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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 6  |  Issue : 4  |  Page : 215-221

Leakage radiation and workplace monitoring of a cobalt-60 teletherapy facility in South-West Nigeria: Is the dose significant?


1 Department of Radiation Biology, Radiotherapy, Radiodiagnosis and Radiography, College of Medicine, Lagos University Teaching Hospital, Lagos, Nigeria
2 Department of Radiology, Medical Physics Unit, Federal Medical Center, Asaba, Delta State, Nigeria
3 Department of Radiology, Medical Physics Unit, University College Hospital, Ibadan, Nigeria

Date of Web Publication12-Dec-2017

Correspondence Address:
Mr. Akintayo Daniel Omojola
Department of Radiology, Medical Physics Unit, Federal Medical Center, Asaba, Delta State
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijhas.IJHAS_91_17

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  Abstract 


BACKGROUND: Cobalt-60 (Co-60) teletherapy machines are still in use in most developing countries because of their minimal power requirements, reduced operational cost, and since the source does not vary in energy, the amount of quality assurance required to ensure “good beam” is dramatically reduced. Although as the machine wear, the chances of leakage radiation increase.
AIMS AND OBJECTIVES: The aim of this study was to determine if leakage radiation at 5 cm and 1 m from the Co-60 teletherapy source head is within the acceptable tolerance limit set by the International Electro-technical Commission (IEC) and to determine if controlled and supervised areas within working hours were within the Institute of Physics and Engineering in Medicine (IPEM) limit.
MATERIALS AND METHODS: The machine used was a Theratron® Phoenix Cobalt 60 Teletherapy machine. A RadEye B20-ER Multi-Purpose Survey Meter was used to measure mean time-average dose rate (TADR) at various points in the controlled and supervised areas. Instantaneous dose rate (IDR) for leakage radiation was measured at 5 cm from the source head using the same Survey Meter and a measuring tape. In addition, measurement was made at 1 m from the normal treatment distance in patient and nonpatient planes.
RESULTS: The mean TADR at beam OFF position in the controlled area at four different areas was 2.15 ± 0.48 μSv/h, which was <7.5 μSv/h IPEM limit and mean TADR in the supervised area at six different areas was 1.70 ± 0.45 μSv/h, which was also <2.5 μSv/h IPEM limit. The percentage IDR leakage radiation at beam OFF position at 5 cm and 1 m was within 200 μSv/h and 20 μSv/h IEC tolerance limit, respectively. Percentage leakage radiation at beam ON in patient plane was below the maximum and average IEC tolerance limit and nonpatient planes at 1 m was below 0.5% IEC limit.
CONCLUSION: Supervised and controlled areas were within the acceptable range. Leakage radiation was within the tolerance limit.

Keywords: Cobalt-60 teletherapy machine, dose rate, instantaneous dose rate, medical physicist, radiographer, survey meter, time-average dose rate


How to cite this article:
Akpochafor MO, Omojola AD, Habeebu MY, Adeneye SO, Madu CB, Aweda MA, Orotoye T. Leakage radiation and workplace monitoring of a cobalt-60 teletherapy facility in South-West Nigeria: Is the dose significant?. Int J Health Allied Sci 2017;6:215-21

How to cite this URL:
Akpochafor MO, Omojola AD, Habeebu MY, Adeneye SO, Madu CB, Aweda MA, Orotoye T. Leakage radiation and workplace monitoring of a cobalt-60 teletherapy facility in South-West Nigeria: Is the dose significant?. Int J Health Allied Sci [serial online] 2017 [cited 2018 Jan 23];6:215-21. Available from: http://www.ijhas.in/text.asp?2017/6/4/215/220528




