Thymic cyst: Is attenuation artifactually increased on contrast-enhanced CT?

Abstract

Background: Thymic cysts are often misinterpreted as thymomas or lymph nodes, then leading to unnecessary thymectomy. The purpose of this study was to investigate how the adjacent large vessels artifactually affected attenuation values of thymic cysts on contrast-enhanced CT (CE-CT).

Methods: In this retrospective study, a total of 84 patients were included with pathological diagnosis of thymic cysts and preoperative CE-CT. Quantitative measurements of the size, CT attenuation of thymic cysts and CT attenuation of adjacent large vessels were performed on preoperative CE-CT. According to the absolute change in attenuation of the cysts between contrast-enhanced and nonenhanced CT, the patients were classified into the groups of artifactual hyper-density, unchanged density, and artifactual hypo-density. CT characteristics were compared between the three groups. Furthermore, multivariable logistic regression analysis was performed to determine the independent factors for artifactual hyper-density.

Results: The group of artifactual hyper-density had smaller short diameter of the cysts, higher postcontrast attenuation values and lower nonenhanced attenuation values of the adjacent large vessel. Furthermore, the multivariable logistic analysis showed that artifactual hyper-density of thymic cysts was negatively associated with nonenhanced attenuation of adjacent large vessel, and positively associated with postcontrast attenuation of adjacent large vessel and postcontrast attenuation of cysts.

Conclusions: Most cases with >20 HU nonenhanced CT attenuation in surgically resected cases. Artifactual hyper-density─pseudo-enhancement phenomenon of thymic cysts was more apparent in higher increasing attenuation of adjacent large vessels on CE-CT. A well understanding of this phenomenon can help reduce preoperative misdiagnosis and unnecessary thymectomy.

Keywords: CT; artifacts; misdiagnosis; pseudoenhancement; thymic cysts.

Figure 1

(A-D) Images of a thymic cyst with increased postcontrast attenuation values on CE-CT in 62-year-old woman (age in 2018). Imaging shows (A) a thymic cyst measuring 11.6×8.8 mm, (B) 12.9HU with aortic arch measuring 51.3HU on nonenhanced CT, (C) 28.2HU with aortic arch measuring 337.7HU on arterial phase, and (D) 24.1HU with aortic arch measuring 197.0HU on venous phase of CE-CT. (E-H) Images of a thymic cyst with stable postcontrast attenuation values on CE-CT in 65-year-old man (age in 2017). Imaging shows (E) a thymic cyst measuring 78.3×34 mm, (F) 9.0HU with main pulmonary artery measuring 40.5HU on nonenhanced CT, (G) 9.0HU with main pulmonary artery measuring 233.8HU on arterial phase, and (H) 10.0HU with main pulmonary artery measuring 105.6HU on venous phase of CE-CT. (I-L) Images of a thymic cyst with decreased postcontrast attenuation values on CE-CT in 53-year-old woman (age in 2018). Imaging shows (I) a thymic cyst measuring 20.1×17.0 mm, (J) 45.4HU with brachiocephalic trunk measuring 54.1HU on nonenhanced CT, (K) 29.0HU with brachiocephalic trunk measuring 288.5HU on arterial phase, and (L) 38.8HU with brachiocephalic trunk measuring 148.6HU on venous phase of CE-CT.

Figure 2

The group difference between the groups of artifactual hyper-density, unchanged density, and artifactual hypo-density according to the CT attenuation change of thymic cysts between contrast-enhanced and nonenhanced CT. (A) The group of > 5 HU had smaller short diameter (p=0.008), (B) lower nonenhanced attenuation values of adjacent large vessel than the group of -5~5 HU (p < 0.001) and the group of <-5 HU (p=0.001), (C) higher postcontrast attenuation values of adjacent large vessel than the group of -5~5 HU (p < 0.001), and (D) higher postcontrast attenuation values of cysts than the group of -5~5 HU (p < 0.001) and the group of <-5 HU (p < 0.001). * indicates significant differences.