Radiation safety in interventional radiology and cardiology

Mark Hanson
East Kent Hospitals NHS Trust, Canterbury, Kent, UK
Address for correspondence:
Mark Hanson
Head of Radiology Physics
East Kent Hospitals NHS Trust
Canterbury, Kent, UK
Tel: +44 (0)1227 864148
Fax: +44 (0)1227 783137
Email: Mark.Hanson@ekht.nhs.uk

Abstract
Interventional radiology and cardiology can be associated with a significant radiation hazard to staff. A risk assessment developed for each facility can be used to assess the radiation produced and to define the steps required to minimise the radiation exposure of staff. A hierarchy of control measures should be implemented, including the consideration of environmental and equipment features, working practices and personal protection. Monitoring, particularly relating to radiation exposure at different locations, is essential to ensure the adequacy of safety measures. Co-operation between radiation safety experts and clinical colleagues to define and maintain radiation safety conditions will enable appropriate protection of specialist staff.

Overview
Common features of interventional radiology and cardiology are the extensive use of fluoroscopy and fluorography - relatively lengthy dynamic procedures that can involve staff being close to the patient for long periods - and the use of open couch X-ray systems. This combination of factors presents a significant radiation hazard that demands careful measures to restrict the radiation exposure of staff. Patients can also be exposed to high radiation doses. The frequent need to image for long periods over the same region of skin can lead to deterministic effects such as erythema or dermal necrosis, the thresholds for which are 2 Gy and 20 Gy respectively [1] Consequently, all acceptable measures that restrict the use of radiation are desirable and beneficial to patients and staff alike.

Understanding the radiation hazard

Figure 1. Scatter simulation in a cardiology laboratory. A stack of polymethyl methacrylate (similar in atomic number and density to tissue) is used to simulate the patient and radiation measurement is made with a large volume ionisation chamber.

The radiation hazard to staff stems predominantly from scattered radiation and needs to be determined either through theoretical prediction or experimental simulation (see Figure 1). Published data in the UK [2] gives a broad indication that a vascular or cardiac facility may generate a weekly dose-area product (DAP) reading in the range 1000-2000 Gy cm². The International Commission on Radiological Protection (ICRP) has published [1] suggested isodose curves for a typical interventional facility, indicating a hazard of 8 µSv per Gy cm² at 1.0 m. Local studies indicate that the position of the lead clinician will often be closer, e.g. around 70 cm from the scattering centre, indicating an approximate doubling of the suggested isodose level at 1.0 m. Putting this information together, the predicted dose at the 70 cm position over one year is in excess of 1200 mSv. A comparison of this value with existing dose limits [3-5] of 20 mSv (effective dose), 150 mSv (eye lens) and 500 mSv (extremities) demonstrates clearly how dose limits may be a small fraction of the potential hazardous radiation field and hence that safety is an essential issue to address.

Risk assessment
Realistically, safety conditions may only be properly understood through the development of a specific risk assessment for a facility [6,7]. The risk assessment should take account of:

• all relevant equipment factors (i.e. equipment settings, scatter dose rate)
• the hierarchy of safety control (see below)
• staff activity (where staff stand, how work is shared), and
• workload

to predict staff exposure.

In doing this, it is important to incorporate accurate information whenever possible, linked with clearly defined and reasonable assumptions where necessary. Remember that the results of personal monitoring indicate what doses staff actually receive; the risk assessment helps understanding of how the doses can arise and how they may be reduced.

Creating safe conditions
To attain a high standard of radiation safety, it is essential to follow an appropriate hierarchy of control measures [6,7].





Attention should be paid to the degree of protection that each option may provide and its practicality. For example, the transmission of X-rays through lead aprons typically ranges from 1.5-7% for 0.35 mm lead equivalence to 0.5-3.5% for 0.5 mm of lead equivalence, over the expected spectrum of X-rays. For protection, a lead equivalence of 0.5 mm is preferred for demanding work, though aprons can prove uncomfortable if worn for long periods. Two-piece garments can improve comfort and provide the added benefit of a double layer of wrapped protection across the chest.

The protective effect of spectacles rests not only on the lead equivalence (which is typically 0.5 mm) but also on the area of the lens: with small lenses, there is an increased risk of eye-lens exposure to a backscatter of radiation arising from the surrounding tissues.

Monitoring
Monitoring of personnel is required to ensure that the safety measures in place are effective. In this context, direct or indirect monitoring of dose to the eye lens, thyroid and extremities may prove to be essential, in addition to whole body monitoring. It is important to note that dosemeters used to cover work at more than one facility can provide assurance but cannot reveal where contributory exposure has taken place, making linking to risk assessment difficult. There is an immediate need to correlate dose monitoring results with work at different locations. Any failure to sum the radiation doses for individuals, or instances where the monitoring is inadequate or absent, can lead to a potentially serious risk to the individual and a breach of the law.

Worked example
The information shown in Table 1 relates to a cardiology laboratory that receives safety support from the author’s team. This is an informative example because the dose monitoring information is particularly reliable, excellent safety standards are maintained in the laboratory, and the outcomes can be correlated with a detailed risk assessment. The information (see Table 1) demonstrates how actual results should relate to risk assessment, how dose reduction measures are important, and how the potential for relatively high dose is significant. The dose values may be compared with the annual dose limit of 20 mSv and the classification limit of 6 mSv. The key outcome is how the low maximum entrance dose of 0.26 mSv could rise as high as 4.2 mSv if poor safety standards were maintained, bearing in mind that no account is given here of any radiation exposure that may be received by the same staff working at other facilities.

Conclusions
There is an unquestionable need for radiation safety experts and clinical colleagues to work in close co-operation to define and maintain appropriate radiation safety conditions in interventional and cardiological facilities. High risk patients and specialist staff are both precious: we need to look after them with equal care



References

1. ICRP Publication 85: Avoidance of Radiation Injuries for Medical Interventional Procedures. Annals of the ICRP Vol 30/2. 2001.
2. Radiation Shielding for Diagnostic X-rays. Joint BIR/IPEM Report. British Institute of Radiology. 2000.
3. ICRP Publication 60: 1990 Recommendations of the International Commission on Radiological Protection. Annals of the ICRP Vol 21/1-3. 1991.
4. Risks Associated with Ionising Radiation. Annals of the ICRP Vol 22/1. 1991.
5. National Radiological Protection Board. Risk of Radiation-induced Cancer at Low Doses and Low Dose Rates for Radiation Protection Purposes. Documents of the NRPB;Vol. 6 No.1 1995.
6. Health and Safety Commission.Working with ionising radiation- Approved Code of Practice and Guidance (Ionising Radiations Regulations 1999).
7. The Ionising Radiations Regulations 1999. Statutory Instruments 1999, No. 3232.

06-2004 JB1009/MB000776/OS

 

Source: C2I2, Summer 2004

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