  Introduction Top


Radiation protection is concerned with controlling exposures to ionizing radiation so that the risk of radiation-induced cancer and hereditary disease (termed stochastic effects) is limited to acceptable levels and tissue reactions are prevented. It is estimated that 50% of all cancer patients diagnosed receive radiotherapy as a primary treatment, or in combination with other treatment modalities either as palliative or cure, putting in mind radiation safety.[1],[2] The effect of radiation was detected in early workers who were involved in the use of ionizing radiation due to overexposure. Radiation monitoring plays an important role in estimating the amount of dose received by an individual over a period of time, knowing well that we cannot smell or see these ionizing particles.[3],[4],[5],[6]

Teletherapy machines (which use gamma rays) have been in use since the early 1950s for the treatment of different cases of cancer and other malignancy. Unlike linear accelerators that are produced by electron interaction, Co-60 is generated from the nucleus of an atom.[7] The cobalt-60 (Co-60) source is produced by irradiating ordinary stable Co-59 with neutrons in a reactor, the co-60 decays to Nickel-60, by the emission of beta particle. The activated nickel nucleus emits two gamma ray photons with energies of 1.17 MeV and 1.33 MeV resulting in an average beam energy of 1.25 MeV.[8] Teletherapy units are widely used in developing countries for cancer treatment and are preferred over medical linear accelerators (linacs) because of their low cost, low maintenance cost, lower power requirements, and less downtime. It is recognized that medical linac has some notable advantages over teletherapy machines such as variable dose rates, multiple photon and electron beams and energies, and smaller beam penumbra.[9],[10],[11],[12]

In Nigeria, today, there are only four teletherapy facilities that are available. One out of the four teletherapy machines is used as a secondary calibration center in the National Institute of Radiation Protection and Research, Ibadan, an arm of the Nigerian Nuclear Regulatory Authority.

Some standard organizations have been put in place to checkmate and control leakage radiation while using high-level gamma beam. One among such is the International Electro-technical Commission (IEC) which serves as a nonprofit and nongovernmental international standard organization. It helps to prepare and publish international standard for all electrical, electronic, and related technologies. IEC standards cover a vast range of technologies from power generation, transmission, and distribution to home appliances and office equipment, semiconductors, fiber optics, batteries, solar energy, nanotechnology, and marine energy as well as many others. Of interest to us is the IEC 60601-2-11 standard which applies to the basic safety and essential performance of gamma beam therapy equipment, including multi-source stereotactic radiotherapy equipment. This particular standard of the 60,601 series establishes requirements to be complied with by manufacturers in the design and construction of gamma beam therapy equipment. It states tolerance limits beyond which interlocks must prevent, interrupt, or terminate irradiation to avoid an unsafe condition. Its purpose is to identify those features of design which are regarded at the present time as essential for the safe operation of such equipment. It places limits on the degradation of equipment performance at which it can be presumed that a fault condition applies, for example, a component failure, and where an interlock then operates to prevent continued operation.[13],[14],[15]

In addition, another document that is of interest to us is the Medical and Dental Guidance Notes titled “A good practice guide on all aspects of ionizing radiation protection in the clinical environment” prepared by IPEM with support from other bodies like National Radiological Protection Board and Health and Safety Executives. This document explains in detail the recommended values for designated radiation areas.[16]

This study was carried out in a radiotherapy center in Lagos, Nigeria, where a Theratron® Phoenix Cobalt-60 teletherapy machine is currently in use. The agitations that led to this study were borne out of the fact that leakage radiations from the source head at the recommended IEC standard have not been verified. Furthermore, the controlled area which comprises of the treatment room and the control console and supervised areas which include patient waiting area, physicist's office, radiographer's office, nurse's office, reception area, and corridors have not been equally assessed. In the same vein, studies have shown that there are small permanent leakage and streaming radiation emanating from the housing of the Co-60 teletherapy unit, even after the source is retracted to the shielded OFF position.[17] It is clear to note that Co-60 teletherapy unit cannot activate any element by causing photonuclear reactions unlike high-energy linear accelerators where the head and other parts of the accelerators get X-ray-induced activity by photonuclear reactions, which remains even after the beam is switched off.[18],[19],[20],[21] Several articles on the safety of personnel working with teletherapy machine have been done; one of such studies was the re-evaluation of radiation safety at a teletherapy unit in Accra, Ghana, in which the measured scatter dose rate was compared with the acceptable dose limit.[22] Also, another study by Opoku et al. critically accessed the dose rate to the staff and public when using a Co-60 teletherapy machine whose source strength was 222 TBq (Terabecquerel).[23]

