Can Low-Dose Unenhanced Chest CT Be Used for Follow-Up of Lung Nodules?

OBJECTIVE. The purpose of this study is to establish the difference in lung nodule volume between standard-dose contrast-enhanced and low-dose unenhanced CT. SUBJECTS AND METHODS. Twenty patients with known pulmonary metastases underwent three CT examinations on 1 day: two unenhanced low-dose (120 kVp and 30 mAs) and a standard-dose (120-140 kVp and 75-200 mAs) contrast-enhanced chest CT examinations. For nodules < 1000 mm(3), nodule volume was quantified using dedicated software from the CT manufacturer. Wilcoxon's signed rank tests were used for analysis of nodules <= 200 mm(3) and > 200 mm(3) (approximately diameter of 8 mm). RESULTS. One hundred one nodules (n = 69 = 200 mm3) were analyzed in 15 of these subjects. Measured volume of nodules = 200 mm(3) was systematically lower on both low-dose unenhanced CT examinations when compared with standard-dose contrast-enhanced CT (differences, 13.7% and 15.5%, respectively; p < 0.0001), but nodule volume was not different between low-dose CT (median difference, 1.0%; p = 0.10). Nodule volume was not systematically different between the protocols for nodules > 200 mm(3) (p > 0.30). CONCLUSION. For lung nodules <= 200 mm(3) (approximately 8 mm) the measured volume on low-dose unenhanced CT is significantly lower when compared with standard-dose contrast-enhanced CT. This effect is likely due to contrast administration rather than other imaging parameters, which should be taken into account in the follow-up of lung nodules because growth can remain undetected or doubling time underestimated.