuclear medicine as long as it can be measured effectively in the apparatus.
3.1. III.1 Radioactivity measurements As discussed in section II.1, continuous measurements were taken every day for three months for sampled work areas in the hospital. This produced 1850 measurements for every area. The measurements were averaged calculating the sample mean and variance with Eqs. (1) and (2):
zk =
σk2 =
1 1850
1850
1 1850 2
∑ xik i =1
(1)
References
1850
∑ σ x2ik i =1
Britcher, A.R., Strong, R., 1994. Personal air sampling-a technique for the assessment of chronic low level exposure? Radiat. Prot. Dosim. 53 (1–4), 59–62. Calandrino, R., del Vecchio, A., Todde, S., Fazio, F., 2007. Measurement and control of the air contamination generated in a medical cyclotron facility for PET radiopharmaceuticals. Health Phys. 92 (5), S70–S77. http://dx.doi.org/10.1097/ 01.HP.0000253941.56400.76. Calandrino, R., del Vecchio, A., Savi, A., Todde, S., Belloli, S., 2009. Intake risk and dose evaluation methods for workers in radiochemistry labs of a medical cyclotron facility. Health Phys. 97 (4), 315–321. http://dx.doi.org/10.1097/HP.0b013e3181ad8192. EURATOM 1996: Council Directive 96/29/Euratom of 13 May 1996. laying down basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. ICRP Publication 66, 1994. Human respiratory tract model for radiological protection. Ann. ICRP 24 (1–3). ICRP Publication 60, 1991. 1990 Recommendations of the International commission on radiological protection. Ann. ICRP 21 (1–3). Kocher, D.C., 2000. Assessment of occupational exposure due to intakes of radionuclides. Health Phys. 78 (5), 567. http://dx.doi.org/10.1097/00004032-200005000-00018. MecMurphil web site - 〈http://www.molimag.eu/mecmurphil.html〉 (accessed Jul 2016). Sarnelli, A., Negrini, M., D’Errico, V., Bianchini, D., Strigari, L., Mezzenga, E., Menghi, E., Marcocci, F., Benassi, M., 2015. Monte Carlo based calibration of an air monitoring system for gamma and beta+ radiation. Appl. Radiat. Isot. 105, 273–277. http://dx.doi.org/10.1016/j.apradiso.2015.08.040. Terranova, N., Testoni, R., Cicoria, G., Mostacci, D., Marengo, M., 2011. Assessment of internal contamination hazard and fast monitoring for workers involved in maintenance operations on PET cyclotrons. Radiat. Prot. Dosim. 144, 468–472. http://dx.doi.org/10.1093/rpd/ncq327. Valentin, J., 2007. The 2007 Recommendations of the International commission on radiological protection. ICRP Publication 103 (ed.)Ann. ICRP 37.
(2)
with the index k running over the different areas and the index i running over the single measurements in every area. The results, assumed to be representative of the entire year, are presented in Table 3. Propagating the error, and considering a 5% incertitude in the efficiency calibration, an overall error of ca. 7% (1 sd.) was determined for the radiopharmacy value, 9% for the hot lab. 3.2. Inhalation results Occupancy time and activity conducted were timed for hospital staff: they are reported in Table 4. From these data and the ventilation rates of Table 2 average ventilation rates were assessed, for all the hospital staff categories: results are presented in Table 5; total air inhalation was calculated, for all personnel, in every area. 3.3. Committed dose results The conversion coefficients prescribed in the Italian legislation have values that depend on the absorption type (Slow, Medium, Fast) and on
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