The aim of this research work was to determine leakage radiation at the treatment room at 5 cm and 100cm(1m) away from the source head at both phantom plane and other planes in relation to IEC recommended standard and to determine whether or not the dose rates at the controlled and supervised area are within the Institute of Physics and Engineering in Medicine (IPEM) dose rate guidelines.


  Materials and Methods Top


This prospective study was conducted for 6 months in a privately owned hospital-based radiotherapy department in South-West Nigeria. The assessment of this facility was spear headed by the Clinically Certified Medical Physicist in charge of Radiation Safety. Other personnel involved were oncologist, radiographers, technologist, and nurses. The medical physicist carried out various measurements from the control areas: at distances of 2 m and 4 m from the source head, entrance door of the treatment room and control console, and supervised areas: patient waiting area, medical physicist's office, radiographer's office, corridor, and receptionist's desk. Data were collected mostly during weekends when patients do not have appointment, to allow for better and efficient assessment. No human subject was used.

The properties of the Theratron® Phoenix Cobalt-60 teletherapy machine with half-life ~5.27 years have a source strength of 3545 Curies (Ci) which is approximately 131 TBq (Terabecquerel) and a RadEye B20-ER Multi-Purpose Survey Meter which has the capacity to measure alpha, beta, gamma, and X-rays. The measuring range (gamma dose rate) of the survey meter was 0–100 mSv/h (0–10 rem/h), energy range of 17 keV–1.3 MeV, and a detector made of Pancake GM-tube, of window diameter 44 mm, 1.8–2.0 mg/cm².

In this study, the RadEye B20-ER Multi-Purpose Survey Meter was used to measure mean time-average dose rate (TADR) at beam OFF position by positioning it at 15 different points in both the controlled and supervised areas. Instantaneous dose rate (IDR) for leakage radiation was measured at 5 cm from the source head using a survey meter and a measuring tape at beam OFF position. In addition, measurement was made from normal treatment distance and nonpatient plane using the same instrument at beam ON position [Figure 1]. A water phantom which is tissue equivalent was positioned at the isocenter to act as if a normal patient was being treated. Data for this study were analyzed using descriptive statistics by determining mean dose rate and standard deviation.
Figure 1: Theratron® phoenix cobalt-60 machine with phantom position at patient plane

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The study used IPEM guidelines as follows: areas where TADR exceeds 7.5 μSv/h[16],[24] should normally be designated as controlled with appropriate control to ensure “as low as reasonably practicable below the individual limit and areas where TADR exceeds 2.5 μSv/h (but <7.5 μSv/h) can normally be designated as supervised area provided TADR2000 <3.0 μSv/h.[16],[24] IEC guideline was used as follows: leakage radiation within the treatment room at 5 cm shall not exceed 200 μSv/h and at 100 cm (1 m) shall not exceed 20 μSv/h.[17] Also, IEC guideline other than patient plane shall not exceed 0.5% of the maximum absorbed dose rate on the radiation beam axis measured at 1 m from the source head. In the same vein, the maximum radiation leakage and average radiation leakage in patient plane shall not exceed 0.2% and 0.1%, respectively, for field size of 10 cm × 10 cm.[17]

Statistical analysis

Data analysis was done using Microsoft Excel, descriptive statistics, and with 95% level of significance. P < 0.05 was considered statistically significant.


