Objective:

To investigate the imaging characteristics of thymoma and mediastinal lymphoma using spectral CT and evaluate whether the quantitative information can improve the differential diagnosis of these diseases.

Methods:

This retrospective study was approved by the institutional review board, and written informed consent was obtained from all patients. Overall, 39 patients with mediastinal tumors (24 thymomas and 15 mediastinal lymphomas) were examined with CT spectral imaging during the arterial phase (AP) and venous phase (VP). Iodine concentrations were derived from iodine-based material-decomposition CT images and normalized to the iodine concentration in the aorta. The difference in normalized iodine concentrations (NICs), HU curve slop(λ_HU), and the differences between AP and VP for CT values of lesions in 70 Kev were calculated. The two-sample t-test was performed to compare quantitative parameters, and non-quantitative parameters were compared with the Chi-square test (Fisher exact). Receiver operating characteristic (ROC) curves were generated to help establish threshold values for the parameters required for the significant differentiation of thymomas from mediastinal lymphomas. Two readers qualitatively assessed the lesion types according to the imaging features. The sensitivity and specificity of the qualitative and quantitative studies were compared.

Results:

NICs during the VP and λ_HU during the AP in patients with mediastinal lymphomas differed significantly from those in patients with thymomas. The mean NICs during the VP were 0.28 ± 0.08  mg ml^-1 (±standard deviation) vs 0.49 ± 0.15  mg ml^-1, respectively. The λ_HU during the AP was 0.69 ± 0.17 vs 1.26 ± 0.74, respectively. The NICs during the VP and λ_HU during the AP had high sensitivity and specificity in differentiating mediastinal lymphomas from thymomas. The tumor location, margin, necrosis, presence of swollen mediastinal lymph nodes, relationship with adjacent vessels, and enhancement pattern differed significantly between the groups (p < 0.05). The combination of NICs and λ_HU had higher sensitivity and specificity than did those of conventional qualitative CT image analysis during the combined phases.

Conclusion:

CT spectral imaging has promising potential for the diagnostic differentiation of mediastinal lymphomas and thymomas. The iodine content and λ_HU could be valuable parameters for differentiating thymomas and mediastinal lymphomas.

Advances in knowledge:

The iodine content and λ_HU, provided by spectral CT, could be used as new parameters to distinguish mediastinal lymphomas from thymomas.