Estimation.html
Estimation of annual occupational effective doses from external ionising radiation at medical institutions in Kenya
G K Korir, BSc, MSc (Nuclear Science), PhD (Radiological Science)
Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, Massachusetts, USA
J S Wambani, MB ChB, M Med (Rad), Pediatric (Rad)
Radiology Department, Kenyatta National Hospital, Nairobi, Kenya
I K Korir, BSc, MSc (Nuclear Science), PhD (Nuclear Physics)
National Nuclear Regulator, Centurion, Pretoria
Corresponding author: G Korir (chumo2009@gmail.com)
Abstract
This
study details the distribution and trends of doses from occupational
radiation exposure among radiation workers from participating medical
institutions in Kenya, where monthly dose measurements were collected
for a period of one year (January to December 2007) using
thermoluminescent dosimeters. A total of 367 medical radiation workers
were monitored, comprising 27% radiologists, 2% oncologists, 4%
dentists, 5% physicists, 45% technologists, 4% nurses, 3% film
processor technicians, 4% auxiliary staff, and 5% radiology office
staff. The average annual effective dose for all subjects ranged from
1.19 to 2.52 mSv. Among these workers, technologists received the
largest annual effective dose. The study forms the initiation stage of
wider, comprehensive and more frequent monitoring of occupational
radiation exposures and long-term investigations into its accumulation
patterns, which could form the basis of future records on the
detrimental effects of radiation, characteristic of workers in the
medical sector, and other co-factors in a developing country such as
Kenya.
Introduction
The discovery of X-rays in 1895 by Wilhelm Conrad
Röntgen (for which he won the Nobel Prize in Physics in 1901)
wrought a revolution in medicine and medical care. Diagnostic and
experimental radiation exposures in the early 1900s revealed the
deterministic effects of radiation such as skin erythema and radiation
burns. Owing to this recognised harmful effect of radiation on patients
and experimental subjects, significant modifications took place in the
design of X-ray machines and patient positioning. Other researchers who
used radiation also suffered deterministic effects. In 1905, guidelines
on the safety of workers handling patients for diagnostic X-ray were
introduced for the first time.1
X-ray equipment was first installed in Kenya in 1936 at the current
grounds of the Kenya Medical Training College within the Kenyatta
National Hospital grounds. It was housed in a wooden structure;
radiation safety and protection was not considered at that time.
Kenya is a developing nation, with about 1 000
radiation-producing facilities country-wide of which 80% are for
medical applications. There are around 500 large X-ray machines for
diagnostic radiology, 150 for dental imaging, 27 for CT scans, 18 for
mammography and bone densitometer units, 3 cobalt radiotherapy units, 3
Linac accelerators for radiotherapy, over 100 fluoroscopy units, 5
interventional units, 2 brachytherapy units, and 3 gamma cameras. There
are less than 10 airport security cargo scanners, and a few dozen
radioactive sources are estimated to be used in agriculture, as well as
in industrial gamma radiography. A few sources with low activities are
found at the in vitro
biomedical research and teaching institutions. Medical use accounts for
the largest proportion of ionising radiation use in Kenya. It is on
this basis that the present study focused on occupational exposure in
the medical sector.
The legal framework for radiation protection in Kenya is based on laws governing radiation protection;2 subsequent regulations are being revised to ensure compliance with current international practices and safety standards.3
The regulations that govern the radiation protection of persons working
in radiation areas is covered internationally under the prescribed dose
limits derived from quantitative estimates of human studies on the
effects of acute high doses, such as the Hiroshima and Nagasaki nuclear
bomb survivors, who have demonstrated increased deaths from
circulatory, respiratory and digestive diseases associated with
radiation exposure.4
However, current regulations in Kenya do not classify radiation workers
according to recognised occupational dose limits criteria. The Type A
radiation worker conditions allow the possibility of receiving in
excess of 30% of the annual effective dose limit, and require a
mandatory medical examination each year as well as individual
monitoring of exposure levels. Type B radiation workers are highly
unlikely to receive more than 30% of the annual effective dose limit
and therefore do not undergo mandatory medical examination or have
individual exposure monitoring requirements.5
,
6
Personnel radiation monitoring in the USA is required for workers who
are likely to receive more than 10% of the occupational dose limits.7
Overall, the Kenyan regulatory requirements subject all radiation
workers to the same medical examination, and require occupationally
exposed persons to incorporate the sum of external and, where relevant,
internal radiation exposures into the dose limitation criteria.