  Results Top


The TADR which is taken as 8 h for a day at control area at 2 m, 4 m, entrance door of the treatment room, and control console from the source head was 2.85 ± 0.46 μSv/h, 2.09 ± 0.28 μSv/h, 1.82 ± 0.33 μSv/h, and 1.83 ± 0.53 μSv/h, respectively [Table 1]. In addition, TADR which is taken as 8 h for a day at supervised area at patient waiting area, medical physicist's office, radiographer's office, nurse's office, corridor, and receptionist's desk from the source head was 1.29 ± 0.17 μSv/h, 2.10 ± 0.53 μSv/h, 2.40 ± 0.38 μSv/h, 1.49 ± 0.31 μSv/h, and 1.51 ± 0.32 μSv/h, respectively [Table 2].
Table 1: Time-average dose rate for points within the controlled areas

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Table 2: Time-average dose rate for points within the supervised areas

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Measurement of leakage radiation for the study at beam OFF position at 5 cm from the source head by positioning the survey meter and taking measurement at 15 points round the source was 24.24, 30.92, 20.76, 21.40, 22.04, 31.08, 24.88, 17.04, 15.12, 19.08, 16.52, 22.76, 20.28, 23.80, and 28.56 μSv/h. The result was then compared to IEC tolerance limit of 200 μSv/h [Table 3]. Similarly, measurement of leakage radiation for the study at beam OFF position at 100cm(1m) from the source head by positioning the survey meter and taking measurement at 15 points round the source was 9.17, 9.33, 8.84, 7.22, 8.04, 11.58, 10.18, 9.32, 11.92, 12.01, 10.22, 9.36, 11.66, 7.57, and 13.02 μSv/h. The result was then compared to IEC tolerance limit of 20 usv/hr [Table 4].
Table 3: Leakage radiation for this study and recommended International Electro-technical Commission standard at beam OFF condition at 5 cm from source head

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Table 4: Leakage radiation for this study and recommended International Electrotechnical Commission standard at beam OFF condition at 1 m from source head

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In addition, the percentage dose rates for leakage radiation from the source-head at 100cm(1m) at beam ON position other than patient plane were 0.0612, 0.0596, 0.0588, 0.0620, 0.0552, 0.0604, 0.0644, 0.0552, 0.0556, 0.0504, 0.0576, 0.0588, 0.0564, 0.0540, and 0.0628. These values were seen to be below 0.5% IEC tolerance limit [Table 5].
Table 5: Radiation leakage from source head at beam ON position at 1 m in nonpatient/phantom plane

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Furthermore, the percentage dose rates for leakage radiation in patient/phantom plane were 0.0169, 0.0127, 0.0131, 0.0154, 0.0127, 0.0181, 0.0175, 0.0141, 0.0107, 0.0120, 0.0131, 0.0142, 0.0158, 0.0117, and 0.0149. The maximum percentage dose rate was 0.0181 and the average leakage radiation was 0.0142 [Table 6].
Table 6: Radiation leakage from source head at beam ON position at 1 m in patient/phantom plane for a 10 cm × 10 cm field size

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  Discussion Top


There was a gradual reduction in dose rate from 2 m away from source head down to the control console which is in accordance to the inverse square law which gives explanation on how dose reduces with increased distance and vice versa [Table 1]. The control console was seen to have the least dose rate due to the fact that it is the farthest area to the source head based on the design of the facility. The highest dose rate was recorded at 2 m away from the source head with a value of 2.85 ± 0.46 μSv/h. It was seen to be <7.5 μSv/h IPEM limit. The result at the treatment room for this study was seen to be consistent with Opoku et al. whose inter comparison value of measured dose rate in the treatment room at the vertical and horizontal planes for 18 points was <7.5 μSv/h. Although two points out of the 18 were >7.5 μSv/h, the highest TADR at the control console was 2.64 μSv/h, this value was seen to be close to those obtained by Opoku et al. whose highest IDR at the control console was 0.3 μSv/h, which can be estimated over 8 h to give a TADR value of 2.4 μSv/h. The highest measurement at the controlled area for this study was 3.44 μSv/h and that of Opoku et al. was 2.5 μSv/h.[22] The results for an unpaired t-test for equal variance show that there was statistically significant difference from 2 m and 4 m distance to the source head (P = 0.000). There was no statistically significant difference in TADR between the entrance door and the control console (P = 0.947). There was significant difference from 2 m distance and the control console (P = 0.000). In general, the mean TADR was statistically different (P = 0.003) for points of measurement from the source head.

