Key Point

Annual CT is associated with a risk of radiation-induced mortality in patients with cystic fibrosis. This risk, which rises modestly with patient survival, can be reduced by conducting biennial CT or by reducing the radiation dose.

NEW YORK CITY—Routine, lifelong annual CT carries a low risk of radiation-induced mortality in patients with cystic fibrosis, according to Pim A. de Jong, MD, PhD, and colleagues. However, as the overall survival increases for patients with cystic fibrosis, the risk of radiation-induced mortality may modestly increase.¹ The researchers found that this risk could be lowered by conducting biennial CT or by reducing the radiation dose. They said their findings "indicate that radiation dose must be considered in routine CT imaging strategies for patients with cystic fibrosis, to ensure that benefits outweigh the risks."

WEIGHING RISKS AND BENEFITS

According to the researchers, "the argument in favor of routine CT scanning in cystic fibrosis is that earlier disease detection would lead to more aggressive treatment and to prolonged survival. The major concern associated with CT scanning—namely, cancers arising from low-dose radiation exposure emitted during the procedure—has been minimal since there is a significant lag time between exposure and cancer occurrence and, traditionally, patients with cystic fibrosis did not live long enough to develop radiation-related cancers. Thus, the assumption has been that the potential survival advantage of earlier detection would outweigh the potential radiation risks of serial CT scanning."

A MODEL FOR RISK ASSESSMENT

To assess the risk of all-cause and cancer-specific mortality associated with repeated CT in patients with cystic fibrosis, the researchers calculated CT dose, according to published cystic fibrosis surveillance CT scanning protocol for biennial CT, and estimated the risk per scan using atom-bomb survivor data. They designed a computational model, which also evaluated the effects of background survival, scanning interval, and radiation dose. Seven variations of the model were developed, assessing survival reductions associated with CT in patients who had median survival periods of 26, 32, and 50 years. "An important assumption in our model was that CT scans would not provide clinical benefits that would improve survival in patients with cystic fibrosis," said the researchers.

According to Dr. de Jong and colleagues, the average radiation dose in the published CT protocol was 1 millisievert. Annual CT from age 2 until death was associated with survival reductions of approximately one and two months in patients whose median survival was 26 and 32 years, respectively. However, in patients with a median survival of 50 years, annual CT was associated with a survival reduction of two years. Corresponding cancer mortality was approximately 2% and 13% at ages 40 and 65, respectively.

The researchers pointed out that biennial CT reduced all-cause as well as cancer-specific mortality by half. "Moreover," they said, "by reducing the radiation dose fivefold, the risks can also be substantially reduced. This shows that it is important to consider the dose and frequency of CT scanning for routine clinical purposes."

LIMITATIONS AND FUTURE DIRECTIVES

Dr. de Jong’s research team noted several limitations to their study: First, they may have overestimated the mortality risks associated with low-dose radiation exposure, since the atom-bomb survivor data were obtained before the advent of newer therapies that have improved the survival of patients with cancer; second, they did not consider the negative effects of radiation on noncancer mortality, such as that from heart/blood vessel disease; third, they did not take costs or potential benefits of CT scanning into account; and fourth, the data on cancer risk were taken from the general population and not the cystic fibrosis population.

In an editorial, Amy Berrington de Gonzalez, DPhil, and Jonathan M. Samet, MD, MS, noted that there appears to be no direct evidence that using CT as a monitoring tool actually prolongs survival in patients with cystic fibrosis.² "Studies of the potential survival benefits, preferably randomized controlled trials, therefore now need to be initiated so that any future risk of cancer will be justified by the benefits of the test," they said.

Using similar reasoning, Dr. de Jong and colleagues commented that, "Additional investigations in dose reduction as well as the optimal timing of routine CT scans are needed to further mitigate the risk of radiation-induced morbidity and mortality." They noted that "a growing number of cystic fibrosis centers have adopted a strategy to use routine CT scans every second or third year in managing patients with cystic fibrosis during childhood."

—Karen L. Spittler

Reference
1. de Jong PA, Mayo JR, Golmohammadi K, et al. Estimation of cancer mortality associated with repetitive computed tomography scanning. Am J Respir Crit Care Med. 2006;173:199-203.
2. Berrington de Gonzalez A, Samet JM. What are the cancer risks from using chest computed tomography to manage cystic fibrosis? Am J Respir Crit Care Med. 2006;173:139-142.

Return to table of contents