Physicist downplays risk of prenatal radiation exposure

CT scanning of pregnant women is the source of considerable anxiety. But data indicate that many types of CT exams result in a negligible radiation dose to the fetus and should be performed when clinically necessary, according to a physicist's presentation on Wednesday at the Stanford MDCT meeting.

Cynthia McCollough, Ph.D., an associate professor of radiologic physics at the Mayo Clinic College of Medicine, started off her talk by showing a CT image of a fetus inside a pregnant woman and asked her audience the provocative question, "Is this a bad thing?"

In this particular case, CT revealed a ruptured appendix following an inconclusive ultrasound. In hindsight, it was clear that the benefit to mother and child outweighed the risk. But often in clinical practice, the appropriateness is not nearly so clear-cut. For example, CT of suspected appendicitis may result in a large number of negative results.

The lack of recent data complicates matters, according to McCollough. Most data about the risks of fetal exposure to radiation are drawn from studies of female atomic bomb survivors. Of 2800 pregnancies in the study pool, 500 had exposures greater than 10 mGy. Possible harmful effects include prenatal death, retardation, and small head size. The available data indicate the biggest risk is childhood cancer.

When counseling pregnant women who have been exposed to CT, McCollough says that rather than telling them about an increase in risk, she advises them of the likelihood that no harm will result.

The most sensitive period is the first trimester of pregnancy. Following exposure of 50 mGy during that period, the chance there will be no childhood cancer drops to 99.12% from 99.93% absent prenatal exposure.

"That is a very different way of communicating the exact same data. Patients realize it is not as scary as they thought," she said.

Mayo Clinic staff assessed the fetal radiation dose for a range of studies performed in the radiology department. For extra-abdominal radiography, such as head CT, the exposure was close to zero or a small fraction of 1 mGy. Chest CT to rule out PE resulted in exposure of .2 mGy, and CT of the arteries was merely .1 mGy.

"These are negligible dose levels," McCollough said.

In abdominal radiography that does not involve exposure to the pelvis or uterus, dose might reach 3.4 mGy for an excretory urogram, which involves multiple films. With exposure to the pelvis, risk rises but is still thought to be relatively low. Pelvic arterial embolization, a serious procedure, results in exposure of 20 mGy, and some nuclear medicine scans deliver a dose of 10 mGy to 15 mGy.

CT is not infrequently performed in pregnant women to look for renal stones, and a routine protocol delivers a dose of 25 mGy. But the Mayo Clinic has developed a new protocol that calls for a dose as low as 5 mGy, because stones are bright and easy to pick up, McCollough said.

A CT angiogram with thin slices of the whole chest, abdomen, and pelvis results in a prenatal dose of 34 mGy. But that is still much lower than the 50 mGy point at which the risk of childhood cancer increases by less than 1%.

During her presentation, McCollough noted that official National Council on Radiation Protection and Measurements (NCRP) guidelines advise that the risk of abnormality is negligible at 50 mGy or less. The risk of malformation is significantly increased over control levels only at doses above 150 mGy.

The American College of Obstetricians and Gynecologists advises that exposure of less than 50 mGy has not been shown to increase fetal anomalies or pregnancy loss.

Even though many exams involve a negligible dose, practitioners cannot be cavalier with respect to CT ordering. It's important to set protocols for appropriate use to ensure consistent care and to help residents and fellows manage trauma cases in the middle of the night. McCollough stressed that guidelines should be "data driven" and based on a "thorough review of the scientific literature."

In the case of CT to rule out pulmonary embolism for trauma patients, the radiation risk is negligible and CT is more effective than the alternative ventilation/perfusion scan, she said.

McCollough added that some procedures call for greater oversight and control than others:

"In acute abdominal pain, we won't order studies willy-nilly. We want the surgeon to come and evaluate a patient before we do a CT for suspected appendicitis," she said.