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Computed tomography (CT) exposes patients to hazardous ionizing radiation, which carry the risk to damage the genetic material in the cells, leading to stochastic health effects in the form of heritable genetic mutations and increased cancer risk. These probabilistic, long-term carcinogenic effects of radiation can be seen over a lifetime and may sometimes take several decades to manifest.
Computed tomography (CT) has come a long way since its introduction in 1972 and it has revolutionized the diagnostic radiology [1]. CT is a noninvasive imaging modality that creates cross-sectional and three-dimensional (3D) images of the internal anatomical structures of the body, leading to improved diagnosis, and in turn, saving many lives [2][3]. There has been an exponential increase in the number of CT examinations in the last two decades. In 2019, more than 90 million CT scans were performed in the United States [4], up from 85 million in 2011 [5], 62 million in 2007 [6] and 57 million in 2000 [7]. CT is the major source of radiation exposure to the general population from medical imaging, which is evident from the fact that while CT represents only ~6.3% of all diagnostic radiologic procedures, it contributes to ~43.2% of the collective radiation dose given to the patients [8]. This has become a matter of growing concern as these harmful ionizing radiations can lead to DNA damage, mutagenesis and carcinogenesis in the exposed individuals [9].
Some epidemiological studies have shown a small yet significant increase in cancer risk at typical CT doses [10][11][12][13][14][15]. One report estimated that 0.9% of cancer cases in the United States could be related to low-dose diagnostic X-rays performed between 1991–1996 [16]. Given the increasing use of CT, Brenner and Ha`ll translated these figures to 1.5–2% of the 2007 United States cancer cases [6]. Since the novel coronavirus disease 2019 (COVID-19) pandemic, the role of chest CT has garnered increased attention for screening, diagnosis and management of patients with suspected or known COVID-19, as well as for monitoring the disease progress and its complications [17][18]. To date, more than 616 million cases of COVID-19 have been identified worldwide [19][20], many of whom were subjected to CT scanning [21] and some even underwent repeat CT examinations ranging between 2–8 scans [21][22][23][24][25]. The dramatic increase in the number of CT scans in a short span of time has raised concerns about patient safety [21].
The general population is at some risk for cancer and associated mortality during their lifetime, even without being exposed to medical radiation. This risk is called the lifetime baseline risk (LBR) for cancer. In the United States, the sex-averaged LBR of cancer incidence and mortality (including solid cancers and leukemia) is about 42% and 20%, respectively [8]. According to the American Cancer Society, based on 2016–2018 data, the average lifetime risk of developing cancer from other causes stands at 40.14 and 38.7% in men and women, respectively [31]. The additional cancer risk above and beyond LBR due to radiation exposure is called the lifetime attributable risk (LAR) [32][33]. Table 1 and Table 2 represent qualitative approaches to communicate the LAR of cancer incidence and mortality compared to LBR [8].
Table 1. A qualitative approach to communicate different levels of cancer incidence associated with radiation exposure compared with the lifetime baseline risk of cancer incidence.
Risk Qualification |
LAR of Cancer Incidence per 100,000 People |
LBR a (%) |
% LBR + % LAR b |
Negligible |
<0.2 |
42 |
42.00 |
Minimal |
0.2–2 |
42 |
42.00 |
Very low |
2–20 |
42 |
42.02 |
Low |
20–200 |
42 |
42.25 |
Moderate |
200–400 |
42 |
42.50 |
LAR: Lifetime attributable risk; LBR: lifetime baseline risk. a: Sex-averaged lifetime attributable risk of cancer incidence in general population; b: probability of cancer incidence in general population. Adopted with permission from Ref. [8]. 2019, World health organizations.
Table 2. A qualitative approach to communicate different levels of cancer mortality associated with radiation exposure compared with the lifetime baseline risk of cancer mortality.
Risk Qualification |
LAR of Fatal Cancer per 100,000 People |
LBR a (%) |
% LBR + % LAR b |
Negligible |
<0.1 |
20 |
20.00 |
Minimal |
0.1–1 |
20 |
20.00 |
Very low |
1–10 |
20 |
20.01 |
Low |
10–100 |
20 |
20.10 |
Moderate |
100–200 |
20 |
20.20 |
LAR: lifetime attributable risk; LBR: lifetime baseline risk. a: Sex-averaged lifetime attributable risk of fatal cancer in the general population; b: probability of fatal cancer in the general population. Adopted with permission from Ref. [8]. 2019, World health organizations.
Table 3. The BEIR VII preferred estimates of the lifetime attributable risk of cancer incidence and mortality from exposure to 100 mSv per 100,000 persons (95% subjective confidence interval).
|
All Solid Cancers |
Leukemia |
||
|
Males |
Females |
Males |
Females |
Excess cancer cases |
800 (400–1600) |
1300 (690–2500) |
100 (30–300) |
70 (20–250) |
Excess deaths |
410 (200–830) |
610 (300–1200) |
70 (20–220) |
50 (10–190) |
Adapted with permission from Ref. [32]. 2022, Biologic Effects of Ionizing Radiation (BEIR) VII report.
With the given controversies and uncertainties in dose–response models, there is currently no consensus on LAR estimates for low-dose radiation exposures [8] and radiation protection policies [10]. It is likely that the risk of some cancers could be overestimated, while those of others is underestimated [50]. Moreover, a subset of individuals can be more susceptible and genetically predisposed to the carcinogenic effects of radiation, such as those with congenital/acquired genetic mutations or defective genes [53].
Thus, with the understanding of radiation-related cancer risk still evolving, and till the time we obtain clear answers, a conservative policy needs to be adopted to ensure patients’ safety by following the basic ALARA (as low as reasonably achievable) principle of radiation exposure through the process of justification and optimization [8].