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AAPM Response in Regards to CT Radiation Dose and its Effects

December 17, 2009


The American Association of Physicists in Medicine (AAPM) is a scientific and professional society comprised of scientists (medical physicists) who establish radiation measurement procedures and perform them on radiation emitting devices, including computed tomography (CT) scanners. There have been a number of CT related issues in the news over the past months pertaining to radiation dose, however there have been several misleading statements made with respect to radiation hazards from CT scanning. The AAPM believes in an open discussion, but one that is based on facts. The goal of this statement is to present these facts.

We should state from the outset that medical physicists are partnering with technologists, radiologists, regulators, manufacturers, administrators and others to strive for CT scans that are medically indicated; and when they are performed that the minimum amount of radiation is used to obtain the diagnostic information for which the CT scan was ordered.

CT brain perfusion overexposures

The Food and Drug Administration (FDA) issued an alert in regards to high dose levels used in head CT perfusion studies at a hospital in Southern California(1). Over 200 patients apparently received excess radiation during these time-lapse (repeated) CT studies of the head. Subsequently, similar incidents have been identified at two other hospitals in Southern California and potentially in other locations as well.  Early investigations of these incidents revealed a misunderstanding of some of the automated dose selection features on the scanner, and this led to an estimated 8 fold increase in radiation to the patient.  This was discovered when a number of the patients experienced some temporary hair loss (epilation) and skin reddening (erythema).

This incident apparently resulted from a lack of adequate training of CT technologists, and perhaps an overreliance on the use of preselected CT protocols. There is no excuse for such radiation overexposures, and improved training as well as machine interface features may need to be improved to prevent future occurrences. News of these incidents has led to a nationwide mobilization of medical physicists, working with hospital administrators, radiologists, and CT technologists to get a better handle on CT protocols at each individual institution. Longer term, the AAPM has responded to this incident by developing a scientific symposium on this topic to be held in late April 2010, which will be led by two medical physicists who have vast experience with developing and managing CT protocols at large institutions. This course will be open to lead CT technologists, radiology managers, radiologists, medical physicists, and all others interested in learning more about CT protocol optimization and management. (www.aapm.org).

Cancer Risks from CT in the United States

Two articles were published back-to-back in the Archives of Internal Medicine (2,3) recently, suggesting that increased use of diagnostic CT leads to the cancer deaths of tens of thousands of Americans each year. The fact that large radiation exposures to an individual can cause cancer is not controversial, however the supposition that much smaller radiation exposures (such as with CT) to many individuals can cause substantial increases in cancer incidence is certainly controversial and not universally accepted. Indeed, many of the series of assumptions used in these articles (and their source materials) make use of worst case scenarios and most conservative assumptions. One example of this is in the Smith-Bindman article(2), where the risk of cancer was illustrated in Figure 2 for 20 year old women. The authors acknowledge that this is an extreme example because younger women are the most susceptible group to radiation induced cancers, even though the median age for women undergoing CT scans is well into the 5th decade(3); in fact CT scanning of women in their 20s is relatively uncommon.

If we accept the claim that 29,000 cancers were caused by CT in 2007 among the 70 million people in the U.S. receiving about 13.8 mSv from one CT session as reported in the Berrington de Gonzalez article(3), then it follows that 21,000 cancers are likely to be induced from background radiation levels of 3.1 mSv to the other 230 million Americans who have not had CT. The average background level of 3.1 mSv per year is 22% (3.1/13.8) of the average effective dose from CT.

Predicting cancer deaths from radiation is not the same as assessing deaths from other causes such as automobile accidents or gun shots – in these latter cases the victims can be counted without much ambiguity in the cause of death. Because radiation induced cancers are exactly the same clinically as normally occurring cancers, there is no way to know who died from a radiation induced cancer and who died from a naturally occurring cancer. This issue is compounded by the fact that the number of predicted radiation induced cancers is tiny compared to the very large cancer incidence rate in humans (~25-30%), making the impact of radiation on cancer rate very hard to measure.

Observations and Recommendations in Regards to CT Examinations

Most of the 70 million CT scans performed each year in the U.S. are medically indicated, resulting in more accurate diagnostic assessment of patient health, which in turn results in more appropriate treatment and better health outcomes. Many CT scans, however, are ordered without sufficient medical justification and the most efficacious way to reduce CT radiation levels to the U.S. population is to substantially reduce unnecessary CT scans. Patients and their referring physicians should discuss the risks of a CT scan, as well as the risks of not having a CT scan (i.e. potentially compromising an accurate diagnosis). A radiologist should be consulted if there remains any ambiguity as to whether or not a CT scan should be performed. By confirming the presence or absence of disease or injury, an appropriately-ordered CT examination is of tremendous benefit to the individual patient, and far outweighs the radiation risks in the vast majority of cases.

Providers of CT scanning services – hospitals, clinics, and radiologists – have in general made good progress in reducing the dose levels of CT scanning, however the patient should ask the CT technologist if all appropriate measures for dose reduction for a particular CT study have been used – and if an adequate answer is not obtained from the technologist, they should insist on talking to the radiologist prior to the scan. Patients and referring physicians should inquire if their CT facility is accredited by a recognized accrediting body - if so, this is an excellent way of assuring that the CT facility is practicing state of the art, low dose CT.

For a patient undergoing a specific CT scan, the factors which need to be considered for reducing dose include (1) the scanned area should be limited to the region of the body where the suspicion exists, (2) the CT technique factors should be adjusted according to the size of the patient’s body – newer scanners can adjust radiation output automatically, which is useful, and (3) repeated CT scans should be avoided whenever possible, and certainly if the scans are only being repeated because the physician does not have access to the images from a recent CT scan.

The patients who experienced hair loss and skin reddening from head CT perfusion studies are in general gravely ill, many are comatose, and a large fraction will die from their head injury or stroke. Indeed, the procedure itself is one way of assessing brain death. The CT perfusion study gives practitioners essential guidance as to the need for or success of interventional procedures such as angioplasty or surgery. By comparison, patients with cancer routinely lose all of their hair when treated with some forms of chemotherapy, but this is presumed to be an acceptable consequence of the treatment. While there is no excuse for unnecessarily high radiation levels in CT perfusion, hair loss and skin reddening can and will occur even with appropriate levels of radiation when the procedure is repeated or is combined with other x-ray examinations such as interventional angiography.

SUMMARY

CT scans are a very important tool for diagnosis and assessment of response to treatment in the practice of medicine. The detailed assessment of anatomy and function that CT imaging provides does require the use of x-rays, which do result in some small, but not zero, risk to patients. Medical Physicists are working with technologists, radiologists, regulators, and manufacturers to assure that CT is practiced uniformly across the U.S. in a low dose manner.

AAPM Science Council
AAPM Executive Committee

(1) FDA Safety Investigation of CT Brain Perfusion Scans: Update 12/8/2009, accessed 16 Dec 2009.

(2) Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer, R Smith-Bindman, J Lipson, R Marcus, et Al., Arch Intern Med 169(22); 2078-2086 (2009)

(3) Projected cancer risks from computed tomographic scans performed in the United States in 2007, A Berrington de Gonzalez, M Mahesh, K-P Kim, et Al., Arch Intern Med 169(22); 2071-2077 (2009)