ABSTRACT

Incremental dynamic CT (IDCT) has been used recently to determine whether malignant solitary pulmonary nodules (SPNs) can be differentiated from benign SPNs. In IDCT, CT number of SPNs is calculated using circular region of interest (60% size of SPN diameter) at noncontrast early (30 sec after administration of contrast media), middle (2 minute) and late (5 minute) enhancement phases. The difference of CT number between noncontrast and peak enhancement phase (maximum attenuation) is calculated as an indicator of enhancement of SPN. Malignant sign is considered as maximum attenuation ≥25 HU, benign as ≤ 15 HU ans indeterminate as 16-24 HU. This criterion is based on the fact that malignant SPNs are more enhanced than benign ones.

In our recent analysis, maximum attenuation of malignant SPNs (mean 49 ± standard deviation  34 HU; range, 25 to 91 HU) was significantly higher than that of tuberculoma (20 ± 5 HU;-7 to 78 HU; p<.0001) hamartoma (27 ± 29 HU; 2 to 71 HU; p=.006), and bronchogenic cyst (2 ± 10 HU; -8 to 11 HU; p=.0003) however, it was significantly lower than that of organizing pneumonia (64 ± 25 HU; 36 to 112 HU; p=.035), intrapulmonary lymph node (71 ± 20 HU; 33 to 90 HU; p=.02) and rounded atelectasis (106 ± 29 JU; 77 to 145 HU; p<.0001). If malignant range of maximum attenuation  was assumed ≥ 25 HU and benign range ≤ 15 HU, all (39/39) malignant SPNs and about a half (29/52) were diagnosed exactly, another half (29/52) of benign SPNs were not diagnosed exactly and one benign SPN was indeterminate (sensitive 100%, specificity 43%, accuracy 67%, indeterminate rate 1%)

Maximum attenuation correlated well (r=0.77) with tumor vascularity evaluated as number of small tumor vessels (0.02-0.10 mm in diameter) distributed in tumor interstitium containing elastic fiber. This result proved that maximum attenuation reflected the tumor vascularity. Benign SPN also seems to show well enhancement if it has a large amount of vascularity like lung carcinoma.

In IDCT analysis of SPN, there are several points to be considered carefully and we discussed intratumoral necrosis of lung carcinoma. transient decreased attenuation (transient CT number decreases in early or middle enhancement phases) and how and what size of region of interest should be established in IDCT.

In conclusion, enhancement appeared to be a good indicator of malignancy, however, about  a half of benign SPNs were not exactly diagnosed. To differentiate these enhanced benign SPNs from malignant ones, information besides enhancement  (morphologic data) will be needed.