However, assessment of internal radiation exposures is not yet well
established in Kenya.
There are indications from epidemiological studies
that radiologists and other medical X-ray workers may experience
increased mortality from cancer and leukaemia.8
Cytogenetic studies of hospital workers occupationally exposed to low
doses of ionising radiation have revealed enhanced baseline levels of
chromosomal aberrations, compared with the control populations.9
Cytogenetic monitoring of persons who accidentally had large exposures
is of special value in biodosimetry, and the measurement technique may
be extended to personnel in hospitals as well as workers in the nuclear
and radiopharmaceutical industries. Without proper calibration
references for personal dosimeters, individual monitoring of radiation
exposure may result in underestimation of the actual occupational
exposure. Thermoluminescence dosemeters (TLDs) are easy to calibrate
and give reliable dose measurements, and have been the basis of many
important studies, including national dose surveys in Sweden10 and the UK.11
A radiation safety programme should lend support to
all radiation users by promoting radiation safety at the equipment
performance level and a safe working environment. The programme
objectives require accurate and reliable monitoring of radiation
workers to effectively manage radiation protection and quality
assurance. The use of TLDs in dose measurements offers several
advantages in radiation protection monitoring programmes. TLDs are
small, robust dosimeters, allowing accurate positioning and reasonable
spatial detail in dose measurement, and they are suitable for wide
ranges of dose and dose rate values. Some TLD materials, especially Li2B407,
have nearly the same effective atomic numbers as soft tissue, and their
energy responses to absorbed radiation show little variation over wide
ranges of photon energy. The energy stored in TLD crystals following
exposure can be retained over long periods of time before read-out. TLD
cards can be re-used after suitable thermal treatment, making them
cost-effective and viable in the long term.
In Kenya, there is no recorded evidence in the
literature of studies on occupational radiation exposure, and personnel
monitoring programmes are not yet fully established, except during this
study. The aim of the study was to evaluate the dose delivered to the
various groups of radiation workers as a result of external exposure to
ionising radiation and to compare the results with dose limits stated
by international safety standards.5
,
6
Materials and methods
This study was carried out over one year by
monitoring occupationally exposed individuals working at medical
institutions that agreed to participate in the International Atomic
Energy Agency (IAEA) Project RAF/9/033 on Medical Exposure Control.12
A list of medical radiation workers indicating job group and age was
submitted by each participating hospital. Each worker was assigned 2
pairs of individual TLDs (TLD-100) with a facility and a personal
identification number (PIN) for traceability. Via hospital management,
radiation safety officers were provided with dosimeter user
instructions that included strict adherence to wearing of TLD badges on
the upper torso, between the neck and waist, and outside protective
gear when undertaking exposure-related activities. Hospital management
assigned one person to deliver the dosimeters for monthly reading and
collection of newly annealed TLD badges. Natural background radiation
levels from control TLD samples were used to correct for the actual
individual dose received by each worker.
The TLD-100 is fabricated from lithium fluoride
elements assembled in bar-coded cards encapsulated in Teflon (Harshaw
Model 0110); units were provided with the Harshaw Model 8814 card
holder to each radiologist, oncologist, dentist, physicist,
technologist, nurse, film processor, auxillary staff (cleaners in the
department) and radiology office staff in participating medical
institutions. A TLD reader (Harshaw Model 4500 operating under WinREMS
software) was used to process the TLD signals. The TLD-100 has a
radiation dose measurement range of 0.05 mSv - 10 Sv. The calibration
factor RCF used was 0.024 nC/µSv for the radiation to which
workers in the medical sector were exposed, as determined using the
manufacturer’s instruction manual and recommendations in the IAEA
Standard.12
Dosimeter
read-outs were done at the National Radiation Protection Laboratories
on the Kenyatta National Hospital grounds. Accumulated dose from TLD
cards not submitted on time for reading was excluded and an appropriate
value of the individual measured monthly mean dose was assigned
instead. For penetrating external ionising radiation, personal deep
dose equivalent (which is scientifically recommended for operational
deep dose quantity) was adopted in this study. The measured dose and
details of the data collected were entered into an Excel spreadsheet
for analysis. The collective effective dose was estimated from the
number of persons multiplied by the average effective dose. An analysis
of the average annual effective doses received for medical radiation
workers according to gender was also determined.
