One of the biggest medical controversies in recent years concerns claims about radiation risks from CT (computed tomography) imaging. Although experts have questioned certain facets of such claims, an especially powerful riposte was published in an article late last year in the \u2018American Journal of Clinical Oncology\u2019. The authors took great pains to explain that the \u201cwidespread belief\u201d about a link between medical imaging and cancer was founded on \u201can unproven\u201d and \u201cillegitimate\u201d theoretical model, dating to the 1940s.<\/b><\/p>\n

Media fuels emotion, fear<\/b>
The emotive nature of the CT and cancer debate may be best illustrated by an Op-Ed in the \u2018New York Times\u2019 on January 31, 2014. Its authors are two Professors at the University of California, San Francisco Medical Center – radiologist Rebecca Smith-Bindman and cardiologist Rita Redberg.
The headline of the article, in one of the world\u2019s most influential publications, clearly seeks to draw maximum attention. \u201cWe Are Giving Ourselves Cancer,\u201d it says. The opening paragraph continues in the same vein, closing with the observation that \u201cwe\u201d are \u201csilently irradiating ourselves to death.\u201d So too does the final sentence of the Op-Ed – that \u201cwe\u201d must find ways to use CTs \u201cwithout killing people in the process.\u201d<\/p>\n

Expert oversights<\/b>
Professors Redberg and Smith-Bindman acknowledge that \u201cmedical imaging can be lifesaving.\u201d Their principal argument is that once rare, CTs have now become routine and that the \u201ccurrent high rate of scans\u201d do not correlate to \u201cbetter health outcomes.\u201d The reasons for growth in CT use are both good and bad: \u201cdesire for early diagnoses, higher quality imaging technology,\u201d as well as the \u201cfinancial interests of doctors and imaging centers.\u201d However, the authors do not even attempt an informed guess about whether good motives outweigh the bad.
In contrast, they state conclusively that there is \u201cevidence of its harms.\u201d This evidence consists of two clinical studies in Britain and Australia where the risks of CT, they say, were \u201cdirectly demonstrated,\u201d especially in children.
One of their biggest oversights was to avoid mentioning the conclusions of the two studies. Both, in fact, took great care to qualify their verdicts. <\/p>\n

Key studies in Britain and Australia qualify judgements<\/b>
The British study, published in \u2018Lancet\u2019 in August 2012, was titled \u2018Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study\u2019. It used data on 175,000 children and young adults and found a three-fold increase in the risk of brain tumoUrs and leukemia. However, the cumulative 10-year risk was one excess case of leukemia and one excess case of brain tumour per 10,000 head CT scans. <\/p>\n

On its part, the Australian study was published in the \u2018British Medical Journal\u2019 in May 2013 and titled \u2018Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians.\u2019 The authors found a 24% increase in childhood cancer risk over a 10-year period – from 39 per 10,000 young people to 45, after a CT scan. These findings were, like the British study, put in perspective by an accompanying editorial: the incidence of cancer in children \u201cis extremely small and so a 24% increase makes this risk just slightly less small.\u201d In addition, the authors observed that almost 60% of CT scans were of the brain and \u201cin some cases the brain cancer may have led to the scan rather than vice versa.\u201d <\/p>\n

Indeed, unlike the \u2018New York Times\u2019 Op-Ed, the two studies provided a balanced view. Above all, they took great care to avoid drawing alarmist conclusions.<\/p>\n

The need for balance<\/b>
The British study stated that \u201cimmediate benefits of CT outweigh the long-term risks in many settings and because of CT\u2019s diagnostic accuracy and speed of scanning …, it will remain in widespread practice for the foreseeable future.\u201d Lead author Mark S. Pearce of Newcastle University\u2019s Institute of Health and Society echoed this forcefully, noting that \u201cCT can be highly beneficial for early diagnosis, for clinical decision-making, and for saving lives. However, greater efforts should be made to ensure clinical justification and to keep doses as low as reasonably achievable.\u201d
The Australian study, too, concluded that practitioners \u201cwill increasingly need to weigh the undoubted benefits of CT scans in clinical practice against the potential risks to justify each CT scan decision.\u201d
\nDosage: wide margins for error <\/b>
Meanwhile, one of the biggest issues of concern with CT is a lack of clarity – and some uncertainty – about dosage and exposure. The British study, for example, underscored that the increase in risk followed \u201ctwo or three CT scans of the head\u201d under \u201ccurrent scanner settings\u201d for brain tumours and \u201cfive to 10 head CT scans\u201d for leukemia.
Radiologists have in fact not reached a consensus on how to define a dose, according to Michael McNitt-Gray, an associate professor of radiological sciences at the University of California, Los Angeles. Doses per indication vary by institution and by patient size. As a result, no national average is available.
An additional problem is that effective, organ-specific doses relate to how much radiation the body absorbs. However, CT scanners do not report the absorbed fraction, says McNitt-Gray. Instead, they report only what the machine emits, which is less than what is absorbed by the body. <\/p>\n

Australian study urges validation of risk model<\/b>
Nevertheless, in general, there is widespread agreement that CT scans should be limited to situations with a definite clinical indication and that scans should be optimiZed to provide a diagnostic CT image at the lowest possible radiation dose. The editorial accompanying the Australian study in the \u2018British Medical Journal\u2019 called for further validation of risk models, and stated that \u201cmore accurate risk assessment \u201ccan be performed to \u201cbetter inform imaging decisions.\u201d<\/p>\n

