Diagnostic radiation exposure and cancer risk

Diagnostic and therapeutic radiological investigations are an essential part of the workup of patients with a number of clinical problems across a variety of medical specialties. Although new non-x ray technologies have started to replace traditional investigations these have not lead to a reduction in radiation exposure. In contrast, based on global statistics and projections, radiation exposure of patients is increasing, in particular as a result of new indications and use in cross sectional imaging.¹ In addition, multiple investigations of patients with chronic disease can lead to substantial individual radiation exposure as surgical practice increasingly relies on the use of cross sectional imaging to aid diagnosis and treatment.² New imaging techniques, in particular computed tomography (CT) colonography, have become attractive alternatives to conventional colonoscopy.³ However, the necessity for both prone and supine scanning means that radiation exposure is double that of a conventional abdominal scan which can lead to a theoretical increase in the risk of exposure related cancer and death.⁴

The awareness of hospital doctors about radiation exposure and associated cancer risk is poor.⁵ From personal experience, many gastroenterologists involved in diagnostic and therapeutic procedures using ionising radiation do not routinely wear full protective clothing (0.35 mm lead equivalent aprons, thyroid shield, lead glasses) on a regular basis. This is also shown in a survey of endoscopic retrograde cholangiopancreatography (ERCP) practices by Campbell et al in which only 52% of respondents reported wearing a thyroid shield all of the time.⁶ An audit of radiation exposure to personnel performing ERCP found that both patients and staff are exposed to significant radiation exposure. This was equivalent to an estimated additional lifetime fatal cancer risk of between 1 in 3500 and 1 in 7000.⁷ These studies highlight the substantial underestimation by medical staff of patient and operator related radiation induced cancer risk.

The National Radiological Protection Board (NRBP) has recently revised the radiation dose for typical x ray examinations. For example, an abdominal/pelvic CT scan would typically lead to an effective dose of 10 mSv, which is an equivalent of 4.5 years of natural background radiation.⁸ This radiation exposure was estimated to carry a 1:2000 risk of fatal cancer in the 16–69 year old patient population (personal communication from NRPB). For older patients, this may be halved but for younger patients increased up to fivefold. Put another way, this is equal to 250–300 fatal cancers for every 1 million abdominal/pelvic CT scans.

These values are calculated using a probability coefficient, which was developed by the International Commission on Radiological Protection (ICRP), based on historical epidemiological cohorts and other research.⁹ It is worth noting that the cancer risk attributed to radiation exposure has constantly risen over time as longer term follow up information from the Hiroshima and Nagasaki cohorts have become available. It is therefore possible that radiation attributed cancer risk will continue to rise in the future.

In the UK, diagnostic x rays related cumulative risk of cancer to age 75 years was recently estimated at 0.6%, which is equivalent to approximately 700 cases of cancer per year.¹⁰ In the same report, this rate was estimated to be up to five times higher for countries with a higher use of diagnostic x rays. The British Society of Radiology has made specific recommendations to reduce radiation exposure.¹¹ Adherence to these guidelines may well be an explanation for the comparatively low frequency of diagnostic x ray in UK practice.¹² Clinicians should use these recommendations when considering radiological investigations. Protection of operators and nursing staff using recommended protective clothing should also be followed. Change in clinical practice may not be easy to achieve as, for example, endoscopic capacity to reduce the number of alternative radiological investigations, such as barium enemas, is limited. In contrast, barium enemas are often used to reduce the demand on endoscopic services. New technologies and methods may well reduce radiation exposure. Examples in gastroenterology include magnetic resonance cholangiopancreatography or endoscopic ultrasound instead of ERCP and magnetic resonance enteroclysis instead of small bowel enema. Technological advances, in particular low dose helical CT colonography, may reduce radiation exposure by 40–70%.¹³ However, availability of these technologies is limited or only slowly increasing and it is therefore unlikely that their use will influence radiation exposure in the near future.

What remains is the judicious use of radiological investigations and close liaison with radiologists in order to keep the radiation exposure of patients and staff as low as possible.