CANCER
January 25, 2010

CT Scans Get a Closer Look

CT scans have benefits, but they also have some very real cancer risks.

While CT (computed tomography) scans are undeniably useful, it may be time to reassess when they are necessary. Each scan exposes the patient to high amounts of radiation. This radiation is likely causing a large number of additional cancers. Because cancer from radiation takes a number of years to develop, it is particularly important to minimize unnecessary CT scans in younger patients.

The average radiation dose from a single CT scan was as high as what an individual would receive from 74 mammograms or 442 chest x−rays.

Two recent studies dealt with various aspects of this problem. One attempted to estimate the number of cancers caused by CT scans in the U.S. The second sought to determine how much radiation patients are exposed to during CT scans, how much this radiation varied from procedure to procedure and to calculate the risk of a patient developing cancer from an individual CT procedure.

Background: Too Much of A Good Thing
Computed tomography is a procedure using specialized x−ray equipment which provides detailed images of bones, organs and tissues. A CT scan takes many x−rays at different angles to produce a three−dimensional image. Because many x−rays are taken, the radiation exposure from a CT procedure is higher than from a conventional x−ray. A chest CT scan typically delivers more than 100 times as much radiation as a conventional chest x−ray.

Physicians don't seem to be taking this into account in deciding when to have a CT scan taken. An example is in patients who are known to have kidney stones. While a CT scan may provide information about the size and location of a kidney stone, it is unlikely to change the treatment. This is exactly the type of scan that may be doing a patient more harm than good. And some patients go directly from the emergency room to the CT scanner before they are even seen by a physician.

Because CT reveals information that might otherwise require exploratory surgery, its use has soared. There were approximately 3 million CT scans performed in 1980; by 2007 that number had increased to around 72 million. This means that patients are being exposed to an increasing amount of radiation. And radiation exposure at this level leads to increased numbers of cancers. The risks to individuals may be small, but because of the sheer number of persons exposed annually, this can add up to a considerable number of total cancers.

Determining the Level of Cancer Risk Posed By CT Scan Exposure
Using data from a large commercial insurance database, Medicare claims data, and IMV Medical Information Division survey data, Amy Berrington de Gonzalez, PhD estimated that 72 million CT scans were performed in the U.S. in 2007. Combining this data with the known effects of ionizing radiation on cancer, Berrington de Gonzalez, an investigator at the National Cancer Institute, estimated that the scans performed in 2007 would cause 29,000 new cancers. Assuming 50% fatality, that means 14,500 deaths from scans performed in a single year. Nearly two−thirds of these cancers are expected to occur in women, both because more scans (60%) are done on women and because a higher proportion of scans done on women are of the chest.

CT scans may be causing a large number of new cancers every year, and doctors and patients should consider whether some scans are doing more harm than good.

Because cancers caused by radiation can take 20−30 years to appear, any study that attempted to show the actual numbers of cancers caused by CT scans would take many years to complete. Though these studies will be performed, for now, the number of CT−caused cancers can only be estimated using a mathematical risk projection model. While some scientists are certain to question the assumptions and calculations used in this model (this happens with all models), these objections are unlikely to change the study's message: CT scans may be causing a large number of new cancers every year, and doctors and patients should consider whether some scans are doing more harm than good.

This is especially true for scans of younger, healthier patients for whom the risks may far outweigh the benefits.

Berrington de Gonzalez's results strongly suggest that unnecessary CT scans should not be performed and that the radiation dosage from individual CT scans should be lowered.

Finding an Enormous Range in Radiation Doses From Different Scanners
Rebecca Smith−Bindman, MD headed the second study, which investigated how much radiation patients are being exposed to in CT scans. This is a first step in determining how to lower radiation exposure. She also used these data to estimate the lifetime attributable risk (LAR) of cancer associated with these tests.

Smith−Bindman focused on the 11 most common CT procedures and looked at how these procedures were performed on 1,119 patients at four medical centers in the San Francisco Bay Area. Among the most striking findings were that radiation doses from these procedures were higher than had previously been reported, that doses from the same procedure varied an average of 13−fold at these institutions. The average radiation dose from a single CT scan was as high as what an individual would receive from 74 mammograms or 442 chest x−rays.

The main reason that radiation dosage from the same procedure varied so much was that the length, scope, number of scans taken at different institutions varied greatly. There was no consistent pattern for which institution had the highest radiation dose; rather, each site had the highest dose for at least one of the included study types.

The main reason that radiation dosage from the same procedure varied so much was that the length, scope, number of scans taken at different institutions varied greatly.

In other words, there is very little standardization in how a particular CT scan is performed and therefore the kind of radiation to which patients are exposed.

In the past, many reported radiation doses for individual procedures have come from tests on plastic models, not from actual human beings. And reported doses come from assuming that a procedure is standardized, while in actual clinical settings, procedures may be carried out for longer times or additional scans taken, based on what was seen in the original scan. These factors may explain why Smith−Bindman observed average patient radiation dose to be higher, sometimes four times higher, than the previously reported doses.

The observed radiation doses were estimated to be more likely to cause cancers in younger patients than in older patients and in women than in men. So radiation exposure in young women is a prime concern.

It is commonly quoted in the literature that the increased risk of developing cancer from a CT scan is roughly 1 in 2,000. Smith−Bindman's data indicates that a 20−year old woman undergoing any of three different types of CT procedures (a CT for suspected pulmonary embolism, a CT coronary angiography or a multiphase abdomen and pelvis CT scan) has an actual increased risk of developing cancer as high as 1 in 80.

This is one reason that Smith−Bindman, an associate professor in the department of radiology at UCSF who also works in the department of epidemiology and biostatistics, strongly recommends that lowering radiation exposure from CT scans should be made a high priority, especially in younger, female patients. She suggests three ways to accomplish this.

  • The first involves creating specific standards for CT examinations and requiring their adoption. This would likely lower how often multiple scans are done in a single procedure. She points out that there has been much standardization in Europe, and one recent study indicates that European CT procedures average half the effective radiation dose that she found in her San Francisco−based study.
  • A second approach to minimize medical radiation exposure should focus on reducing the number of CT examinations, especially scans that will not affect the treatment of a medical condition.
  • A third approach involves better tracking of radiation doses received by individual patients. This is particularly important for patients who undergo repeated scans; they are at higher risk of developing cancer.

Both studies along with a short editorial which summarizes the results were published in the December 14/28, 2009 issue of Archives of Internal Medicine.

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