The effective radiation dose of kidney stone protocol CT exams for obese patients has been shown to be more than three times the dose for a non-obese patient, demonstrating the importance of considering patient body mass index (BMI) when ordering CTs, according to the results of a study presented at the American Urological Association’s annual meeting, May 19-23, in Atlanta.

After the meeting, Michael E. Lipkin, MD, of the department of urology at Duke University School of Medicine in Durham, N.C., told Health Imaging that it’s important to understand radiation dose because while it is important to limit radiation risks, he worries the pendulum might have swung too far toward concerns about lowering dose.

“There’s a balance that you need to strike between radiation dose and image quality,” he said. “That’s something that radiologists and urologists need to discuss on an institutional basis.”

To determine the amount of energy required to penetrate the tissues of an obese stone patient compared with a non-obese patient, Lipkin and colleagues scanned an anthropomorphic male phantom with dosimeters to measure organ specific radiation. When sitting in for a non-obese patient, the model had a BMI of 24 kg/m², but when wrapped in two custom fat layers to model an obese patient, the phantom had a BMI of 30 kg/m².

Results showed the mean effective dose for the non-obese model was 3.04 mSv compared with an effective dose of 9.67 mSv for the obese model.

In both models, the bone marrow received the highest dose and the skin received the second highest dose, according to the authors.

At Duke, the kidney stone protocol CT utilized a variable tube current, which leads to higher doses in obese patients as more energy is required to penetrate the extra tissue. By fixing the tube current, radiation dose can be capped, though this also leads to images of lower quality.

All risks are essentially theoretical, said Lipkin, and there no definitive way to determine whether a single CT scan will cause cancer, but if a patient has a stone, and could benefit from CT, he advised not sacrificing good care in an effort to limit radiation risks.

“We do these studies for a reason. There’s no guideline for a safe amount of radiation in medicine and in each individual case it’s a risk-benefit assessment.”

A better understanding of the effect obesity has on radiation dose and image quality, however, can lead to more effective cooperation between physicians caring for a stone patient.

“You need to have a discussion between the urologist and the radiologist,” said Lipkin. “The radiologist is going to have to tell the urologist what they’re comfortable reading. The urologist is going to have to be comfortable that with the image they’re getting they are going to be able to make the diagnosis they need to make, and it’s really a discussion that needs to happen at the institutional level because every hospital has different CT scanners, every radiology department has a different threshold on what they are willing to read and what they’re not. Same with urologists. What’s really critical is the communication.”