What are the Radiation Risks from CT?
As in many aspects of medicine, there are both benefits and risks associated with the use of CT. The main risks are those associated with
- test results that demonstrate a benign or incidental finding, leading to unneeded, possibly invasive, follow-up tests that may present additional risks and
- the increased possibility of cancer induction from x-ray radiation exposure.
The probability for absorbed x-rays to induce cancer or heritable mutations leading to genetically associated diseases in offspring is thought to be very small for radiation doses of the magnitude that are associated with CT procedures. Such estimates of cancer and genetically heritable risk from x-ray exposure have a broad range of statistical uncertainty, and there is some scientific controversy regarding the effects from very low doses and dose rates as discussed below. To date, there is no evidence of genetically heritable risk in humans from exposure to x-rays. Under some rare circumstances of prolonged, high-dose exposure, x-rays can cause other adverse health effects, such as skin erythema (reddening), skin tissue injury, and birth defects following in-utero exposure. But at the exposure levels associated with most medical imaging procedures, including most CT procedures, these other adverse effects do not occur.
Because of the rapidly growing use of pediatric CT and the potential for increased radiation exposure to children undergoing these scans, special considerations should be applied when using pediatric CT. Among children who have undergone CT scans, approximately one-third have had at least three scans. The National Cancer Institute and The Society for Pediatric Radiology developed a brochure, Radiation Risks and Pediatric Computed Tomography: A Guide for Health Care Providers, and the FDA issued a Public Health Notification, Reducing Radiation Risk from Computed Tomography for Pediatric and Small Adult Patients, that discuss the value of CT and the importance of minimizing the radiation dose, especially in children.
See a recent article from the New England Journal of Medicine (NEJM) titled, "Computed Tomography (CT) - An Increasing Source of Radiation Exposure".
Radiation Dose from CT Examinations
The quantity most relevant for assessing the risk of cancer detriment from a CT procedure is the "effective dose". The unit of measurement for effective dose is millisieverts (abbreviated mSv). Effective dose allows for comparison of the risk estimates associated with partial or whole-body radiation exposures. It also incorporates the different radiation sensitivities of the various organs in the body.
Radiation dose from CT procedures varies from patient to patient. The particular radiation dose will depend on the size of the body part examined, the type of procedure, and the type of CT equipment and its operation. Typical values cited for radiation dose should be considered as estimates that cannot be precisely associated with any individual patient, examination, or type of CT system. The actual dose from a procedure could be two or three times larger or smaller than the estimates. Facilities performing "screening" procedures may adjust the radiation dose used to levels less (by factors such as 1/2 to 1/5 for so called "low dose CT scans") than those typically used for diagnostic CT procedures. However, no comprehensive data is available to permit estimation of the extent of this practice and reducing the dose can have an adverse impact on the image quality produced. Such reduced image quality may be acceptable in certain imaging applications.
Estimates of the effective dose from a diagnostic CT procedure can vary by a factor of 10 or more depending on the type of CT procedure, patient size and the CT system and its operating technique. A list of representative diagnostic procedures and associated doses are given in Table 1.
|Diagnostic Procedure||Typical Effective Dose (mSv)1|
|Chest x-ray (PA film)||0.02|
|Upper G.I. exam||6|
|Coronary artery calcification CT||3|
|Coronary CT angiogram||16|
1. Average effective dose in millisieverts (mSv) from McCollough CH, Bushberg JT, Fletcher JG, Eckel LJ. Answers to common questions about the use and safety of CT scans. Mayo Clin Proc. 2015;90(10):1380-92.
The effective doses from diagnostic CT procedures are typically estimated to be in the range of 1 to 10 mSv. This range is not much less than the lowest doses of 5 to 20 mSv received by some of the Japanese survivors of the atomic bombs. These survivors, who are estimated to have experienced doses only slightly larger than those encountered in CT, have demonstrated a small but increased radiation-related excess relative risk for cancer mortality.
The effective doses from diagnostic CT procedures are typically estimated to be in the range of 1 to 10 mSv. This range is not much less than the lowest doses of 5 to 20 mSv estimated to have been received by some of the Japanese survivors of the atomic bombs. These survivors, who are estimated to have experienced doses slightly larger than those encountered in CT, have demonstrated a small but increased radiation-related excess relative risk for cancer mortality.
The risk of developing cancer as a result of exposure to radiation depends on the part of the body exposed, the individual’s age at exposure, and the individual’s gender. For the purpose of radiation protection, a conservative approach that is generally used is to assume that the risk for adverse health effects from cancer is proportional to the amount of radiation dose absorbed and that there is no amount of radiation that is completely without risk. This conservative approach is called the “linear non-threshold” model. The amount of dose depends on the type of x-ray examination. A CT examination with an effective dose of 10 millisieverts (abbreviated mSv; 1 mSv = 1 mGy in the case of x-rays.) may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000. This increase in the possibility of a fatal cancer from radiation can be compared to the natural incidence of fatal cancer in the U.S. population, about 1 chance in 5 (equal to 400 chances in 2000). In other words, for any one person the risk of radiation-induced cancer is much smaller than the natural risk of cancer. If you combine the natural risk of a fatal cancer and the estimated risk from a 10 mSv CT scan, the total risk may increase from 400 chances in 2000 to 401 chances in 2000. Nevertheless, this small increase in radiation-associated cancer risk for an individual can become a public health concern if large numbers of people undergo increased numbers of CT screening procedures of uncertain benefit.
There is considerable uncertainty regarding the risk estimates for low levels of radiation exposure as commonly experienced in diagnostic radiology procedures. This is because the risk is quite low compared to the natural risk of cancer. At low doses, the radiation-related excess risk, which is thought to be proportional to dose, tends to be dwarfed by statistical and other variation in the background risk level. To obtain adequate evidence for a statistically valid estimate of cancer risk from exposure to low doses of radiation would require studying millions of people for many years. There are some who question whether there is adequate evidence for a risk of cancer induction at low doses. Some scientists believe that low doses of radiation do not increase the risk of developing cancer at all, but this is a minority view.