AJR 2003; 181:1504
© American Roentgen Ray Society
Radiologic-Pathologic Conferences of The University of Texas
M. D. Anderson Cancer Center |
Invasive Thymoma
Mylene T. Truong1,
Bradley S. Sabloff,
Gregory W. Gladish,
Gary J. Whitman and
Reginald F. Munden
1 All authors: Division of Diagnostic Imaging, The University of Texas M. D.
Anderson Cancer Center, 1515 Holcombe Blvd., Box 57, Houston, TX 77030.
Received February 6, 2003;
accepted after revision February 12, 2003.
Address correspondence to M. T. Truong.
A43-year-old woman presented with a 2-day history of chest pain and
shortness of breath. Chest radiography showed left mediastinal widening and
left pleural thickening (Fig.
1A). Chest CT showed a homogeneous lobulated left anterior
mediastinal mass adjacent to the aortic arch
(Fig. 1B). Nodular left pleural
thickening extended into the major fissure and involved the diaphragm.
CT-guided core needle biopsy revealed a lymphoid-predominant thymoma
(Fig. 1C). The patient was
treated with cyclophosphamide, doxorubicin and cisplatin, followed by surgical
resection. An extrapleural pneumonectomy with diaphragmatic reconstruction was
performed. Results of surgical pathology showed invasive thymoma involving the
lung, pleura, and diaphragm.
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Fig. 1A. —43-year-old woman with stage IV invasive thymoma.
Posteroanterior chest radiograph shows contour abnormality of left anterior
mediastinum (arrow) with circumferential nodular left pleural
thickening.
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Fig. 1B. —43-year-old woman with stage IV invasive thymoma. Chest CT
scan confirms lobulated left anterior mediastinal mass (curved arrow)
associated with nodular pleural implant (straight arrow) along
lateral aspect of left hemithorax.
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Fig. 1C. —43-year-old woman with stage IV invasive thymoma.
Photomicrograph of histopathologic specimens shows lymphocytes and epithelial
cell clusters with thick fibrous bands, consistent with thymoma. (H and E, low
power)
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Thymoma is an indolent epithelial neoplasm of the thymus that accounts for
nearly half of all primary tumors of the anterior mediastinum
[1]. The peak incidence of
thymoma is in the fifth and sixth decades of life with no predilection for sex
[2]. Thymoma is rarely seen in
children. Approximately half of the patients are asymptomatic, with the
diagnosis made incidentally through chest radiographs obtained for other
reasons. Symptoms may occur because of mass effect of the tumor on the
trachea, the recurrent laryngeal nerve, the esophagus, or the superior vena
cava. Patients may present with chest pain, cough, dyspnea, respiratory tract
infections, hoarseness, dysphagia, or signs of superior vena cava syndrome.
Forty percent of patients with thymomas have paraneoplastic syndromes. These
syndromes include myasthenia gravis, pure red cell aplasia,
hypogammaglobulinemia, endocrinopathy, and connective tissues disorders.
Approximately 30–40% of thymoma patients have myasthenia gravis, and
10–15% of patients with myasthenia gravis have thymomas
[3].
Although most thymomas are encapsulated with a histologically bland
appearance, some may be locally invasive or metastasize to distant organs.
Infiltration of the pleura, lung, great vessels, pericardium, chest wall, or
diaphragm occurs in 30–50% of cases. The most widely used staging
classification of thymoma was devised by Masaoka et al.
[4], based on the presence of
capsular invasion. Stage I thymomas are completely encapsulated
macroscopically with no microscopic capsular invasion. Stage II thymomas show
invasion through the capsule into the mediastinal fat or pleura. Stage III
thymomas show macroscopic invasion into neighboring organs. Stage IV thymomas
are associated with pleural or pericardial implants or distant metastases.
Histologically, thymomas can be classified according to the predominant
cell type. Mixed lymphoepithelial cell type is the most common, followed by
lymphoid predominant; epithelial cell predominant is the rarest form of
thymoma. Marino and Muller-Hermelink
[5] subdivided thymomas on the
basis of the epithelial cell ontogeny. Subdivisions include cortical,
medullary, and mixed types.
On radiographs, thymomas are usually seen as round or oval anterior
mediastinal masses, which can protrude to one side or extend to both
hemithoraces. Although most thymomas arise in the anterior superior
mediastinum, they can be found in the neck, the posterior mediastinum, the
pleura, or the lung. Calcification may be detected in the capsule or scattered
throughout the tumor. Thymomas usually measure between 5 and 10 cm in diameter
and commonly have smooth lobulated borders. On CT, thymomas are soft-tissue
lesions that often enhance uniformly with the IV administration of contrast
material. Necrosis and hemorrhage may occur. CT findings suggestive of tumor
invasion include obliteration of fat planes in the mediastinum, pleural or
pericardial thickening, encasement of the mediastinal vessels, and irregular
interface with the adjacent lung. On MRI, thymomas have intermediate signal
intensity on T1-weighted images and show increased signal intensity on
T2-weighted images, approaching that of fat. MRI can be used to evaluate for
evidence of vascular invasion and diaphragmatic involvement.
For stage I thymomas, surgical resection is curative. Up to 12% of cases
without gross or microscopic evidence of invasion recur locally. En bloc
removal of the thymus and adjacent mediastinal fat has also been used to treat
myasthenia gravis with or without an associated thymoma. Thymomas are
radiosensitive, and radiation therapy is used as an adjunct to surgical
resection for invasive disease. Recent trials of combined-modality therapy
suggest that neoadjuvant and adjuvant chemotherapy may further improve
long-term survival in patients with advanced-stage thymomas.
References
- Morgenthaler TI, Brown LR, Colby TV, Harper CM Jr, Coles DT.
Thymoma. Mayo Clin Proc1993; 68:1110
–1123[Medline]
- Wilkins EW Jr, Grillo HC, Scannell JG, Moncure AC, Mathisen DJ.
Role of staging in prognosis and management of thymoma. Ann Thorac
Surg 1991;51:888
–892[Abstract]
- Lara PN Jr. Malignant thymoma: current status and future
directions. Cancer Treat Rev2000; 26:127
–131[Medline]
- Masaoka A, Monden Y, Nakahara K, Tanioka T. Follow-up study of
thymomas with special reference to their clinical stages.
Cancer 1981;48:2485
–2492[Medline]
- Marino M, Muller-Hermelink HK. Thymoma and thymic carcinoma:
relation of thymoma epithelial cells to the cortical and medullary
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Histopathol 1985;407:119
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