Korean J Radiol.  2019 Apr;20(4):671-682. 10.3348/kjr.2018.0400.

Distinguishing between Thymic Epithelial Tumors and Benign Cysts via Computed Tomography

• Lee SH
• Yoon SH
• Nam JG
• Kim HJ
• Ahn SY
• Kim HK
• Lee HJ
• Lee HH
• Cheon GJ
• Goo JM

• KMID: 2440490
• DOI: http://doi.org/10.3348/kjr.2018.0400

Abstract

OBJECTIVE
To investigate whether computed tomography (CT) and fluorine-18-labeled fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) may be applied to distinguish thymic epithelial tumors (TETs) from benign cysts in the anterior mediastinum.
MATERIALS AND METHODS
We included 262 consecutive patients with pathologically proven TETs and benign cysts 5 cm or smaller who underwent preoperative CT scans. In addition to conventional morphological and ancillary CT findings, the relationship between the lesion and the adjacent mediastinal pleura was evaluated qualitatively and quantitatively. Mean lesion attenuation was measured on CT images. The maximum standardized uptake value (SUVmax) was obtained with FDG-PET scans in 40 patients. CT predictors for TETs were identified with multivariate logistic regression analysis. For validation, we assessed the diagnostic accuracy and inter-observer agreement between four radiologists in a size-matched set of 24 cysts and 24 TETs using a receiver operating characteristic curve before and after being informed of the study findings.
RESULTS
The multivariate analysis showed that post-contrast attenuation of 60 Hounsfield unit or higher (odds ratio [OR], 12.734; 95% confidence interval [CI], 2.506-64.705; p = 0.002) and the presence of protrusion from the mediastinal pleura (OR, 9.855; 95% CI, 1.749-55.535; p = 0.009) were the strongest CT predictors for TETs. SUVmax was significantly higher in TETs than in cysts (5.3 ± 2.4 vs. 1.1 ± 0.3; p < 0.001). After being informed of the study findings, the readers' area under the curve improved from 0.872-0.955 to 0.949-0.999 (p = 0.066-0.149). Inter-observer kappa values for protrusion were 0.630-0.941.
CONCLUSION
Post-contrast CT attenuation, protrusion from the mediastinal pleura, and SUVmax were useful imaging features for distinguishing TETs from cysts in the anterior mediastinum.

Keyword

Mediastinum; Cystic mass; Thymic tumor; Computed tomography; FDG-PET/CT

MeSH Terms

Humans
Logistic Models
Mediastinum
Multivariate Analysis
Pleura
Positron-Emission Tomography
ROC Curve
Thymus Neoplasms
Tomography, X-Ray Computed

Figure

• 

  Fig. 1 Study diagram for patient inclusion.CT = computed tomography, TET = thymic epithelial tumor

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  Fig. 2 Protrusion length and protrusion ratio on axial CT image.A. 58-year-old female with 3.5-cm thymic cyst (arrow). Lesion abuts to adjacent mediastinal pleura but does not protrude: both protrusion length and protrusion ratio are 0. B. 64-year-old male with 3.4-cm thymic carcinoma (arrow), showing protrusion to mediastinal pleura. Protrusion length = 12.7 mm, protrusion ratio = (12.7 / 31.6) × 100 = 40.2%.

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  Fig. 3 Proportion of TETs and benign cysts that showed protrusion from adjacent mediastinal pleura according to lesion size.No TET was smaller than 1 cm. Protrusion was absent in 9 thymic cysts smaller than 1 cm.

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  Fig. 4 Distribution of (A) pre- and (B) post-contrast CT attenuation of TETs and benign cysts.HU = Hounsfield unit

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  Fig. 5 Pseudoenhancement in thymic cysts.A. 61-year-old male with 3.5-cm thymic cyst showing pseudoenhancement. Lesion is located at prevascular space, abutting left innominate vein. Pre-contrast study (left) shows round shaped, smooth-margined mass with internal fluid attenuation. After contrast enhancement (right), beam-hardening artifacts from left innominate vein with injected dense contrast medium create bright streaky high attenuations on mass, causing pseudoenhancement. Mean CT attenuation values measured at two phases were 14 HU and 49 HU, respectively, resulting in absolute enhancement of 35 HU. B. 52-year-old male with 2.2-cm thymic cyst without enhancement. Lesion is also abutting left innominate vein, but contrast medium is being injected through right upper extremity vein. Mean CT attenuation values measured at two phases were 44 HU and 42 HU, respectively.

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  Fig. 6 ROC curves of four radiologists before and after instruction of study results.AUC = areas under ROC curve, ROC = receiver operating characteristic

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