Results
Table I indicates the distribution of age and
annual occupational dose for different groups of medical radiation
workers in Kenya in 2007. The measured natural radiation background was
0.10 mSv, and the measurement range for annual absorbed dose was 0.32
mSv to 6.78 mSv. The largest to the smallest radiation exposure was
observed in the following worker groups respectively: technologists,
physicists, radiologists, dentists, nurses, oncologists, film
processors, auxillary staff, and radiology office staff. The annual
collective effective dose from occupational exposure in the medical
sector was estimated as 0.8 person-Sv.
Fig. 1 indicates the trends over time for the
individual worker groups. The monthly percentage range was 1 - 4%, with
an average of 2%. The spread was less than 5% for each worker group. A
standard deviation of 34% indicates the variation in exposure among the
different groups of workers involved in the medical sector.
Fig. 2 indicates the distribution of occupational
dose among the 29% female and 71% male radiation workers. The female
mean annual dose was 2.16 mSv, the male 2.14 mSv. The level of mean
annual dose for female radiation workers was higher than the fetal dose
limit of 1 mSv per year, and a study is therefore necessary to ensure
that the working environment is safe for pregnant workers.
Fig. 3 indicates the distribution of annual
personal dose; 17% were below 1 mSv and 81% between 1 mSv and 5 mSv.
For all the subjects monitored, the doses were well below the
internationally recommended limit of 20 mSv per year.5
,
6
In all the individual doses received by the radiation workers, none of
the workers qualified to be classified as type A. Only 4% of the
workers received more than 10% (5 mSv) of the annual occupational dose
limits.
Discussion
Annual average occupational dose values in Kenya are higher than those reported among South Korean medical radiation workers.13
The former country’s average annual effective dose was found to
be 2.15 mSv, which was larger than the average annual dose of 0.80 mSv
for equivalent radiation workers in South Korea for 2006. The Korean
means also were smaller than those of 3.6 mSv, 4.7 mSv and 7.7 mSv
reported for radiation workers in Nigeria for 1999, 2000 and 2001,
respectively.14 The
distribution of annual dose, however, was similar to that reported for
Portugal (1986 - 1988), which showed that 97.8% of the personnel
monitored received doses below 5 mSv.15
The average dose to all radiation workers, corrected for the natural
radiation exposure, was 4 times larger than the 2000 - 2002 estimated
value of 0.5 mSv.16 The
technologist group exhibited the larger amount of radiation exposure
owing to increased patient workload as well as the lack of physical or
engineering radiation safety measures in the working environment. The
technologist sample size produced consistent dose trends and the least
spread among the group studied. The results of this study will
consequently form the baseline for optimisation of radiation protection.
The monthly dose trend indicates a reduction of
average dose over the study period. The personal monitoring effort
therefore made radiation workers more aware, and led to improvement, of
some of their radiation protection practices. The study showed that
providing each worker with the measured monthly dose can have a
positive influence on improving radiation safety measures. Radiation
workers who, like physicists, have fundamental understanding and
knowledge of radiation safety, can derive the most benefit from these
studies because their measured monthly dose showed the largest spread
in distribution. The trend also indicates that working behaviour
changed when radiation workers realised that they would be subjected to
detailed analysis of their monthly exposure.
The level of mean annual dose to female radiation
workers exceeded the fetal dose limit of 1 mSv per year; the working
environment therefore did not comply with regulations for pregnant
radiation workers. Additional radiation safety measures were necessary
for this category of worker. Seventeen per cent of radiation workers
(comprising radiologists and technologists) worked in 2 medical
facilities and consequently received twice-larger doses than the annual
average doses for the respective groups. About 17% of the workers
monitored (mainly radiology office staff) had doses within the
permissible limits. However, some of the occupational doses for this
group were above the third quartile value obtained in the study, which
emphasises the importance of radiation safety training for all workers
in medical irradiating facilities.
Conclusion
A representative sample of occupationally exposed
workers was surveyed in an effort to determine levels of radiation
exposure in the medical industry in Kenya. The study found that annual
exposure levels ranged from 0.32 - 6.98 mSv with a skewed annual
distribution showing a median value of 1.5 mSv. Technologists were in
the upper quartile in this radiation exposure distribution, therefore
being the largest exposed group in the medical sector. The study also
found shortcomings in various regulations governing radiation exposure
of workers, wherein additional safety measures for pregnant radiation
workers was lacking. Lastly, this study will form the basis for a
national database of exposures for radiation workers that can be used
to assess potential adverse radiation effects.
Acknowledgements. We
thank the Ministry of Health, the management and radiology staff of all
the private and public facilities who agreed to participate in the IAEA
project (RAF/9/033 – Strengthening Radiological Protection of Patient and Medical Exposure Control), the National Council for Science and Technology, and the IAEA for their support.
