Weight watchLow patient weight hinders stone detection on low-dose CT

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The detection of kidney stones using extremely low-dose CT is more difficult in obese patients, but new research has shown that the sensitivity and specificity for the detection of ureteral calculi on CT is decreased for underweight patients as well, according to a study published in the July issue of the Journal of Urology.

The authors explained that CT has become the de facto standard for stone diagnosis, with radiation dose concerns prompting implementation of low-dose (effective dose of 3 mSv or less) CT protocols. “Patients being evaluated for urinary stones often undergo multiple CT scans, which places them in a higher risk group for the development of radiation-induced neoplasms,” wrote Jonathan P. Heldt, BS, and colleagues from Loma Linda University Medical Center in Loma Linda, Calif.

Recently, studies have demonstrated that ultra-low dose protocols (less than 1 mSv) do not significantly decrease accuracy for detecting stones in average weight cadavers. Heldt et al wanted to evaluate the accuracy of low-dose CT protocols in patients at both extremes of weight.

The researchers studied three cadavers weighing 55, 85 and 115 kg (121, 187, and 254 lbs, respectivey). A total of 721 calcium oxalate stone ranging from 3 to 7 mm were placed in 33 random configurations in the urinary tracts, and the cadavers were scanned at seven different radiation settings. A blinded review by a single radiologist was used to assess the scans.

“Our results support the existing literature that body weight has a significant role in the radiation dose required for accurate diagnosis of ureteral calculi using unenhanced multidetector CT,” wrote the authors. “At any given radiation level the highest weight cadaver consistently had the lowest [area under the curve] for ureteral calculi detection, although this difference was only significant at the lowest radiation settings (5 and 7.5 mAs).”

On the other end of the spectrum, the lowest weight cadavers also demonstrated lower area under the curve values than average weight cadavers. The authors said the finding was “of particular interest” because it ran contrary to the idea of a linear relationship between diagnostic accuracy and body weight.

“While seemingly counterintuitive, it is probable that the lack of perinephric and periureteral fat in this low body fat cadaver could have made the ureters more difficult for the radiologist to trace, thereby negating the beneficial effect of less abdominal fat,” wrote Heldt and colleagues. “In addition, the presence of phleboliths in the pelvis can be mistaken for urinary calculi if the ureter has minimal periureteral adipose tissue to distinguish it from surrounding structures.”

While the sensitivity and specificity for stone detection in underweight and overweight subjects was decreased on extremely low dose radiation settings of less than 1 mSv, low-dose protocols of 2 mSv could still be used in this patient group, according to the authors.