A New England Journal of Medicine review article published Thursday targets the cancer risks of CT at the same time that hundreds of scientific presentations and new products at the 2007 RSNA meeting are fueling multislice CT's continued growth.
David J. Brenner, Ph.D., and Eric J. Hall, Ph.D., both associated with the Center for Radiological Research at Columbia University, conclude that the growth of medical CT utilization may be responsible for 1.5% to 2% of cancer cases in the U.S. They estimate that perhaps 20 million adults and more than one million children per year in the U.S. are irradiated unnecessarily from medical CT.
The authors examined CT physics and often-quoted historical data from survivors of the Hiroshima and Nagasaki atomic bomb blasts.
The rapid growth of CT utilization — from three million procedures in 1980 to 62 million per year in the mid-2000s — forms the basis for Brenner and Hall's concerns. They say the risks for any one person are not large, but increasing exposure to radiation may be a future public health issue.
Their analysis pegs that growth to the proliferation of MSCT technology and screening applications for adults and children. The authors express concern about the growing popularity of CT for presurgical diagnosis of appendicitis in children, for example, because diagnostic ultrasound, a modality that involves no ionizing radiation, is probably equally effective for the same procedure. They cite estimates indicating that between 6% and 11% of CT studies are performed on children. A chart in the NEJM review indicates a neonatal abdominal CT scan exposes a newborn infant to 20 mSv of radiation.
Screening is also an important motivation for increased CT use in asymptomatic adults, according to the authors. They predict that future utilization growth will arise from virtual CT colonoscopy, CT lung cancer screening, cardiac screening, and whole-body screening.
Citing the atomic bomb survivor data, Brenner and Hall point out that a substantial proportion of the 25,000 survivors received less than 50 mSv exposure, about the equivalent of three cardiac CT scans. A significant increase in the overall risk of cancer was recorded for the subgroup of atom bomb survivors who received low doses of radiation in the 5 to 50-mSv range.
"There is direct evidence from epidemiologic studies that the organ doses corresponding to a common CT study (two or three scans) result in an increased risk of cancer," they wrote. "The evidence is reasonably convincing for adults and very convincing for children."
They recommend better equipment and techniques to lower radiation exposure and the alternative use of MRI and ultrasound, especially for infants and children. They also urge physicians to avoid inappropriate CT utilization.
The American College of Radiology expressed skepticism about the article. A release from the ACR noted that no published research exists to support Brenner and Hall's claim that radiation from CT may cause up to 2% of all cancers in the U.S. It challenged the correlation between atomic bomb blast data and controlled exposure to radiation involved with CT scanning.
Dr. Arl Van Moore Jr., chair of the ACR board of chancellors, worried that patients may mistakenly avoid what the ACR characterized as "life-saving medical imaging care," because of the article.
"They may be terribly confused and unduly distressed by some of the statements in this study," he said.
In contrast, Dr. Stephen Baker, president of the Society of Chairmen of Academic Radiology Departments, is pleased that the radiation protection battle has again been joined. He believes that a link does exist between higher cancer rates and CT and points to rising incidence of thyroid cancer as evidence.
Practicing radiologists see examples of inappropriate utilization every day, according to Baker. He said it is generally known that about one-third of diagnostic imaging is medically unnecessary.
The problem can solved through communication between radiologists and referring physicians to winnow out applications known not to contribute to accurate diagnosis and proper patient management, Baker said.
"The utilization management companies already do this," he said. "It would be better for collaboration between radiologists and referring physicians to play this role rather than some bureaucracy."
Many radiologists agree that inappropriate CT scans are performed because of medicolegal concerns and exploitation by profit-driven referring physicians and commercial imaging providers, said Dr. Michael Brant-Zawadzki, a member of the RSNA's public information advisor's network.
The theoretical risk of carcinogenesis from radiation discussed by Brenner and Hall has been debated for decades. Experts disagree over whether carcinogenesis occurs at the low-dose levels generated by CT scanners, said Brant-Zawadzki, medical director of radiology at Hoag Memorial Hospital in Newport Beach, CA.
CT has become the diagnostic workhorse in hospital and emergency room settings and has dramatically increased the diagnostic productivity of physicians, he said. In the ER, CT has replaced the physical exam to speed patient triage.
In contrast to Brenner and Hall's public health concerns about CT, Brant-Zawadzki stressed its contributions.
"It has saved countless thousands of lives," he said.