The highest TADR at the supervised area in this study was recorded at the radiographers' office which is 2.40 ± 0.38 μSv/h; next to it was the physicist's office which was 2.10 ± 0.53 μSv/h. The two areas (that is radiographer and physicist office) were the most closest to the treatment room of the teletherapy machine. The TADR at patient waiting area, oncology nurse office, corridor, and receptionist's desk was >0.5 μSv/h as per the IPEM guideline for unsupervised public areas. This implies that all areas outside the controlled zone should still be monitored regularly since their TADR is >0.5 μSv/h but <2.5 μSv/h. There was statistically significant difference in the mean TADR in the supervised areas (P = 0.000).

The average leakage radiation at 5 cm and 100cm (1m) from the source head at beam OFF position was 23.57 ± 4.88 μSv/h and 9.96 ± 1.75 μSv/h, respectively, which was <200 μSv/h and 20 μSv/h of the IEC standard. This study's mean IDR value was below than that of Sahani et al's.[25] study which compared Bhabhatron-II and Theratron® Equnox-80 teletherapy units. The head leakages during source OFF condition at 5 cm and 100 cm from unit head surface (extrapolated for 555 TBq of 60Co source) were 132.2 μSv/h and 141.5 μSv/h and 18.4 μSv/h and 17.3 μSv/h, respectively. One of the reasons for increased dose rate in Sahani et al's. study was attributed to its high source strength. In another related study by Athiyaman et al.,[26] average leakage radiation at 5 cm and 100cm (1m) from a Bhabhatron-II source head at beam OFF position was 60.68 μSv/h and 10.30 μSv/h, respectively, which was <200 μSv/h and 20 μSv/h of the IEC standard.

Maximum radiation leakage for this study at 1 m from the source head under beam ON condition other than patient plane for this study was 0.056% against 0.5% tolerance limit. The maximum radiation leakage in Sahani et al's.[25] study was found to be 0.026% against 0.5% of the maximum absorbed dose at 1 m from the source in other than the patient plane. In light of this, our study's maximum radiation leakage other than patient plane was higher. Better accuracy was noticed in this study than the study by Athiyaman et al. whose maximum leakage other than patient plane (nonpatient plane) at beam ON was 0.593% against 0.5%.

Furthermore, the maximum leakage radiation in the patient/phantom plane in this study was 0.0181% against 0.2% IEC limit and the average leakage radiation was 0.0142% against 0.1% IEC limit. Sahani et al's. study which compared two sources (Bhabhatron-II and Theratron® Equnox-80 teletherapy units) had a maximum leakage radiation in the patient plane to be 0.019% and 0.133%, respectively, against 0.2% IEC limit and the average leakage radiation was 0.0097% and 0.0678%, respectively, against 0.1% IEC limit. Athiyaman et al. had a maximum leakage radiation in the patient plane to be 0.002% against 0.2% IEC limit and the average leakage radiation was 0.005% against 0.1% IEC, which was better in terms of accuracy than this study.


  Conclusion Top


The study showed that both the supervised and controlled areas were safe and were in line with the IPEM guidelines. Leakage radiation emanating from the source head was also within acceptable IEC tolerance limit. A recheck must always be done from time to time, and should in case a replacement of source is to be made, reassurance of safety in and around the source head, controlled, supervised, and other unsupervised public areas must be met.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]



 

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