Imaging:
The chest radiograph shows an anterior mediastinal mass that is round or oval with either smooth or lobulated margins.

CT is superior to radiography at demonstrating anatomic relationships with surrounding structures.

MRI has similar success at defining the tumor and the surrounding mediastianal structures.

http://www.vh.org/Providers/TeachingFiles/ITTR/Thymoma/Thymoma.html
Modified: Tue Nov 30 11:23:18 1999

Thymoma

Jeffrey R. Galvin, M.D.
Michael P. D'Alessandro, M.D.
Yasayuki Kurihara, M.D.
Peer Review Status: Internally Peer Reviewed Clinical Sx:

Thymomas are epithelial neoplasms of the thymus and are the most common primary neoplasms of the anterior superior mediastinum.
The majority of patients are middle-aged adults. There is no sex predominance.
Approximately one half of the patients are asymptomatic, 25-30% of patients have symptoms related to compression of adjacent mediastinal structures including cough, chest pain, and shortness of breath.
Patients with thymoma may have myasthenia gravis (30-50%), pure red cell aplasia, hypogammoglobulinemia, endocrine disorders. About 15% of patients with myasthenia gravis have a thymoma.

Etiology/Pathophysiology:
Thymomas arise from thymic epithelium.

Pathology:
Thymomas can be completely encapsulated (benign) or locally invasive without a fibrous capsule.
On macroscopic view, they are classified by predominant cell types: epithelial, lymphoid, or biphasic. Most thymomas are slow-growing tumors. Approximately one- third of thymomas are invasive and may grow into the surrounding mediastinal structures. This is determined at surgery and is not a histologic diagnosis. Local invasion of the pleura occurs frequently. Distant metastases are infrequent.

The signal characteristics are not adequate, however, to differentiate benign from malignant thymomas. Calcification is demonstrated in a minority of patients with thymoma.

DDx:
Lymphoma
Thymic carcinoma
Intrathoracic goiter
Germ cell tumor
Mesothelioma

Key references:
Rosado-de-Christenson M, Galobardes J, Moran C: Thymoma: Radiologic-Pathologic Correlation. RadioGraphics 12:151-168 1992

Sakai F, Sone S, Kiyono K et al: MR imaging of Thymoma: radiologic-pathologic correlation AJR 158:751-756

All contents copyright © 1992-2000 the Author(s) and The University of Iowa. All rights reserved.

Thymoma

Malignant tumor (mesenchymal)

(c) 1st Institute of Pathology and Experimental Cancer Research, Semmelweis University of Medical Sciences, Budapest, Hungary

Thymoma

Malignant tumor (mesenchymal)

(c) 1st Institute of Pathology and Experimental Cancer Research, Semmelweis University of Medical Sciences, Budapest, Hungary

Thymoma

Malignant tumor (mesenchymal)

(c) 1st Institute of Pathology and Experimental Cancer Research, Semmelweis University of Medical Sciences, Budapest, Hungary

Thymus, T-cell non-Hodgkin's lymphoma

(c) 1st Institute of Pathology and Experimental Cancer Research, Semmelweis University of Medical Sciences, Budapest, Hungary

Granulomatous thymoma

(c) 1st Institute of Pathology and Experimental Cancer Research, Semmelweis University of Medical Sciences, Budapest, Hungary

Thymoma

Malignant tumor (mesenchymal)

Electron microscopic

(c) 1st Institute of Pathology and Experimental Cancer Research, Semmelweis University of Medical Sciences, Budapest, Hungary

FRONTIERS IN BIOSCIENCE;
RECURRENT MEDIASTINAL THYMOMA

See the ultrastructural features of this tumor at low power

from bioscience.igh.cnrs.fr

FRONTIERS IN BIOSCIENCE;
RECURRENT MEDIASTINAL THYMOMA

See the ultrastructural features of this tumor at high power

from bioscience.igh.cnrs.fr

Figure 1. Smear preparation of FNAB with many lymphocytes and the occasional larger epithelioid appearing tumor cells, some of which are also evident in the clump of tumor cells on the right.

Figure 2. Cell block preparation of FNAB. Many normal appearing lymphocytes with larger cells scattered in between.

Figure 3. A relatively large nucleus is surrounded by remnants of cell membrane and cytoplasmic contents (C). A few desmosomes appear to join small cytoplasmic fragments (arrows).