Missing nuances<\/b>
Such nuances seem to have been missed out by the Op-Ed in the \u2018New York Times\u2019.
A \u201csingle CT scan,\u201d the authors wrote, \u201cexposes a patient to the amount of radiation that epidemiologic evidence shows can be cancer-causing.\u201d They also asserted that CT radiation was \u201c100 to 1,000 times higher than conventional X-rays.\u201d
Their certainty stands in some contrast to a quote from the British study used to justify the authors\u2019 views: The \u201camount of radiation delivered during a single CT scan,\u201d it said, \u201ccan still vary greatly and is often up to 10 times higher than that delivered in a conventional X-ray procedure.\u201d<\/p>\n

French study in 2014 raises new doubts<\/b>
In October 2014, nine months after the \u2018New York Times\u2019 Op-Ed, a French study of 67,274 children raised further doubts about the strength of the correlation between CT and childhood cancer.  The objective of the study, published in the \u2018British Journal of Cancer\u2019, was to estimate how cancer-predisposing factors (PFs) affected assessment of radiation-related risk in CTs.
The authors found that adjusting for PF \u201creduced the excess risk estimates related to cumulative doses from CT scans\u201d and that \u201cno significant excess risk was observed in relation to CT exposures.\u201d The study concluded  that there was a need \u201cto avoid overestimation of the cancer risks associated with CT scans.\u201d<\/p>\n

The 2009 NCI study<\/b>
Aside from the British and Australian studies, an authoritative and oft-cited source for most of the alarms about CT have their origins in a 2009 report from the US National Cancer Institute (NCI). This report was referenced by the authors of the \u2018New York Times\u2019 Op Ed, in their statement that  \u201cCT scans conducted in 2007 will cause a projected 29,000 excess cancer cases and 14,500 excess deaths over the lifetime of those exposed.\u201d<\/p>\n

BEIR and Lifetime Attributable Risk<\/b>
The NCI figures were based on the US National Research Council\u2019s Biological Effects of Ionizing Radiation (BEIR) report in 2006 and estimated the mean number of radiation-related incident cancers with 95% uncertainty limits (UL). 
The so-called BEIR VII model generates what it calls lifetime attributable risk (LAR) factors. These estimate the likelihood of cancer in hypothetical individuals as a function of dose. Multiplying LAR by the number of people exposed to a given dose yields an estimate of expected cancers from that exposure in the population.
BEIR VII was also used by Smith-Bindman, one of the authors of the \u2018New York Times\u2019 Op-Ed, who used it in a study of four San Francisco facilities to estimate that one cancer might appear for every 270 middle-aged women undergoing CT coronary angiography, and that women aged 20 who underwent the procedure had twice the risk as middle-aged women.<\/p>\n

BEIR and linear no-threshold: assumptions challenged<\/b>
The so-called BEIR VII model postulates that there is no safe level of ionizing radiation exposure. As a result, it is assumed that carcinogenic effects follow a linear dose response – which means that even the smallest exposure carries some level of cancer risk. It is precisely this baseline assumption which has been challenged by a recent article in the \u2018American Journal of Clinical Oncology\u2019. The article, published in November 2015,  is titled \u2018The Birth of the Illegitimate Linear No-Threshold Model: An Invalid Paradigm for Estimating Risk Following Low-dose Radiation Exposure.\u2019
BEIR VII is based on the linear no-threshold (LNT) model. However, risk estimates based on LNT \u201care only theoretical\u201d and \u201chave never been conclusively demonstrated by empirical evidence,\u201d according to lead author James Welsh, Professor in the Department of Radiation Oncology of Loyola University Chicago Stritch School of Medicine. <\/p>\n

Dosing fruit flies after 70 years <\/b>
The article painstakingly re-examines the original studies, dating back over 70 years, which led to adoption of LNT. The experiments involved exposure of fruit flies to various doses of radiation, and concluding there was no \u2018safe\u2019 level of radiation – the basis of the LNT model which is used to this day. However, such a conclusion was unwarranted as the experiments had not been done at truly low doses and there is growing evidence that the human body has evolved the ability to repair damage from low-dose radiation – such as that which occurs naturally in the environment.
Indeed, as the authors argue, the first study to expose fruit flies to low-dose radiation was conducted only in 2009 and its findings did not support the LNT model. Other sources too suggest that the dose-response relationship between radiation and somatic mutation has a threshold, and that biological defence mechanisms come into play at low radiation levels. <\/p>\n

Abandoning LNT<\/b>
Use of the LNT model, according to the authors of the article in the \u2018American Journal of Clinical Oncology\u2019, dissuades many physicians from using appropriate imaging techniques and \u201cdiscourages many in the public from getting proper and needed imaging, all in the name of avoiding any radiation exposure.\u201d They conclude that the LNT model \u201cshould finally and decisively be abandoned.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"

One of the biggest medical controversies in recent years concerns claims about radiation risks from CT (computed tomography) imaging. Although experts have questioned certain facets of such claims, an especially powerful riposte was published in an article late last year in the \u2018American Journal of Clinical Oncology\u2019. The authors took great pains to explain that […]<\/p>\n","protected":false},"author":2,"featured_media":6538,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[26],"tags":[],"class_list":["post-3329","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-featured-articles"],"yoast_head":"\n