[Pathology] Atlas of Mixed Lymphoepithelial Thymoma (WHO Type B2), Atlas of Mixed Lymphoepithelial Thymoma, Mixed Lymphoepithelial Thymoma, Atlas of Mediastinal Pathology
Fig. 2.63 Mixed lymphoepithelial thymoma (WHO type B2) is
characterized by an approximately equal admixture of lymphocytes vis-Ã -
vis the thymic epithelial cells. This diagnosis can be made on low-power
examination by recognizing the abundance of larger thymic epithelial
cells admixed with the lymphocytes
Fig. 2.64 Higher magnification of mixed lymphoepithelial thymoma
(WHO type B2) shows an approximately equal admixture of epithelial
cells and lymphocytes
Fig. 2.65 High magnification of mixed lymphoepithelial thymoma
(WHO type B2) shows higher numbers of thymic epithelial cells than in
type B1 thymoma admixed with the background lymphocytes. Notice
that the nuclear features of the epithelial cells do not differ significantly
from those of type B1 thymomas. There can be a range of nuclear features
seen in B1 and B2 thymomas, ranging from some cells displaying
large vesicular nuclei with no discernible cytoplasm to cells that have
slightly smaller nuclei and abundant eosinophilic cytoplasm
Fig. 2.66 Mixed lymphoepithelial thymomas (WHO type B2) can also
show striking lobulation separated by dense fi brous bands. There is a
balanced admixture of small lymphocytes and larger epithelial cells
Fig. 2.67 High-power magnification of the preceding field in a WHO
type B2 thymoma shows abundant thymic epithelial cells, with large
vesicular nuclei and prominent eosinophilic nucleoli admixed with the
smaller lymphocytes. The epithelial cells in the type B2 thymoma can
show great variability in size and shape, but generally they do not differ
significantly from those in type B1 thymoma; they are simply more abundant
Fig. 2.68 Another unusual histologic appearance in mixed lymphoepithelial
thymoma (WHO type B2) is characterized by sharp segregation
between the epithelial cells and the lymphocytes, with the larger epithelial
cells crowding around dilated perivascular spaces and the lymphocytes
situated in-between; this imparts a distinctly biphasic,
compartmentalized appearance
Fig. 2.69 Another feature of mixed lymphoepithelial thymoma (WHO
type B2) is the presence of dilated perivascular spaces, which can be
quite abundant in these tumors. In type B2 thymomas, the perivascular
spaces are rimmed by a layer of thymic epithelial cells, unlike in type
B1 thymomas, in which they appear naked
Fig. 2.70 Cystic changes with hemorrhage and areas of necrosis and
infarction are also features that can be seen in all types of thymoma
(including all A and B types), but they are most often encountered in
types B1 and B2 thymomas. These can present as focal cystic areas of
necrosis and infarction that may or may not be surrounded by a thin
residual layer of epithelial cells
Fig. 2.71 Another area of necrosis and infarction in mixed lymphoepithelial
thymoma shows a central area of necrosis with abundant karyorrhectic
material mostly composed of dead small lymphocytes. Notice
the absence of an epithelial lining and the dense connective tissue and
chronic infl ammation surrounding the area of infarction. Areas such as
this can raise the suggestion of a more aggressive behavior in these
tumors; however, prognosis is not affected by this feature
Fig. 2.72 Along with the areas of hemorrhage, necrosis, and infarction,
these tumors can also show dilatation of residual non-neoplastic
thymic epithelium; this may lead to a multiloculated cystic mass that
may closely resemble an acquired multilocular thymic cyst. Careful and
extensive sampling may be necessary to identify the neoplastic component in such lesions
Fig. 2.73 Gross appearance of mixed lymphoepithelial thymoma
(WHO type B2) with cystic and hemorrhagic changes and areas of
necrosis. The tumor shows a relatively well-circumscribed and encapsulated
mass with large areas of cystic degeneration and foci of hemorrhage and necrosis
Fig. 2.74 Gross appearance of a mixed lymphoepithelial thymoma
(WHO B2) invading the lung and pleura. All types of thymoma have the
capability for invasion and metastases. The majority of invasive thymomas
involve surrounding structures, including the pleura, pericardium,
lung, and great vessels. The modified Masaoka staging proposed by
Koga and colleagues is the most commonly used system for the staging
of these tumors
Fig. 2.75 Extensive infiltration of the pleura is seen by mixed lymphoepithelial
thymoma (WHO type B2). The pleura is thickened and irregularly infiltrated
by lobules of type B2 thymoma
Fig. 2.76 Distant metastases in thymoma are rare but can occur with
all histologic types. The fi gure illustrates a metastasis to the kidney in a
53-year-old woman with a mixed lymphoepithelial (type B2) thymoma.
The most common sites of metastases for these tumors, however, are
the lungs and mediastinal lymph nodes. “Drop metastases” or implants
in the pleura and diaphragm are also frequently encountered
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