Figure 4. Higher magnification verifies that numerous desmosomes (arrows) join tissue fragments and that some of the desmosmes are closely associated with each other.

Anterior Mediastinal Mass
Is it Thymoma?

Irving Dardick Department of Laboratory Medicine and Pathobiology University of Toronto, Toronto, Canada

History:
A 55-year-old woman was discovered to have an anterior mediastinal mass and had a fine-needle aspiration biopsy as the initial phase of her investigation. Light microscopy: The smear (Fig. 1) and cell block (Fig. 2) preparations were largely composed of small, normal-appearing lymphocytes with the occasional larger cell scattered among the lymphocytes. The nuclei of what seemed to be epithelial cells generally had dispersed heterochromatin and rather inconspicuous nucleoli. Antibodies to cytokeratins were all negative raising doubts about the diagnosis of thymoma.

As a concurrent part of the diagnostic investigation, electron microscopy was done on samples of glutaraldhyde-fixed tissue obtained from the needle rinse following preparation of the smears and cell block.

Electron Microscopy:
As seen in Figure 3, the majority of the cells were small, mature lymphocytes. Scattered among thse were a few poorly preserved cells with relatively large nuclei having markedly disaggregated heterochromatin and a tiny nucleolus. Little cytoplasm remained, but some cellular frgaments appeared to be joined by desmosomes. At higher magnification (Fig. 4), many tissue fragments were indeed joined by well-formed desmosomes, some in close proximity to each other.

Diagnosis and Discussion: The ultrastructural features, particularly the nuclear characteristics of the larger, partially fragmented tumor cells and the numerous desmosmes, which are sometimes in a row, confirm that the diagnosis of an anterior mediastinal thymoma is indeed correct. The case illustrates one of the limitations of immunohistochemistry and a situation that resulted in a false negative immunostain with cytokeratin antibodies. Probably as a result of somewhat excessive aspiration pressure during the FNAB, the more senstive epithelial cells ruptured. This dispersed the cytoplasmic contents, including intermediate filaments, into the supernatant fluids, which were lost during the preparation of smears and the cell block. Cellular fragments, including cell membranes and their the desmosmes, however, were retained and these had features typical for the epithelial cells of thymoma. Immunohistochemistry is not sufficiently sensitive to visualize intermediate filaments associated with such desmosomes.

Radiograph of the chest, anteroposterior view

Radiograph of the chest, lateral view

Close-ups of relevant area

Close-ups of relevant area

Thymoma

Vivek David, MD Robert D Pugatch, MD

Presentation
A 26-year-old woman with a history of myasthenia gravis presented for a routine chest radiograph. An abnormality was noted in the study.

Imaging Findings
Radiograph of the chest, anteroposterior view
Radiograph of the chest, lateral view
Close-ups of relevant area

Anteroposterior and lateral radiographs of the chest show a smooth, rounded anterior mediastinal mass.

Differential Diagnosis
The differential diagnosis of an anterior mediastinal mass includes a thymic mass (eg, thymoma, thymolipoma, thymic cyst, carcinoid), teratoma, a mass arising from the thyroid, lymphoma, and a vascular lesion. In this patient, a reasonable differential diagnosis would include these entities although a mass related to thyroid tissue would be unlikely given the lack of anatomic connection to the thyroid. The history of myasthenia gravis suggests the diagnosis of thymoma.

Diagnosis
Thymoma

Discussion
Thymomas are the most common primary thymic tumor. They are slow-growing lesions and usually behave in a benign fashion. A subgroup of these tumors have an aggressive nature and are called invasive thymomas. Invasive thymomas typically invade locally and spread along pleural and pericardial surfaces, but they can exhibit extrathoracic metastasis. A thymoma can appear at any age, but most patients are over the age of 40. Radiographically, thymomas are typically round or oval, well-marginated masses that may contain some peripheral calcification.

The association with myasthenia gravis: 50% of patients with thymoma have myasthenia gravis, and 15% of patients with myasthenia gravis have a thymoma. Among patients with myasthenia gravis and a thymoma, 25% show remission of their myasthenic symptoms after thymectomy.

References 1. Ellis K, Austi JHM, Ill AJ. Radiologic detection of thymoma in patients with myasthenia gravis. AJR 1988; 151: 873. 2. Fraser RG, Pare JAP, Pare PD, Fraser RS, Genereux GP. Diagnosis of diseases of the chest. 3rd ed. Philadelphia: WB Saunders, 1991: 2820-2831