ly m p h o id tissue fragm ents c on tain in g a scaffold o f arb orizing
sm all vessels. C ytologically, the fo llic u la r d end ritic cells are
relative ly b la n d -lo o kin g and bear o vo id nuclei, pale chrom atin,
sm all so lita ry n uc le oli, syncytial cytoplasm , and ill-d e fin e d cell
borders. Som etim es, occasional fo llic u la r d end ritic cells m ay
appear atypical and can be m istaken as Reed-Sternberg cells
o n cytologic e xa m in a tio n .37,38 T in g ib le-b o d y macrophages are
rarely id entified . M arked ly p leo m o rp hic cells w ith h ig h m ito tic
activity are n o t seen. C ell b loc k sections m ay reveal the lam el-
lated arrangem ent o f lym phocytes surro un d in g atretic follicles.
T he in te rfo llic u la r areas contain ab und ant sm all vessels.
A lth o u g h the features seen in cytologic preparations are re m i-
niscent o f those seen in histolog ic specimens, a d e fin itive diag-
nosis o f Castlem an's disease is u su ally n o t possible on the basis
o f fine-needle asp iration cytology alone.39 Som etim es, cytologic
d istin ctio n fro m o the r fo llic u la r d endritic cell-rich lesions, such
as ang ioim m u no b la stic T-cell lym p h o m a and fo llic u la r d endritic
cell sarcoma, can be d ifficu lt. In general, fo llic u la r d endritic cell
sarcom a tends to show hig her d end ritic c e llu la rity and m ore dis-
cernible cytologic atypia. A ng io im m u n o b la stic T-cell lym p h om a,
o n the o the r hand, does n o t present as a m ed iastinal mass.
Extramedullary Hemopoiesis
E xtram ed ullary hem opoiesis is a com pensatory p h en om en on
th a t occurs w h e n the n o rm a l fu n c tio n o f the bone m a rro w
is disturbed. It is m ost often seen in patients w ith hem ato -
logic disorders. A lth o u g h the organs m ost fre q u e n tly affected
are spleen, liver, and lym p h nodes, o the r sites, inc lu d in g the
p o sterior m ed iastinum , m ay be in vo lve d.5,40-42 C ytolog ic prepa-
rations are cellular and com posed o f h em op o ie tic elem ents w ith
e ryth ro id hyperplasia. Megakaryocytes are also seen. A correct
cytologic diagnosis can help to guide fu rth e r clinical m anage-
m e n t and avoid unnecessary surgical in te rve n tio n , especially in
asym ptom atic patients.
Neoplastic Lesions
T h y m o m a is a rarely encountered neoplasm located in the
a n te rio r m ed iastinu m and differentiates tow ard th ym ic epi-
th e lia l cells. The m a lig n a n t p o ten tial varies fro m n il to lo w o r
interm ed iate grades. T h y m o m a occurs in a ll age groups and
peaks in the sixth decade. It is extrem ely u n c o m m o n in c h il-
dren and adolescents. There is no sex p red ilection id entified .
Between 10 and 80% o f patients w ith th y m o m a have sym p-
tom s o f m yasthenia gravis. Paraneoplastic pure red cell apla-
sia, hyp o g a m m ag lob u lin em ia (G ood's synd rom e), and o the r
a u to im m u n e diseases have also been reported.
Tra d itio na lly, histologic classification o f th ym o m a is confus-
ing. Several classification systems based o n either h istolog y o r
histogenesis have been proposed (Table 26.2) .43-47 According to
the updated W o rld H ealth O rganization classification, th ym o m a
is categorized histolog ically in to tw o m a jo r types: th ym o m a w ith
spindle b lan d -lo oking neoplastic ep ithelial cells (type A ) and th y -
m om a w ith rou nd o r polygonal neoplastic ep ithelial cells (type
B ) 47 Type B th ym o m a is fu rth e r subdivided in to B1, B2, and B3,
depending on the extent o f the lym p hocytic in filtra te and the
degree o f atypia o f the neoplastic ep ithelial cells. T h ym o m a w ith a
m ixtu re o f type A and type B features is classified as type AB. Type
AB (synonym : m ixed th ym o m a ) and type B2 (synonym : cortical
th ym o m a ) thym om as represent the tw o com m onest subtypes.
T h y m o m a shows a w id e range o f m o rp h o lo g y in fine-needle
aspirates. T he key diagnostic feature is the presence o f dual
p o p ulatio ns o f e p ith elial cells and lym phocytes in various p ro -
p ortions. Precisely, the cytologic appearance depends on the
relative p ro p o rtio n o f (1) e p ith elial versus ly m p h o id cells and
(2) m e d u lla ry versus cortical com ponents. W h ile tu m o rs com -
posed o f dual p o p ulatio ns o f e p ith elial and ly m p h o id cells are
read ily recognizable, th y m o m a w ith a predom inance o f one cell
type m ay be m isdiagnosed.
In general, the cytologic in te rp re ta tio n o f th ym o m a , espe-
c ia lly w h en coupled w ith rad iolog ic findings, cell b loc k exam i-
n a tio n , and im m unocytochem istry, can achieve hig h diagnostic
accuracy.5,29,39,48-55 L im ita tio n s o f fine-needle asp iration cytology,
however, includ e in a b ility in detailed subtyping o f th ym o m a
according to the W o rld H e alth O rg anization classification, as
w e ll as in d eterm in in g capsular invasion.
Thym om a with Predom inance of M edullary
Com ponent
For th y m o m a consisting p re d o m in a n tly o f a m e d u lla ry com -
p o ne nt (such as type A th y m o m a [synonym s: spindle cell
th ym o m a , m e d u lla ry th y m o m a ]), fine-needle asp iration often
yields cohesive sheets o f b la n d -lo o kin g and m ito tic a lly inactive
Table 26.2
Comparison of Different Histologic Classifications of Thymic Epithelial Tumorsa
^ Thymoma w ith predominance of m edullary com ponent
Thymoma w ith predominance of cortical com ponent ^
Updated World
Health Organiza-
tion classification
Type A thymoma
Type AB thymoma
Type B1 thymoma
Type B2 thymoma
Type B3 thymoma
Thymic carcinoma
Mixed thymoma
Cortical thymoma
Thymic carcinoma
cortical thymoma
thymic carcinoma
spindle cell
Thymic carcinoma
mixed thymoma
mixed thymoma
aSome of these categories are not directly comparable, and exceptions may occur. For example, rare cases of type A thymoma are composed of polygonal epithelial cells
and are equivalent to predominantly epithelial thymoma according to the Lattes-Bernatz classification. Some cases of type B3 thymoma correspond to the old category of
predominantly spindle cell thymoma.
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