Do CT scans cause cancer?

Do CT scans cause cancer?

The term “radiation” immediately evokes an emotional response, and one of the first associations that comes to mind will be the term “cancer”. To the layperson, all forms of radiation are the same. However, radiation is a very broad term that covers a spectrum of electromagnetic radiation, from radio waves through X-rays and gamma-rays. It is the latter two which are the potential cancer-causing agents. X-rays and gamma-rays are known as ionizing radiation. They have the ability and energy to strip electrons out of atoms, thus turning the atoms into charged particles known as ions. Ions, in turn, become very chemically reactive, seeking to recapture the missing electrons from other materials in their surroundings. They are the free radicals we are told have the potential to cause disease, ageing and cancer.

In addition, ionizing radiation can also damage the DNA in cells, causing mutations which may manifest as cells which stop functioning correctly, which divide uncontrollably and which ultimately may become tumours.

At high radiation levels there is a strong and undeniable link between ionizing radiation and cancer. However, most of the models used to predict and quantify the link are based on data from people who survived the atom bombs dropped on Hiroshima and Nagasaki at the end of World War 2. The problem with these data is that the radiation levels these survivors were exposed to (several Sieverts) were much higher than the levels experienced during medical imaging (typically 4.5 milliSieverts [mSv] for a CT scan, and 0.150 milliSieverts [mSv] or 150 μSv for a full-body Lodox scan). Extrapolating from these high levels of several Sieverts to the milliSievert or microSievert range is problematic because it requires making assumptions which might not be correct. Therefore, it is not necessarily true that the relationship between the risk of developing cancer vs. the dose is the same for high dose ranges as well as for low dose ranges.

Obviously, it is unethical to expose humans to radiation in order to determine what the potential risks are, and what the likelihood of developing cancer as a result of the exposure is. However, a recent study (Mathews et al., 2013) conducted in Australia, published in the British Medical Journal (BMJ), examined the incidence of cancer in patients following CT scans earlier in life. The results of that study compared well with previous related research in the United Kingdom.

Since the 1980s, when CT scans first became popular, there has been a growing trend to scan patients, despite the relatively high radiation dose. Undoubtedly, the CT scan provides a lot of information which could be critical for diagnosis and treatment planning. However, CT scans are often requested when more traditional approaches or other imaging modalities (ultrasound or MRI) may have been more appropriate.

The study looked at over 680 000 children or adolescents exposed to CT scans when they were aged 0 – 19 years, with a 20-year follow up, out of a total of almost 11 million people in the study. The results of the study showed that the cancer incidence was 24% greater for the exposed people than for the unexposed people, after accounting for age, sex and year or birth.

The main conclusion of the study was that the increased incidence of cancer after CT scan exposure was mostly as a result of irradiation. However, the eventual lifetime risk from CT scans cannot yet be determined. Although radiation doses these days are lower than those during the study years (1985 – 2005), future CT scans should be optimised to provide a diagnostic image at the lowest possible radiation dose, and scans should only be taken when there is a definite clinical indication.

In June the radiology site Aunt Minnie picked up on another related story. A further study (Miglioretti et al., 2013), published by the Journal of the American Medical Association (JAMA), echoed the sentiments of the BMJ article, based on research done in the United States. The conclusion of that study was that the increased use of CT in paediatrics has resulted in many children receiving a high-dose examination. Younger patients were more at risk than older patients, and girls were more at risk than boys. Reducing the doses, especially those in the highest quartile, could dramatically reduce the number of radiation-induced cancers.

So where does the Lodox full-body, low-dose X-ray scanner fit into the picture? Obviously, a planar (2D) image such as that produced by the Lodox scanner will not have the same kind of information provided by a 3D CT scan. However, the question is whether a CT scan is always necessary. A 2D scan may be adequate for a clinical diagnosis. Thus, many patients could be spared the higher doses of a CT scan if a Lodox scan effectively works as a diagnostic tool, providing the necessary diagnostic information for the majority of trauma patients, and as a screening tool, limiting the number of patients who are sent for further imaging in cases of uncertainty.

In terms of effective dose (the weighed dose worked out per organ and its sensitivity to radiation), the Lodox scanner provides diagnostic images at about one tenth of the dose compared to a standard X-ray machine (Maree, Irving & Hering, 2007). These doses are also about four orders of magnitude lower than those from a CT scan (dose from CT scans are about 30 000 times higher than Lodox doses).

In particular, children are more susceptible than adults to ionizing radiation. Lodox has a dedicated paediatric low-dose, full-body scanner in the only dedicated paediatric hospital in South Africa, the Red Cross War Memorial Children’s Hospital in Cape Town.

Not to rest on our laurels, Lodox is conducting ongoing research to reduce the doses even further through effective X-ray beam filtration. Early indications are that a further 30% dose saving can be achieved. Thus, Lodox is still leading with way with the only full-body X-ray scanner, with the lowest dose, available on the market.

 

References:

Maree GJ, Irving BJ, Hering ER; Paediatric dose measurement in a full-body digital radiography unit; Pediatric Radiology, 2007; 37: 990–997

Mathews JD, Forsythe AV, Brady Z, Butler MW, Goergen SK,  Byrnes GB, Giles GG, Wallace AB, Anderson PR, Guiver TA, McGale P, Cain TM, Dowty JG, Bickerstaffe AC, Darby SC; Cancer risk in 680 000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians, British Medical Journal, 2013; 346: f2360

Miglioretti DL, Johnson E,Williams A, Greenlee RT, Weinmann S, Solberg LI, Spencer-Feigelson H, Roblin D, Flynn MJ, Vanneman N, Smith-Bindman R; The Use of Computed Tomography in Pediatrics and the Associated Radiation Exposure and Estimated Cancer Risk, JAMA Pediatrics 2013;(): 1–8. doi:10.1001/jamapediatrics.2013.311

Storrs C; The Science of Health: Do CT Scans Cause Cancer?; Scientific American, July 2013: 24–25

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