Low-dose, but high risk: Malignant lung nodule growth missed on unenhanced CT follow-up
For relatively small lung nodules, low-dose unenhanced CT significantly underestimates volume, increasing the risk that malignant growth rates will be missed at follow-up, according to a study published in the October issue of the American Journal of Roentgenology.

“Recently there has been increased interest in volumetric measurement of lung nodules instead of diameter measurements, but understanding the potential sources of error and variation in these measurements is essential to evaluate growth,” wrote Pim A. de Jong, PhD, University Medical Centre Utrecht, the Netherlands, and colleagues.

The authors explained that the number of incidentally detected small lung nodules has increased with the growth of CT utilization. While follow-up CT studies are recommended, the vast majority of nodules are benign lesions, and low-dose unenhanced CT is recommended for follow-up to reduce radiation and contrast agent risk.

Since pulmonary nodules are often initially detected on standard-dose contrast-enhanced CT, the authors conducted a study involving 20 patients with known pulmonary metastases to compare nodule volume measurements between the two protocols. Patients underwent three CT exams, all in the same day, two of which were unenhanced low-dose scans of 120 kVp and 30 mAs, and the third being a standard dose of 120-140 kVp and 75-200 mAs with contrast enhancement.

A total of 101 nodules in 15 patients were ultimately analyzed, 69 of which were 200 mm3 or smaller. The authors reported that the measured volume of these smaller nodules was systematically lower on both low-dose unenhanced CT exams when compared with the standard-dose exams, with differences of 13.7 percent and 15.5 percent, respectively.

There was no volume difference between low-dose CT scans, and nodule volume was not systematically different between low- and standard-dose protocols for nodules larger than 200 mm3.

The authors put the implications of a 15 percent lower volume measurement into perspective with a hypothetical situation involving a malignant nodule of 100 mm3 with a volume doubling time of 300 days. If follow-up at 100 days was conducted using standard-dose contrast-enhanced CT, the new volume would be measured at 126 mm3, a 26 percent increase representing malignant growth. On the other hand, if follow-up at 100 days was done using low-dose unenhanced CT, the volume would only be measured at 107 mm3, a 7 percent increase that would be considered a benign growth rate. This discrepancy could result in delayed diagnosis.

de Jong and colleagues speculated that the discrepancy in volume is specifically due to the lack of contrast enhancement. “Apparently, the increased nodule density caused by the contrast uptake alters the edge detection of our software, leading to larger nodules.”

To avoid the problem, the authors suggested using the same CT protocol at follow-up that was used at baseline, obtaining a low-dose CT scan as soon as possible after baseline to compare to low-dose follow-up scans, or determining a factor to adjust nodule volume measurements at follow-up to take into account the effect of differing protocols.