[Pathology] Atlas of Atypical Thymoma, Epithelioid, and Spindle-Cell Type (WHO Type B3), Atypical Thymoma, Atlas of Mediastinal Pathology
Fig. 2.81 Gross appearance of an atypical (WHO type B3) thymoma.
These tumors represent a more advanced stage in the malignant progression
of thymoma and are characterized by being more invasive than
the other types and by earlier and multiple recurrences
Fig. 2.82 Atypical thymoma (WHO type B3) is characterized histologically
by its sheetlike growth pattern composed of round to polygonal epithelial
cells with very scant intervening lymphocytes. Synonyms for this tumor
in the past included epithelial-rich thymoma, polygonal cell thymoma,
clear cell thymoma, and well-differentiated thymic carcinoma. T
he last term, in particular, is no longer regarded as a synonym
for this tumor, and its use should be discouraged to avoid confusion
with the entity of thymic carcinoma
Fig. 2.83 Atypical thymoma (WHO type B3) of the epithelioid type is
characterized by sheets of round to polygonal cells with abundant
cytoplasm and sharp cell borders. Organotypical features of thymic
differentiation can still be appreciated in these tumors, including low-power
lobulation, frequent dilated perivascular spaces, and a sprinkling of
small T lymphocytes
Fig. 2.84 In some cases, the cells in atypical thymoma (WHO type B3)
may adopt a squamoid appearance and display a pavement-like architecture
closely reminiscent of immature squamous epithelium. Abortive
foci of keratinization can be seen occasionally in these tumors but
should not be a dominant feature
Fig. 2.85 Higher magnification from an area showing abortive squamous
differentiation in atypical thymoma (center fi eld). Care must be
taken to avoid misinterpreting these areas as evidence supporting a
diagnosis of squamous cell carcinoma
Fig. 2.86 On higher magnification, atypical thymoma (WHO type B3)
can display great variability in cell size, ranging from very small round
to oval cells to large, atypical cells with marked nuclear pleomorphism.
The overriding characteristics, however, are the increase in nuclear-tocytoplasmic
ratio, increased chromatin pattern (compared with B1 and
B2 thymoma), frequent appearance of nucleoli, and the abundance of
densely eosinophilic cytoplasm with sharply defined cell borders
Fig. 2.87 The cytology of atypical thymoma (WHO type B3) can vary
and show different features from case to case. Some of these tumors can
display (a) marked cytologic atypia, with a dense chromatin pattern and
prominent nucleoli; (b) a “raisinoid” appearance of the nuclei, with
crinkled nuclei showing irregular contours; (c) longitudinal (“coffeebean”) nuclei;
and (d) scattered mitotic figures. Although it is claimed by the WHO that type B3
thymoma shows cells that are less atypicalthan those of type B2 thymoma, in
reality the contrary applies, and the cells in type B3 thymoma display
more pronounced cytologic atypia than those in types B1 and B2 thymomas;
this is demonstrated by increased chromatin pattern, high nuclear-to-cytoplasmic
ratios, and scattered mitoses
Fig. 2.88 Atypical thymomas are more often invasive at the time of
initial diagnosis than other types of thymoma. Pleural and lung involvement is
frequently identifi ed in these tumors. The fi gure shows invasion
through the capsule into perithymic fat
Fig. 2.89 Thymomas showing admixtures between WHO types B1
and B2 and WHO type B3 thymoma are commonly encountered.
Transitions can be gradual or abrupt, as in the illustrated case. Atypical
thymoma can also give rise to thymic carcinoma; extensive sampling is
therefore of importance when examining these tumors
Fig. 2.90 Atypical thymoma (WHO type B3) can also be composed of
atypical oval to spindle cells that display a sheetlike growth pattern and
contain abundant eosinophilic cytoplasm. On cursory examination, the
tumors may superficially resemble spindle-cell thymoma (WHO type
A), but on higher magnifi cation, the cells evidence more pronounced
cytologic atypia
Fig. 2.91 Higher magnification from atypical thymoma (WHO type
B3), spindle-cell type, shows an atypical spindle-cell population displaying
large oval to spindle nuclei with increased chromatin pattern
and prominent nucleoli. Scattered mitoses can also be seen. The presence of
nucleoli in any spindle-cell thymoma should raise the possibility of an
atypical spindle-cell thymoma
Fig. 2.92 A closer view of the cytology in atypical thymoma, spindlecell
type (WHO type B3), shows enlarged oval to spindle nuclei with a
dense chromatin pattern and prominent nucleoli. The presence of
prominent nucleoli serves to distinguish these tumors from conventional
spindle-cell thymomas (WHO type A)
Fig. 2.93 Gross appearance of atypical thymoma (WHO type B3)
invading the rib cage. The infi ltrative nature of these tumors often
makes it diffi cult to achieve a complete, margin-free resection, which
accounts for the higher incidence of recurrences
Fig. 2.94 Atypical thymomas can rarely metastasize outside the chest
cavity, usually only in very advanced cases. The section illustrates a
metastasis of an atypical thymoma to the liver in a 56-year-old man
with an invasive atypical thymoma
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