STONE DISEASE

Computed tomography—an increasing source of radiation exposure
Brenner DJ, Hall EJ
Center for Radiological Research, Columbia University Medical Center, New York, NY 10032, USA.
N Engl J Med. 2007 Nov 29;357(22):2277-84

No abstract available.

• Editorial Comment
  The authors reported that 62 million CT scans are performed annually in the United States; including 4 million in children. The radiation dose, a measure of ionizing energy absorbed per unit of mass, is 10 milligrays compared to 0.01 for a PA chest x-ray. The radiation dose from a CT scan depends on the number of scans (for example with and without contrast), the tube current, the scanning time in milliamp-seconds, the size of the patient, the axial scan range, the scan pitch (or degree of overlap between adjacent CT slices), the tube voltage in kilovolt peaks and the scanner design.
  The theoretical risk of ionizing radiation is that it can stimulate the generation of hydroxyl radicals which can then lead to DNA fragmentation or base damage. The authors extrapolate these risks from increased risks of cancer in atomic-bomb survivors (with a mean radiation dose of 40 milligrays) and nuclear industry workers (with a mean radiation dose of 20 milligrays), though in these situations the individuals were exposed to a uniform total body dose, while with CT imaging there is non-uniform exposure with efforts to limit exposure to the focused region of interest. The authors then go on to extrapolate the estimated attributable risk of death from cancer to a single CT scan, and report that the bulk of the risk occurs if the CT imaging is performed prior to the age of 15 years old, and the highest risk is related to digestive system malignancy after abdominal imaging.
  Though this article received dramatic coverage by the press and led to heated discussions in our clinics, it is clear that the article is weak on science and strong on editorial opinion. The authors state that the evidence for an increased risk of cancer after a common CT scan is “reasonably convincing” though in the next sentence state that “no large-scale epidemiologic studies of cancer risk associated with CT scans have been reported.”
  The author’s statement that 2% of cancers in the United States are attributable to CT scan imaging is unsubstantiated. They fail to acknowledge that while CT imaging exposes patients only to x-rays, atomic-blast survivors were exposed to particulate radiation, neutrons and other radioactive materials, the biological significance of which are unknown, and as such it is inaccurate to extrapolate from cancer risk in this cohort.
  The authors acknowledge that though the individual risk estimates for attributable risk of death from cancer is very low, they believe it important from a public health standpoint. What is not calculated is the attributable risk of death by not imaging or by ordering a substandard imaging modality for fear of radiation.
  As CT-scanners and CT-scan imaging becomes assimilated into ambulatory urology clinics, it is imperative for the supervising urologist to become educated on the techniques of adjusting image parameters to minimize radiation dose while maintaining adequate resolution of the image. Though this article emphasizes the importance of evaluating the need for a test before ordering it, it crosses the border of raising awareness into the realm of raising hysteria.

Dr. Manoj Monga
Professor, Department of Urology
University of Minnesota
Edina, Minnesota, USA
E-mail: endourol@yahoo.com