26
Mediastinum
Fig. 26.3 Reactive mesothelial hyperplasia. (A) Monolayered sheets of benign mesothelial cells are identified. The cells possess ovoid nuclei, fine
chromatin, delicate nuclear membrane, focal nuclear grooving, and intercellular "windows.” Direct smear of fine-needle aspirate (H&E x HP). (B) The cells
express mesothelial markers calretinin (nuclear and cytoplasmic staining patterns) (left) and HBME-1 (membranous staining pattern) (right). Cell block sections
of fine-needle aspirate (immunostains for calretinin and HBME-1 x MP and MP, respectively).
Fig. 26.4 Reactive lymphoid hyperplasia of thymus. A heterogeneous
population of lymphoid cells (including lymphocytes and some follicular
center cells) and small number of tingible-body macrophages (arrows) are
present. Direct smear of fine-needle aspirate (Papanicolaou x HP).
plasm a cells, and fo llic u la r center cells (Fig. 2 6.4) . Som e o f the
fo llic u la r center cells tend to arrange in loose aggregates and
are adm ixed w ith sm all n um b er o f fo llic u la r d end ritic cells and
ting ib le-b od y macrophages.
T in g ib le-b o d y macrophages, w h e n present in fine-needle
aspirates o f a ly m p h o id lesion, are o fte n reassuring, as th ey
are seldom seen in m a lig n a n t con d itio ns (such as fo llic u la r
lym p h o m a ). O n the o th e r hand, the p o ssib ility o f fo llic u la r
lym p h o m a needs to be considered i f the heterogeneous ly m -
p h o id p o p u la tio n contains a d isp ro p o rtio n a lly large n um b er o f
"c e n tro b la st-like " cells, w ith a paucity o f ting ib le-b od y m acro-
phages. As fo r th ym ic hyperplasia o r even enlarged thym us w ith
n o rm a l m icroscopy,32 scanty b la n d -lo o k in g e p ith elial cells m ay
be seen in a ly m p h o id background, cytolog ically m im ic k in g
th ym o m a . Som etim es, Hassall corpuscles m ay be sam pled as
w e ll and m istaken as keratin pearls in kera tin izin g squam ous
cell carcinom a. In cytologic preparations, Hassall corpuscles
appear as tig h tly cohesive, concentric w h o rls o f b la n d -lo o kin g
keratinized e p ith elial cells. It is n o te w o rth y th a t Hassall corpus-
cles are seldom id en tifie d in tru e examples o f th ym o m a .
Acute Mediastinitis and chronic Sclerosing
Mediastinitis
Acute m ed iastinitis o fte n occurs in the p osterior m ed iastinum
and is secondary to traum a, surgery, o r perforated viscera. The
subsequent bacterial in fe c tio n m ay result in suppurative in fla m -
m ation . Fine-needle asp iration yield s abund ant p olym orp hs
and necrotic debris, com p atib le w ith features o f abscess.
O n the o th e r hand, chronic sclerosing m ed iastinitis usually
involves a n te rio r m ed iastinu m and is associated w ith a u to im -
m un e con d itio ns o r a h is to ry o f infections (such as tuberculosis
and histop lasm osis).33 N o t surprisingly, fine-needle aspirates
are often paucicellular, consisting o f lym phocytes, plasm a cells,
and sclerotic fib rous strom a. These cytologic find ing s need to be
interpreted w ith caution, however, as p rim a ry m ed iastinal large
B-cell lym p h o m a and H od g kin's lym p h o m a are c o m m o n ly
associated w ith strom al sclerosis and diagnostic cells m ay n o t
be rea d ily sampled.
Granulomatous Lymphadenitis
Tuberculous lym p h ad e nitis m ay present as a m ed iastinal mass.
Fine-needle asp iration o fte n yields sm all loose aggregates o f
e p ith e lio id histiocytes associated w ith scattered m ultinucleated
g iant cells, necrotic debris, and caseous m aterial in the back-
g round (Fig. 26.5) . The nuclei o f e p ith e lio id histiocytes have
a "s a n d a l-lik e " m o rp h o lo g y and do n o t show nuclear polarity.
The diagnosis o f m ycobacterial in fe c tio n can be confirm ed w ith
d e m o nstra tio n o f acid-fast b a c illi b y Z ie h l-N e e lse n stain per-
fo rm e d o n smears o r cell b loc k sections. T he rod-shaped m yco-
bacterial b a c illi m ay also appear as negative images h ig hlig hted
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