CT utilization associated with future cancer risk [Classics Series]

1. Risk projection models estimated 29,000 potential excess cancers related to computed tomography (CT ) scans performed in the United States in 2007.

2. Efforts to reduce overall radiation dose from CT, including reduction of unnecessary scans and reducing dose per procedure, continue to be a high priority.

Original Date of Publication: December 2009

This study summary is an excerpt from the book 2 Minute Medicine’s The Classics in Radiology, 1e (The Classics Series).

Study Rundown: CT has revolutionized medical practice through its speed, availability, and high resolution of anatomical structures free of superimposition. Given its advantages and relative safety over exploratory surgery, its use as a preventative and diagnostic tool for cancer has increased dramatically in the United States since its introduction in the 1970s. However, the nature of CT exposes patients to larger radiation doses than conventional x-ray imaging. As a result, the average radiation dose per United States resident per year has correspondingly increased. The benefits of CT are clear and the radiobiological harm of a single scan are likely insignificant. However, a 2009 retrospective cross-sectional study by Smith-Bindman et al. found that radiation doses from CT scans are higher and more variable than reported. The risk of cumulative CT scans in the patient population may be overlooked and be a cause for public health concern.

While evaluation of the increased risk for cancer from CT is controversial, studies have revived concerns about the risk it poses for developing cancer. In the present cross-sectional study conducted by de González et al., risk models based on cancer rates among survivors of the atomic bomb blasts of World War II (WWII) were used to project the number of future cancers that could be related to radiation from CT scans; these were modeled risks, rather than observed cancer cases. Using both CT scan frequency and organ-specific doses, the study further indicated that cancers associated with the most frequently used CT scans (abdomen and pelvis) and highest-dose scans (chest CT angiography) had large contributions to projected cancer risk. Though potentially misleading due to the data set used, the results indicate a substantial possible increased risk of cancer.

Please click to read study in Archives of Internal Medicine

In-Depth [cross-sectional]: This epidemiologic study primarily used Medicare claims data, the IMV Medical Information survey of CT scan use in 2007, and a national commercial insurance database (NCID) to estimate the age and gender distribution for CT scan frequency in the United States. Linear extrapolation was used to exclude the estimated scans on those who did not survive 5 years and those with an associated diagnosis code of cancer. Organ-specific doses were based on the Food and Drug Administration’s National Evaluation of X -Ray Trends survey and cross checked with an available literature review. Pediatric scans were assumed to follow the recommended lower radiation protocols. Risk models (excluding breast and thyroid) were based on cancer rates among the long -term survivors of the Japanese atomic bomb of WWII. Risk calculations were run using Monte Carlo simulation methods. A linear no-threshold dose-response model was used.

After excluding scans on patients not surviving greater than 5 years and patients with a cancer diagnosis (gross total of 72 million), an estimated 57 million CT scans were used in the calculation for future cancer risk. Overall, an estimated 29,000 (95%UL 15 000 – 45 000) future cancers were linked to CT scans in the United States in 2007. Abdomen and pelvis (n=14 000, 95%UL 6900 – 25 000), chest (n=4100, 95%UL 1900-8100), and head (n=4000, 95%UL 1100-8700) were found to be scan types performed most frequently. Chest CT angiography (n=2700, 95%UL 1300-5000) was found to be the highest-dose scan. Women were found to have increased radiation exposure due to higher frequency of use (60%) and higher incidence of breast and lung cancer, which require higher-dose scans. The risk models showed lung cancer as the most common radiation associated cancer (n=6200, 95%UL 2300-13 000), primarily due to the high frequency of exposure from chest CT . This was followed by colon cancer (n=3500, 95%UL 1000-6800) due to the high frequency of abdominal CT, and leukemia (n=2800, 95%UL 800-4800) due to high organ sensitivity. Multiple measures to reduce overall population dose, including more widespread use of American College of Radiology (ACR) appropriateness criteria, dose reduction software in CT, Image Gently and Image Wisely, and the development of dose registries have all been instituted.

de González AB, Mahesh M, Kim KP, Bhargavan M, Lewis R, Mettler F, Land C . Projected cancer risks from computed tomographic scans performed in the United States in 2007. Archives of Internal Medicine. 2009 Dec;169(22):2071-2077. Additional Review: Smith-Bindman R, Lipson J, Marcus R, Kim KP, Mahesh M, Gould R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Archives of internal medicine. 2009 Dec 14;169(22):2078 – 2086.

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