Lymphocyte-Rich Epithelioid Thymoma (WHO Type B1), [Pathology] Atlas of Lymphocyte-Rich Epithelioid Thymoma (WHO Type B1)
Fig. 2.52 The g ross macroscopic appearance of lymphocyte-rich thymoma
(WHO type B1) shows a well-circumscribed and encapsulated
mass with a gray-white, fleshy, and lobulated cut surface
Fig. 2.53 The histologic appearance of a lymphocyte-rich thymoma
(WHO type B1) is characterized by the abundance of lymphocytes that
may resemble a lymphoma on cursory examination. The WHO
classification separates B-type thymomas into B1, B2, and B3, based on their
decreasing number of lymphocytes and increase in cytologic atypia of
the epithelial cells. B1 thymoma is characterized by the highest number
of lymphocytes and has the least atypia in the epithelial cells
Fig. 2.54 The lymphocyte-rich thymoma (WHO type B1) also is characterized
by lobulation, with broad bands of connective tissue separating the lobules;
this represents one of the main organotypical features of
these tumors, which recapitulates the organotypical architecture of the
mature functional thymus
Fig. 2.55 Another distinctive organotypical feature observed in
lymphocyte- rich thymomas is the presence of dilated perivascular
spaces. Notice the empty dilated spaces, which may contain a few scattered
small lymphocytes and usually harbor a small vessel in the center
Fig. 2.56 Lymphocyte-rich thymoma (WHO type B1) may also show
prominent areas of “medullary differentiation” composed of rounded
areas that appear lighter or paler on scanning magnifi cation because of
the decrease in the number of lymphocytes admixed with the thymic
epithelial cells. This is regarded as another one of the distinctive organotypical f
eatures of thymomas because it presumably recapitulates the
medullary portions in a normal mature thymus
Fig. 2.57 Higher magnification in lymphocyte-rich thymoma (WHO
type B1) shows a few scattered large epithelial cells characterized by
large vesicular nuclei with prominent eosinophilic nucleoli surrounded by
a rim of amphophilic or lightly eosinophilic cytoplasm (center). Contrast
this with the abundance of small T lymphocytes in the background
Fig. 2.58 Another feature frequently seen in lymphocyte-rich thymomas
(WHO type B1) is the presence of a prominent “starry-sky” appearance
characterized by numerous tingible-body macrophages. When this
appearance is present, it may raise the question of the differential diagnosis
with lymphoblastic lymphoma, which frequently displays the same feature
Fig. 2.59 Lymphocyte-rich thymoma (WHO type B1) can also show a
striking microcystic appearance owing to a proliferation of multiple
small abortive perivascular spaces. This differs from the starry-sky
appearance because of the lack of tingible-body macrophages. Instead,
the small empty vacuoles contain a single small lymphocyte
Fig. 2.60 Immunohistochemical staining for pan-cytokeratin in
lymphocyte-rich thymoma highlights a network of scattered thymic
epithelial cells (which correspond to the neoplastic cells in the tumor)
against a sea of keratin-negative lymphocytes. This is the most important feature to
distinguish these tumors from lymphoblastic lymphomas, since the lymphocytes
in the background will mark as CD1a/TdT/
CD99+, a phenotype that is shared with lymphoblastic lymphoma
Fig. 2.61 An unusual histologic appearance that can sometimes be
observed in lymphocyte-rich thymomas (WHO type B1) constitutes
areas that closely resemble the normal thymic medulla (including the
presence of Hassall’s corpuscles) admixed with areas that recapitulate
the normal cortex. Such tumors were designated as “organoid” thymoma or
“predominantly cortical” thymoma in older classification
schemas
Fig. 2.62 Higher magnification of “organoid” thymoma shows a small,
well-formed Hassall’s corpuscle in a medullary-like area surrounded by
a lymphocyte-rich population. This is actually the type of thymoma that
resembles the most normal active thymus of adolescence and thus can
be regarded as the most differentiated type of thymoma
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This is only a part of the book : Atlas of Mediastinal Pathology of authors: Saul Suster. If you want to view the full content of the book and support author. Please buy it here: https://www.amazon.com/Atlas-Mediastinal-Pathology-Anatomic/dp/149392673X
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