Thursday, May 03, 2007

The Radiation Risk of CT Scanning

There is lots of buzz in the cardiology and business community about CT scanning as a means to detect coronary disease non-invasively.
Many patients with chest pain are put through a series of tests, including electrocardiograms and stress tests that are time consuming and expensive: In the U.S. alone, such testing costs $10 billion to $12 billion a year. Other patients go on to have an invasive angiography, or a cardiac catherization to look at the arteries, even though as many as 20% to 40% of these cases reveal no clinically significant narrowing of the arteries.

"Because stress tests are not perfect, there are many patients who go to cath lab who don't need it," said Harvey Hecht, chief of cardiovascular computer tomography at Lenox Hill Hospital in Manhattan. "All of those will be eliminated by doing cardiac CT."
Well, maybe not, Dr. Hecht. A relatively recent article on CT scanning from Circulation demonstrated that such high resolution scanners still could only make a diagnosis 75% of the time at best, since coronary calcium and even a single irregular heart beat could distort an acquired image. But companies want to market to the worried well, offering piece of mind that a heart scan will reassure them they are not at risk of heart disease, and turn a profit.

But what these companies also fail to mention is the radiation exposure these scans supply to the average man and woman. (Yes, ladies, women get much more radiation than men with these scans).

So, in the interest of public disclosure, I thought I'd assemble here some interesting data that I culled from various sources regarding radiation exposures and cancer risks with common diagnostic cardiovascular tests. Hopefully this will serve as a wake-up call to patients considering these tests. It should be mandatory that radiation dose be disclosed with these tests, especially as the number of x-ray elements grows from 64-slice to 128-slice scans. From the FDA's website:
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.

Radiation dose from CT procedures varies from patient to patient. A 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.

The quantity most relevant for assessing the risk of cancer detriment from a CT procedure is the "effective dose" . Effective dose is evaluated in units of millisieverts (abbreviated mSv; 1 mSv = 1 mGy in the case of x rays.) Using the concept of effective dose allows comparison of the risk estimates associated with partial or whole-body radiation exposures. This quantity also incorporates the different radiation sensitivities of the various organs in the body.
So to update the FDA's table, I added a few lines regarding average radiation doses of current cardiovascular tests for comparison:

Radiation Dose Comparison

Diagnostic ProcedureTypical Effective Dose (mSv)Number of Chest
X rays (PA film) for Equivalent Effective Dose
Time Period for Equivalent Effective Dose from Natural Background Radiation
Chest x ray (PA film)0.0212.4 days
Mammogram0.136.515.8 days
CT head2.0100243 days
CT abdomen10.05003.3 years
Coronary Angiogram3.41701.1 years
64-slice CT (male)15.27605.1 years
64-slice CT (females)21.410707.1 years
Dual-isotope (3.0mCi Tl-201+30mCi Tc-99) Thallium scan27.313659.1 years
128-slice CT?????????

So think about this when you get your next CT scan "just to find out" your coronary disease risk: is it worth the radiation exposure?

-Wes

12:21 PM CST: Data for mammogram and dual-isotope Thallium test added. Reference: Thompson RC and Cullom SJ. J Nucl Cardiol 2006; 13: 19-23.

5 comments:

Psyche said...

Thanks for a thought-provoking post. Do we know why women absorb more radiation than men during CT scans? Just curious.

DrWes said...

Psyche:

The article referenced at the bottom of the post gives this explanation:

The issue of radiation dosage is confusing to many clinicians. Part of this confusion is related to the terminology, especially given that radiation-related quantities and effects can be expressed in many different ways and competing terms have been used inconsistently in the past. Radiation dose or absorbed radiation dose refers to the amount of energy deposited by ionizing radiation per unit mass in a patient’s body as a result of exposure. The units of absorbed dose are rads or millirads in conventional units or gray in SI units. A pertinent concept is that of the effective dose, expressed in units of rem or millirem or the SI unit of sievert or millisievert. Effective dose takes into account the different risks of absorbed dose to various organs and is a useful method of comparing risk among different diagnostic tests. The effective dose equivalent is the sum of the products of the effective dose to the organ or tissue and weighting factors specific to each of the body organs or tissues. The weighting factors describe the proportion of the risk of stochastic effects resulting from radiation exposure of that organ or tissue to the total risk of stochastic effects when the whole body is irradiated uniformly."

with that as a basis, the article continues:

"The radiation dosage from CT examinations varies widely depending on the protocol, instrumentation, and patient size. The radiation dosage tends to be higher with 64-slice multidetector CT scanners compared with 16-slice scanners because as the slice thickness is reduced, the radiation dosage must be increased to achieve the same signal-to-noise ratio. The radiation dosage reported in two recent studies of the accuracy of 64-slice CT coronary angiography was 13 to 15 mSv in men and 18 to 21.4 mSv in women. The protocols in these studies, presumably designed to maximize the accuracy of CT coronary angiography, did not use prospective electrocardiography pulse x-ray tube modulation. The effective dose of radiation for cardiac diagnostic tests is also higher in women compared with men because of the radiosensitivity of the female breast and the higher exposure of the female gonads to scattered radiation. If a cardiovascular CT scan of the chest is obtained in a long Z-axis acquisition without radiation reduction protocols in an obese female patient, the effective dosage can easily exceed 20 mSv. A whole-body CT scan may result in an even higher dosage."

I hope this helps explain the difference.

Kim said...

Well! The answer to your question for me is "NO!". It is not worth the risk!

I'm stunned at the other amounts of radiation associated with CTs of other body parts.

Abdominal CTs are ordered like lattes in ERs!

Boy, I now have a much greater respect for radiology tests in general and CTs in particular!

Anonymous said...

Considering the dose to workers (wb) is 5.0 Rem's p/year and 75 Rems p/year extremity, I don't see what the fuss is all about. Those limits are in place for a reason, and can be demonstrated to show that even rad workers will not have a great increase in cancer deaths at the end of a work career. I will take all the rads you want to send my way as a patient/member of the public (who by the way can 'legally' receive 100 mrem from machines/sources NOT being used in treatment p/year and still not expect an increase in cancer deaths) if it means finding a cancer or other life threatening issue as early as possible. You have to weigh the risk/benefits before going off half cocked and declaring radiation scans as a 'neccesary evil'.
Radtec.....

DrWes said...

Radtec-

Thanks for your comment. Your points are well-taken. I would bet, however, that there are issues in this post that are not known by our patients and ER staff who do these "like ordering lattes." I bask in radiation daily (I do afib ablations, need I say more?) and worry about how to minimize my exposure AND my patient's exposure to ionizing radiation. The point here is that there are risks, and the risks are cummulative. Fortunately, device manufacturers, I believe, are becoming more sensitive to this as they stretch the imaging envelope, but this issue warrants careful follow-up as new technologies are introduced.