Radiation exposure a concern in stone patients

October 1, 2008

Management of patients with urolithiasis can place them at risk for significant radiation exposure.

The study, undertaken by researchers at Duke University, Durham, NC, and the University of Texas Southwestern Medical Center, Dallas, identified 108 patients who presented with a primary acute stone episode between 2000 and 2006 and had 1 year of follow-up. Radiation dose received was calculated for each patient based on imaging studies performed related to the stone disease within 1 year of the acute event and using accepted effective radiation dose standards for each type of exam.

During the acute and longer-term management period, one in five patients received a radiation dose of at least 50 mSV, which is the maximum annual dose for occupational exposure recommended by the International Commission on Radiation Protection (ICRP). In addition, another 26% of patients approached the 50 mSV cutoff by receiving a radiation dose >40 mSV in 1 year.

"Our findings suggest that physicians need to be cognizant of the radiation exposure they are giving to patients and should seek alternative imaging strategies wherever possible. In addition, we should try to identify risk factors that may lead to higher exposure levels so that attempts can be made to limit radiation doses in patients with those risk factors."

Dr. Ferrandino noted that a CT scan is currently the gold standard imaging technique for urolithiasis patients. While studies have been done using abdominal x-ray of the kidneys, ureter, and bladder (KUB) and ultrasound in combination and with low-dose CT scans, it is unknown which studies are best for which patients.

The patients in the retrospective study of radiation exposure had a mean age of 48.6 years, and men and women were equally represented. The estimations for radiation doses used in the calculations were 1.7 mSV for abdominal KUB x-ray, 2.5 mSV for intravenous pyelogram, and 20 mSV for a combined abdomino-pelvic CT scan stone protocol.

The average number of exams performed per patient over the 1-year period was four, and the median effective radiation dose per patient was 29.7 mSV.

"One patient in the series received up to 133 mSV in 1 year, which far exceeds the 100 mSV 5-year occupational exposure limit recommended by the IRCP," Dr. Ferrandino noted.

Patients who received a total effective radiation dose of more than 50 mSV received a significantly greater number of CT scans than did those with less radiation exposure (3.5 vs. 1, respectively).

"This difference makes sense considering the much higher dose of radiation that is associated with a CT scan compared to abdominal KUB or IVP," Dr. Ferrandino said.

Variables do not correlate with risk

Analyses were also undertaken to evaluate associations between total radiation dose and various patient, stone, and surgical characteristics. The variables examined included age, race, sex, stone location, stone number, and whether the patient underwent stone removal by percutaneous nephrolithotomy, ureteroscopy, or extracorporeal shock wave lithotripsy. However, none of the variables examined were indicative of increased exposure risk.

Pat F. Fulgham, MD, a private practitioner in Dallas, co-chaired the session.

"This is particularly sobering data, as we must be concerned about the long-term effects of radiation exposure in these patients," he said. "The lag time from radiation exposure to malignancy presentation can be up to 20 years, and patients with stone disease represent a relatively young population. Not only do stone patients frequently develop subsequent stones, but they may also have many CT scans during their lifetime. The effects of radiation exposure are cumulative."