Thymoma, Type AB

Jonathan Levi Streeter, MD - Case Coordinator
Amy Ho Huang, MD - Radiology Discussion
Miguel N Rivera, MD - Pathology Discussion
Philip Costello, MD - Attending Radiologist
Pablo R Ros, MD, MPH - Attending Radiologist

November 25, 2002

Presentation

Imaging Findings

Posteroanterior and lateral views demonstrate an abnormal contour to the right cardiac border, which manifests as a retrosternal opacity on the lateral view. The lungs appear normal.

Differential Diagnosis

Contrast-enhanced CT scan (from outside institution, not available) showed a well-defined soft tissue mass located anterolateral to the right atrial appendage, which demonstrated peripheral calcifications. This is most suggestive of a thymoma or possibly a teratoma. Its distance from the aorta made a vascular abnormality less likely.

Diagnosis

Discussion

The gross images show a lobulated, well-circumscribed mass surrounded by a fibrous capsule and fat. Neither necrosis nor hemorrhage is present. Microscopically, the mass is composed mostly of lymphocytes. At high power, it becomes apparent that the lymphocytes are associated with spindle-shaped cells, which suggests the thymic medulla. Therefore, this mass would be classified as a mixed thymoma.

Radiology Discussion

Thymoma is an epithelial neoplasm of the thymus and the most common anterior mediastinal mass in adults. It usually presents during a patient’s 4th to 6th decade, and the incidence is not gender biased. Approximately 50% of cases are asymptomatic (presenting with incidental discovery on chest x-ray). Forty percent of patients have an associated parathymic syndrome (e.g., myasthenia gravis, aplastic anemia, red-cell aplasia, hypogammaglobulinemia); although 35% of thymoma patients have myasthenia gravis, only 15% of myasthenia gravis patients eventually develop thymoma.

Most (75%) thymomas are noninvasive (i.e. circumscribed), while 25% grow through their capsule to involve the surrounding mediastinum.

On chest radiography, thymoma typically appears as a subtle contour abnormality in the anterior mediastinum. It can sometimes conform to the cardiac contour and simulate cardiac enlargement on a frontal image. Small lesions may be difficult or even impossible to see on a frontal radiograph. For these reasons, a lateral view is often important.

Invasive thymoma may cause pleural disease and is often characterized by an irregular interface between the mass and adjacent lung.

On CT scan, thymoma typically appears as a well-circumscribed soft tissue mass that enhances homogeneously. These masses may display low attenuation foci of necrosis and hemorrhage, and calcification occurs in 20% of cases. Some thymomas have cystic components. On MRI, thymomas are usually isointense on T1 and have increased signal intensity on T2. Malignant (invasive) thymomas typically have an inhomogeneous signal, which may be related to cystic areas or hemorrhage.

CT and MRI are useful for staging of thymomas. The Masaoka Classification is as follows:

Stage 1: No capsular invasion

Stage 2: Capsular/pleural invasion

Stage 3: Invasion of surrounding tissue (lung, pericardium, SVC)

Stage 4a: Pleural or pericardial dissemination

Stage 4b: Lymphogenous or hematogenous metastases

Stage 1 is non-invasive, but stages 2-4b are considered to be malignant.

Surgical resection is the preferred treatment for thymoma. Total thymectomy is associated with complete remission in 80-95% of cases. Radiation therapy is controversial and is usually reserved for invasive thymoma (as an adjunct to surgical resection). Chemotherapy is rarely used, and only for recurrent or metastatic disease.

References

Mani, Sanjeev. Chest Radiology Case 8. IndianRadiologist.com http://www.indianradiologist.com/chestcase8.htm.

Lee JK et al. Computed Body Tomography with MRI Correlation 3rd ed. Lipppincott Williams and Wilkins, Philadelphia, 1998.

Weissleder R et al. Primer of Diagnostic Imaging, 2nd ed. Mosby, St. Louis, 1997.

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