RSNA: Controversial analysis suggests increased breast cancer risk with CT, nuke scans

CHICAGO—There is a small, but real, potential risk of breast cancer associated with cardiac and chest CT exams, and the risk increases with the number of scans, according to researchers who presented a computational analysis Nov. 27 at the annual meeting of the Radiological Society of North America (RSNA). However, due to the nature of the analysis, one expert commenter cautioned against generalizing the findings to a real-life patient population.

Specifically, Richard L. Morin, PhD, of the Mayo Clinic in Jacksonville, Fla., who was not involved with the study, but who served as an expert commenter during RSNA, noted that there are some “concerns” the study might send the wrong message.

The utilization of chest and cardiac CT has grown rapidly over the last 10 years. The study’s lead author, Ginger Merry, MD, MPH, breast imaging fellow at Prentice Women’s Hospital, Northwestern Memorial Hospital in Chicago, reported that CT utilization nearly doubled over the 10-year span of the retrospective analysis, with 99.8 CT scans occurring per 1,000 female enrollees in 2000, and 192.4 CT scans occurring per 1,000 female enrollees in 2010—an annual increase of 6.8 percent.

In 2010, 46 percent of those CT exams exposed the breast to some radiation, reported the study’s senior author Rebecca Smith-Bindman, MD, a professor of radiology and biomedical imaging at the University of California, San Francisco.

Conversely, nuclear medicine imaging decreased from 39.3 scans per 1,000 women in 2000 to 27.5 scans per 1,000 women in 2010 (a 3.5 percent annual decline); however, in 2010, 84 percent of nuclear medicine studies exposed the breast to radiation. Smith-Bindman pointed out that the only exception to the overall trend in decreased utilization was related to PET scans, which had a sharp decline in 2004-2005. The most widespread exam was mammography, followed by chest x-ray.  

Until the current study, Merry noted that the impact of radiation-emitting imaging, such as CT, on radiation exposure to breast tissue and subsequent risk of breast cancer had not been determined.  

For the study, the researchers evaluated imaging and associated radiation exposure (including x-ray, mammography, CT and nuclear medicine studies) among female members enrolled in a large integrated healthcare system between 2000 and 2010, including more than 250,000 enrollees each year.

Merry et al estimated radiation dose differently for the varied modalities. They collected CT dose parameters on 1,656 patients and used a newly developed, automated Monte Carlo computational method to estimate breast and effective doses. For nuclear medicine, data were abstracted on the volume of injected radiopharmaceutical for 5,507 exams and they used the Monte Carlo methods to estimate breast doses.

Using Breast Cancer Surveillance Consortium data, the researchers estimated women’s 10-year risk of breast cancer based on the Gail model and SEER age-specific cancer incidence data and compared the imaging-related risk to the underlying Gail risk. Then, the analysis employed breast-specific absorbed doses and the Preston 2002 pooled model for radiation-effects on breast cancer risk to estimate women’s 10-year risk of developing breast cancer based on age at exposure to CT.

Overall, 124 CT scans and 42 nuclear medicine exams were obtained per 1,000 female enrollees per year. Breast doses from CT were variable, with the highest breast doses delivered by multiple-phase cardiac scan [median 51.6 mGy] and chest CT [median 34 mGy). “The dosage to the breast varied fairly substantially between different women who had a multi-phase scan, and the doses among children were relatively similar to the doses among adults,” said Smith-Bindman.

The researchers noted the wide range in breast dose from CT exams and nuclear medicine exams. However, Smith-Bindman said that for CT, the dose was not always appropriately reduced in children, but for nuclear medicine exams, they observed more appropriate child-sizing.

Based on their analysis, Merry reported that a child or young adult who underwent two cardiac or chest CTs before the age of 23 had a higher 10-year risk of developing breast cancer from these exams than her underlying risk of developing breast cancer in the same period. Thus, according to the study authors, a child or young adult who undergoes two or more chest or cardiac CTs more than doubles her 10-year risk of breast cancer. The analysis also found that the additive risk associated with radiation-emitting scans, such as CT, was less profound as the female participants aged.

Thus, the researchers recommended that imaging providers need to assess the doses associated with each exam, limit multi-phase protocols and optimize dose to minimize exposures and risk.

Morin added that physicians and patients should always discuss the risk/benefit ratio with every radiation-emitting exam.

A hypothetical case based on similar circumstances was the subject of the RSNA Mock Jury Trial.