Diagnostic imaging and cancer risk

Advice on counselling patients about the cancer risks of radiation delivered by X-rays and CT scans.

Radiation dose depends partly on the number of scans undertaken
Radiation dose depends partly on the number of scans undertaken

Medical radiology is used for screening (mammography), diagnosis, mapping (to determine disease extent) and monitoring (follow-up). Radiological imaging is also an integral part of interventional procedures.

The UK average background radiation has been reported to be about 2.2 millisieverts (mSv) per year, with regional averages ranging from 1.5 to 7.5mSv per year.1 A chest X-ray (single PA film) delivers about 0.02mSv, equivalent to about three days of natural background radiation.

The number of CT scans undertaken in hospitals in England each year for the period 1996/97 to 2012/13 steadily increased from just over 1 million to almost 5 million.2

International Commission on Radiological Protection recommendations for workers in healthcare are to monitor and restrict effective radiation doses to 20mSv per year (100mSv every five years), with a maximum 50mSv allowed in any given year.

Potential hazards

Radiological procedures are an important source of exposure to ionising radiation. Although the danger may be small, it is cumulative.

In organs and tissues exposed to X-rays, hydroxyl radicals are formed from water molecules. These radicals interact with the cellular DNA to cause strand breaks or base damage. X-rays can also directly ionise DNA.

Damaged DNA can lead to mutations, gene fusion, chromosome translocations and other abnormalities, which can in turn lead to carcinogenesis.3

A large number of patients are exposed to radiological diagnostic imaging and even small risks could translate into considerable numbers of future cancers.

For the US population, for example, it is estimated that an additional 29,000 future cancers could be related to the approximately 72m CT scans performed in 2007 (there is typically a five-year lag between radiation exposure and cancer diagnosis).4

A basic understanding of diagnostic radiation doses for commonly used imaging, such as plain X-rays, CT scans and coronary angiograms, is very helpful for counselling patients. Awareness of the delayed cancer risk and the cumulative nature of radiation damage can help the patient and the physician to weigh up the risks and benefits of potentially avoidable radiological investigations.

Explaining the risk to patients

The potential harm that might be caused to a patient is determined by a range of factors. The radiation dose is dependent on the size of the patient, the body position and extent of the scan and also the number of scans undertaken. The gender and age of the patient will affect the risk level, with lifetime cancer mortality risks from radiation exposure decreasing with increasing age. Some CT scans have lower doses of radiation than others5, although low-dose CT scanning is not currently widely available in the UK.

Figure 1 shows examples of typical radiation doses from diagnostic procedures, the equivalent number of chest X-rays and equivalent natural background radiation.2

For example a CT scan of the head equates to 90 chest X-rays, CT of the abdomen equates to 370 chest X-rays, and a CT scan of the chest equates to 440 chest X-rays.

The estimated cancer risk for patients can be summed up as:

  • The additional lifetime risk of fatal cancer due to exposure to an ordinary chest X-ray is one in a million
  • The additional lifetime risk of fatal cancer for an abdominal CT examination in an adult is one in 2,0001,6

Figure 1

  • Dr Rajiv Ghurye is a GP on the Isle of Wight; Dr Mrudula Ghurye is a consultant anaesthetist at Portsmouth Hospitals; Dr Bettina Harms is a consultant paediatrician at St Mary’s Hospital, Isle of Wight

  • Acknowledgments: We gratefully acknowledge the assistance given by Dr P Close and Dr A Cave, consultant radiologists, St Mary’s Hospital, Isle of Wight

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  1. The Royal College of Radiologists (2012). iRefer: making the best use of clinical radiology.
  2. Committee on Medical Aspects of Radiation in the Environment (2014). 16th report. Patient radiation dose issues arising from the use of CT in the UK.https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/343836/COMARE_16th_Report.pdf
  3. Brenner DJ, Hall EJ. Computed Tomography – An Increasing Source of Radiation Exposure. N Engl J Med 2007; 357: 2277-84
  4. Berrington de Gonzalez A: Projected cancer risks from computed tomographic scans performed in the United States in 2007. Arch Intern Med 2009; 69(22): 2071-7
  5. Adrian H, Julia L, Lukas E, et al. Performance of ultralow-dose CT with iterative reconstruction in lung cancer screening: limiting radiation exposure to the equivalent of conventional chest X-ray imaging. Eur Radiol. 2016 Jan 26. [Epub ahead of print]
  6. Fazel R, Krumholz HM, Wang Y et al. Exposure to low-dose ionizing radiation from medical imaging procedures. N Engl J Med 2009; 361(9): 849-57

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