1. Calder J. The History of Radiology in Scotland 1896-2000. Edinburgh: Dunedin Academic Press, 2001.
2. The Radiation Protection Act, Chapter 243,
Laws of Kenya. The Radiation Protection (Standards) Regulations, Legal
Notice No. 54. Nairobi: Government Printers, 1986.
3. International Atomic Energy Agency. IAEA Safety Standards. http://www-ns.iaea.org/standards (accessed 22 November 2010).
4. Shimizu Y, Pierce DA, Preston DL, Mabuchi
K. Studies of the mortality of atomic bomb survivors. Report 12, Part
II. Noncancer mortality: 1950 - 1990. Radiat Res 1999;152:374-389.
5. International Commission on Radiation
Protection. Individual Monitoring for Internal Exposure of Workers:
Replacement of ICRP Publication 54. ICRP Publication 78. Ann ICRP
1998;27:3-4.
6. International Atomic Energy Agency.
International Basic Safety Standards for Protection against Ionizing
Radiation and for Safety of Radiation Sources. Safety Series No. 115.
Vienna: IAEA, 1996.
7. Nuclear Regulatory Commission. Standards
for Protection against Radiation. 10 Code of Federal Regulations, Part
20. Washington, DC: NRC, 1988.
8. Aoyama T. Radiations risk of Japanese and Chinese low dose repeatedly irradiated population. J Uni Occup Environ Health 1989;11:432-442.
9. Bengtsson G, Blomgren PG, Bergman K, Aberg
L. Patient exposures and radiations risks in Swedish Diagnostic
Radiology. Acta Radiol Oncol 1978;17:81-105.
10. Wall BF, Fisher ES, Shrimpton PC, Rae S. Current Levels of Gonadal Irradiation from a Selection of Routine Diagnostic X-ray Examinations in Great Britain. NRPB – R105. London: HMSO, 1980.
11. International Atomic Energy Agency.
Radiation Protection of Patients.
http://rpop.iaea.org/RPOP/RPoP/Content/InformationFor/MemberStates/1_RegionalProjects/Task5PatientDose901.htm
(accessed 20 November 2010).
12. International Atomic Energy Agency.
Calibration of Radiation Protection Monitoring Instruments. Safety
Reports Series No. 16. Vienna: IAEA, 2000.
13. Lee WJ, Cha ES, Ha M et al. Occupational radiation doses among diagnostic radiation workers in South Korea, 1996–2006. Radiat Prot Dosimetry 2009;136(1):50-55.
14. Ogundare FO, Balogun FA. Whole body doses of occupationally exposed female workers in Nigeria (1999-2001). J Radiol Prot 2003;23:201-208.
15. Careiro JV, Avelar R. Occupational
exposure in medical and paramedical professions in Portugal. Radiat
Prot Dosimetry 1991;36:233-236.
16. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and Effects of Ionizing Radiation. New York: UNSCEAR, 2008.
Table I. Annual radiation exposure dose measured with TLDs worn by occupationally exposed personnel in the medical sector
Quartile dose mSv
Occupational classification
Age range
Q1
Q2
Q3
Max. annual dose (mSv)
Annual average
dose (mSv)
Number
monitored (
N
)
Person-Sv
1.
Radiologists
37 - 70
1.15
2.01
2.73
5.9
2.18
99
0.22
2.
Oncologists
40 - 68
1.58
1.63
2.00
2.1
1.55
6
0.01
3.
Dentists
30 - 71
1.88
2.27
2.54
3.6
2.04
16
0.03
4.
Physicist
26 -55
1.63
2.00
2.63
6.8
2.33
20
0.05
5.
Technologists
22 - 59
1.37
2.28
3.21
7.4
2.52
166
0.42
6.
Nurses
24 - 53
0.95
1.76
2.27
3.4
1.77
14
0.02
7.
Film processors
45- 54
1.13
1.29
1.73
1.9
1.26
10
0.01
8.
Auxillary
28 - 50
0.62
1.19
1.69
2.2
1.19
16
0.02
9.
Radiology office staff
28 - 55
0.92
1.08
1.25
2.3
1.21
20
0.02
Fig. 1. Distribution of monthly average dose according to work group.
Fig. 2. Percentage distribution of radiation workers by gender in relation to quartile dose in mSv.
Fig. 3. Distribution of annual dose among medical radiation workers by dose range.