Thymic Cyst And Rebound

Thymic cyst

On CT scans, a thymic cyst appears homogeneous with water attenuation. The attenuation may vary, depending on the contents of the cyst. High attenuation may be present if the cyst contains proteinaceous fluid or blood from hemorrhage. A neoplasm with cystic degeneration may closely mimic a thymic cyst; associated soft tissue attenuation may help in their differentiation.

Thymic rebound

After initiation of chemotherapy, CT scans may reveal a decrease in the size of the thymus. Rebound in the form of overgrowth occurs a few months after completion of chemotherapy. Criteria for rebound include an increase in size of greater than 50%, as compared with the baseline volume.

Degree of confidence

Chest CT scanning performed with and without contrast enhancement is clearly superior to routine radiography. A small thymic tumor can easily be missed on a chest radiograph, whereas CT scans distinctly delineate the tumor. In addition, enhancement often helps in clearly differentiating the mass from the surrounding vascular structures. This is helpful in planning surgery.
On imaging studies, bilateral extension of the mass, absence of fat planes, and invasion of adjacent structures are indications of malignancy. MRI may show similar findings. The demonstration of encapsulation of the mass and the homogeneous enhancement of the capsule are indications of benign tumors; these structures are better imaged with CT scanning. MRI and CT scanning often complement each other and can facilitate preoperative diagnosis and staging of the thymus neoplasms.
In cases of lymphoid follicular hyperplasia, the thymus may not always appear enlarged; this may be overlooked on CT scans.

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