Previous articles in this series have centred on the evolution of radiation protection since the discovery of x-rays and radioactivity at the beginning of the last century and the current legal framework in the UK. In the second article, Michael Moores described the operational management system required for implementing radiation safety in a veterinary practice. In this article, I would like to continue with the practical radiation protection theme and discuss the role of personnel dose monitoring within the context of a local radiation safety programme.
System of radiation protection
In the European Union, the Basic Safety Standard Directive, which forms the basis for the Ionising Radiation Regulations 1999 (IRR 99) enacted in UK law has been derived from recommendations from the International Commission for Radiation Protection (ICRP). The cornerstone of modern day radiation protection is ICRP Report 60 and it is in this report we see the introduction of the three key principles of radiation protection. Namely justification, optimisation and the application of dose limits.
In occupational radiation protection the regulations focus mainly on optimisation of occupational doses through application of the ALARA (As Low As Reasonably Achievable) principle and the application of dose limits. In UK law ALARA is implemented as ALARP (As Low As Reasonably Practicable) in the regulations and requires employers to balance risks versus the overall cost of implementing dose reduction measures and is consistent with broader health safety legislation. Practically ALARP is underpinned by appropriate risk assessments and the use of appropriate control measures to ensure exposures to employees and the general public are restricted ALARP. For a small veterinary practice or veterinary hospital this would typically involve the following aspects:
- planned preventive maintenance of equipment
- designation of controlled areas and restriction of access into such areas
- the adoption of local rules and system of work to restrict exposure
- appointment of the Radiation Protection Supervisor
- the use of personal protective equipment
- training programmes
- appointment and consultation of the Radiation Protection Adviser (RPA)
- personal dose monitoring programmes
Role of Personnel Dose Monitoring
Having considered the system of radiation protection and the concepts of optimisation and application of dose limits, we can now consider the role of personal dose monitoring in demonstrating optimal levels of exposure and compliance with dose limits.
Formal legal requirements under the IRR 99 for personal dosimetry of staff are for the most part required for classified radiation workers. Responsibility for classification rests with the employer and is required when the IRR 99 classification levels are likely to be exceeded. In the UK classification is required when the dose to the whole body or an organ is likely to exceed 3/10ths of the IRR 99 dose limit. The whole body dose limit is currently 20 mSv and the classification level for this limit is therefore 6 mSv. It is highly unlikely that any radiation worker in a veterinary practice would ever need to be classified by their employer provided
appropriate ALARP measures are being adopted. However, employers are required to demonstrate that where non classified workers enter controlled areas personal doses are being maintained ALARP. The regulations state this can be done by personal dose monitoring or other suitable measurements. In the veterinary sector personal dose monitoring is almost exclusively used for demonstrating ALARP as it is a cost effective measure. Typically the unit price for a film badge dosemeter or Thermo luminescent Dosemeter (TLD) would not exceed £5.
Arrangements for personal dose monitoring will need to be formalised and it is desirable for these arrangements to be included in the local rules. Formal arrangements must include the following:
- procedures for issuing and returning dosemeters
- where dosemeters should be stored when not in use
- wear the dosemeter should be worn on the trunk
- what to do in the event a dosemeter is lost or misplaced
- what to do if an overexposure or high dose is suspected
- formal annual dose investigation level with action to be taken if exceeded
In addition the above arrangements should be supported by suitable training to communicate the above requirements to all relevant staff. The annual dose investigation level is an important trigger level for optimising staff doses and requires the dose summary reports to be regularly reviewed by the employer or his representative. A typical dose investigation level is 1-2 mSv for the Veterinary sector and would need to be confirmed with the appointed RPA. It is also worth highlighting at this juncture, that there have been incidents reported in the media where employers have failed to adequately review dose summary reports and high individual doses have gone unnoticed, leading to HSE enforcement action for failing to restrict exposure to staff ALARP.
Personal Monitoring Technologies
Currently the most widely adopted dosemeters for low risk environments, like those encountered in veterinary radiology are passive dosemeters in the form of film badges or TLDs worn on the trunk for estimating the whole body effective dose. In larger veterinary practices or hospitals extremity monitoring or eye monitoring may need to be considered if indicated by the radiological risk assessment. The use of image intensifiers for fluoroscopic procedures and the handling of unsealed radioactive materials are key work areas where other body parts may require additional monitoring following consultation with the RPA.
One of the disadvantages of passive dosemeters, is they require returning to a processing facility for reading and it could be several weeks before the dose summary reports (and a potential issue) are received by the veterinary practice. Recently we have seen the introduction of a relatively low cost active dosemeter called ‘Instadose’ supplied by Mirion technologies. This device is similar to a USB pen drive and utilises an ion storage detector to measure personal dose.
This device and other similar devices have the advantage of being read on a PC workstation, when the device is inserted into a USB port, provided the PC has internet access. The device allows a quick and easy reading to be taken and removes time delays for processing, thus allowing employers to more readily demonstrate adherence to ALARP. They can also be worn by pregnant workers to demonstrate and provide reassurance to employers and expectant mothers that fetal dose limits are not being exceeded. This type of dosemeter will ultimately replace passive dosemeters over the next ten years as the benefits from this type of technology are fully understood
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- Practical Radiation Protection – The Role of Personal Dose Monitoring in a Radiation Safety Programme - May 22, 2013