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Thymoma signs

已有 22 次阅读2019-12-2 14:43 |个人分类:TET学习|系统分类:医学科学

Six of the 12 malignant thymomas had a lobulated internal architecture on T2-weighted images,with round or irregularly shaped high-intensity lines.
MR images of the excised specimens revealed the scattered areas of high intensity and lobulated internal architecture seen on preoperative MR images.
Scattered regions of high intensity on T2-weighted images corresponded to cystic regions with or without hemorrhage on pathologic specimens.

Marginal low-intensity areas seen on T2-weighted images of one malignant tymoma corresponded to small mural nodules attached to the wall of a large unilocular cystic mass on pathologic specimens.

Benign thymomas ranged from 1.5 to 8 cm in maximum diameter (average, 3.1 cm). All five showed higher signal intensity than skeletal muscle on T1-weighted images. Signal intensity was increased on T2-weighted images. Four of the five benign thymomas had slightly inhomogeneous intensities, equal to or slightly lower than that of fat.

Microscopically, the lobulated architecture consisted of round or irregular areas composed of mixed lymphocytes and epithelial cells separated by relatively thick fibrous septa.
The lobulated architecture seen on MR images corresponded to the lobulated internal architecture seen macroscopically on the cut surface of the resected specimens.

Six thymomas that showed no distinct lobulated internal architecture had no lobulated configuration or a small lobulation separated by thin fibrous septa on pathologic specimens.

On pathologic specimens of four benign thymomas in which a slightly inhomogenous intensity was seen on MR, areas with lymphocytes and epithelial cells were mingled, without cystic regions. When compared, areas where lymphocytes predominated had the same signal intensities as areas where epithelial cells predominated.
The moderately inhomogenous intensity seen in a small benign thymoma on T2-weighted images corresponded to multiple small cystic foci in the pathologic specimens.

A low-intensity band on the preoperative T2-weighted image of a benign thymoma corresponded to a thick fibrous septum in the pathologic specimen.

Figure legend
T1-weighted axial MR image shows a mass lesion in anterior mediastinum with signal intensity that is higher than that of skeletal muscle but lower than that of fat.

T2-weighted axial MR image shows that signal intensity of a mass is high and slightly inhomogenous. Low-intensity band is identified in medial portion of mass.

T2-weighted MR image of excised specimen shows relatively homogeneous pattern of signal intensity. Thick low-intensity band is seen on medial aspect of tumor.

Histologic specimen shows mixed lymphocyte and epithelial-cell components without cystic regions. Lymphocyte-predominant areas appear dark, while epithelial-cell-predominant areas appear light. Comparison with MR images of specimen shows that there is no distinct difference in signal intensity between lymphocyte and epithelial-cell-predominant areas. Low intensity band seen on MR corresponds to relatively thick fibrous septum.

Disccusion
According to Rosai and Levine[3], all thymomas are composed of a mixture of lymphocytes and epithelial cells varying proportions, with scattered cystic regions of various sizes seen in 40% of thymomas pathologically. Cystic lesions in the tumor are filled with clear, proteinaceous, or bloody fluid. Larger tumors are more likely to exhibit cystic changes. The presence of sharply defined fibrous septa dividing tumors into lobules is one of the most characteristic features of thymomas, and the lobulated architecture was seen in 88% of all thymomas pathologically.

The lobulated internal architecture of thymomas seen on MR images correspond to lobulations caused by fibrous septa in pathologic specimens. Small lobulations or lobulations separated by thin fibrous septa seen in pathologic specimens could not be seen on MR images, owing to the low spatial resolution of MR.

Eleven of the 12 malignant thymoma had an inhomogeneous high-intensity appearance and six had a lobulated internal architecture. Four of five benign thymomas had a slightly inhomogeneous intensity and none had a lobulated architecture. 

The MR appearance of thymoma may be useful in assessing the malignancy of thymomas, but definitive differentiation between malignant and benign thymomas with MR currently is no possible.

Sakai F, Sone S, Kiyono K, et al. MR imaging of thymoma: radiologic-pathologic correlation[J]. AJR Am J Roentgenol, 1992,158:751-756.

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