Clinical History:
A 57 year-old man presented with chest pain and
shortness of breath of several weeks' duration. A chest radiograph demonstrated an anterior mediastinal
mass. A surgical procedure with complete removal of the mediastinal tumor was performed.
Case 2 - Figure 1 - Scanning magnification shows lymphoid tissue with a scattering of nodules composed of sheets of epithelioid cells.
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Case 2 - Figure 2 - Lymphoid stroma surrounding the cell nodules shows lymphoid hyperplasia with prominent germinal centers.
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Case 2 - Figure 3 - Higher magnification of cell islands showing bland-appearing oval to spindle cells with abundant cytoplasm.
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Pathologic Findings:
On low power magnification, the tumor was characterized by
multiple small tumor nodules embedded in an abundant lymphoid stroma. At higher magnification, the
nodules were composed of bland-appearing cells with oval and spindle cell features containing elongated
nuclei and inconspicuous nucleoli. Mitotic activity and necrosis were not observed. The lymphoid
background was composed of small lymphocytes and in some areas lymphoid follicles with germinal centers
were also present. Some of the germinal centers showed an onion-skin like cuffing of lymphocytes with
hyalinization mimicking Castleman's disease.
Immunohistochemical studies demonstrated that the spindle cells forming the nodules
were strongly positive for CAM 5.2 low molecular weight cytokeratin. The lymphoid background showed
positive staining for CD45 and CD20. Kappa and lambda demonstrated a polyclonal population.
Immunohistochemical studies for UCHL-1 and CD99 showed scattered positive cells. Bcl-2 showed strong
positivity in the mantle zone lymphocytes within hyperplasic lymphoid follicles. CD3 and CD5 were
negative
Diagnosis - Micronodular Thymoma with Lymphoid B-cell Hyperplasia
Comment:
Thymomas are the most common epithelial tumors in the anterior mediastinum. Although four main
histopathologic growth patterns are widely recognized, the tumor can also show unusual histopathologic
growth patterns that at times may pose difficulties in diagnosis.
Prominent lymphoid hyperplasia in the thymus is usually seen in the setting of myasthenia gravis or
other autoimmune disorders. However, it can also be seen in tumoral conditions such as seminomas and
multilocular thymic cysts. B-cells in the thymus appear to represent a distinct population of mature,
intraparenchymatous B- lymphocytes present within the normal thymus.
The case depicted here represents a recently described entity corresponding to a distinctively
unusual morphological variant of thymoma characterized by the formation of multiple small tumor nodules
embedded in a prominent lymphoid background with the formation of germinal centers in a patient without
clinical evidence of myasthenia gravis or other immune disorder. Although the cell population is
composed of spindle cells, the overall growth pattern of the tumor is not the usual one for spindle cell
thymoma. The presence of a prominent lymphocyte background composed of reactive B-cell lymphocytes is an
unusual feature in spindle cell thymomas.
Problems posed by this lesion regarding proper interpretation would include considering the
possibility of a metastatic carcinoma to mediastinal lymph nodes. The presence of a prominent lymphoid
background, germinal centers and nodular spindle (epithelial) cells may pose problems on a limited
mediastinoscopic biopsy. On the other hand, the tumor may be confused in a cursory review for primary
thymic carcinoma due to the unusual growth pattern. In both of these circumstances, the treatment and
prognosis may be seriously affected in these patients.
As with other types of thymomas, the outcome of these tumors is based on the staging of the lesion at
the time of diagnosis. In a series of eighteen patients with similar types of thymoma, we noted that
none of the patients included in that study had had recurrence or metastatic disease after a period of 1
to 7 years.
We consider that this type of thymoma needs to be recognized in order to avoid unnecessarily
excessive treatment and also to stress the importance of determining the stage of the tumor at the time
of diagnosis for the management of these patients.
References
- Suster S, Moran CA. Micronodular thymoma with lymphoid B-cell hyperplasia. A clinicopathologic and
immunohistochemical study of eighteen cases of a distinctive morphologic variant of thymic epithelial
neoplasm. Am J Surg Pathol 1999; 23:955-963.
- Suster S, Moran CA. Malignant neoplasm that may mimic benign conditions. Sem Diagn Pathol 1995;
12:98-104.
- Moran CA, Suster S, Przygodzki RM, Koss MN. Primary germ cell tumors of the mediastinum II.
Mediastinal Seminoma: a clinicopathologic and immunohistochemical study of 120 cases. Cancer 1997;
80:691-698.
- Suster S, Moran CA. Primary thymic epithelial neoplasms: current concepts and controversies. In
Fechner RE, Rosen PP, eds. Anatomic Pathology 1997; 2:1-9.
- Suster S, Moran CA. Thymic carcinoma: spectrum of differentiation and histologic types. Pathology
1998; 30:111-122.
- Suster S, Rosai J. Multilocular thymic cysts: an acquired reactive process. Study of 19 cases. Am
J Surg Pathol 1991; 15:388-398.
- Suster S, Moran CA. Primary thymic epithelial neoplasms with combined features of thymoma and thymic
carcinoma. A clinicopathologic study of 22 cases. Am J Surg Pathol 1996; 20:1469-1480.
