18
cytology of Soft Tissue, Bone, and Skin
from lipid accumulation. The cytoplasmic vacuoles must be clear
and sharply delimited and bulge against or distort the nucleus
in order to be recognized as lipidic. Adipocytic tumors represent
the largest group of mesenchymal tumors and liposarcomas rep-
resent the single most common type of soft tissue sarcoma.20
Lipoma
Lipomas usually present between the ages of 40 and 60 years
and are rare in children. The most common sites are the subcu-
taneous tissue and deep soft tissues. They usually present as a
painless soft tissue mass, except when they compress peripheral
nerves. Imaging studies show a homogeneous soft tissue that
demonstrates fat saturation. Conventional lipomas are charac-
terized histologically by lobules of mature adipocytes. FNAB of
lipoma shows the presence of clusters of adipose tissue com-
posed of bland adipocytes, without associated "chicken-wire"
vascular network or myxoid changes. The cells are cytologically
indistinguishable from the surrounding adipose tissue. Clini-
coradiological correlation is required to make the diagnosis of
lipoma and rule out a sampling miss.
Key features of lipoma
• Bland adipocytes; and
• No "chicken-wire" vascular network or myxoid
changes.
Lipoblastoma
Lipoblastomas are localized or diffuse tumors resembling fetal
adipose tissue. They are most common in the first three years of
life and have a male predilection. They are commonly located
in the extremities, but they can be seen in other locations such
as mediastinum and retroperitoneum. The lesions present as
slowly growing masses involving subcutaneous tissue or infil-
trating muscle. Histologically, lipoblastoma is composed of
lobules separated by connective tissue, which are composed
of an admixture of mature adipocytes and lipoblasts. Myxoid
stroma and plexiform vascular network are frequently seen in
this tumor. FNAB of lipoblastoma shows adipocytes, lipoblasts,
and spindle cells in various proportions and arranged in clusters
or single cells. A vascular network is noted inside the clusters.
Abundant myxoid extracellular material and naked oval nuclei
can be seen.21,22 The main differential diagnosis is liposarcoma,
which shows nuclear atypia, a feature not seen in lipoblastoma.
Key features of lipoblastoma
• Presence of both mature adipocytes and lipoblasts;
• Myxoid stroma and plexiform vascular network; and
• No atypia.
Spindle and Pleomorphic Lipoma
Spindle
cell
and
pleomorphic lipomas
are
subcutaneous
lesions, which occur mainly in the head and neck region of
male patients. Most tumors are asymptomatic and occur in the
sixth decade of life. They have a benign behavior and can be
conservatively excised. Spindle cell lipomas are characterized by
spindle cells arranged in parallel clusters between the adipocytes
and associated with thick collagen bundles. Mast cells are fre-
quently noted in between the spindle cells. The cytological find-
ings of spindle cell lipomas include a population of adipocytes,
spindle cells, and collagen bundles. The spindle cells have mild
nuclear enlargement, focal nuclear irregularities, rare intra-
nuclear inclusions, and occasional small nucleoli. No lipoblasts
are present.23-25 Pleomorphic lipomas are clinically similar to
spindle cell lipoma. Pleomorphic lipoma is a term that should
be used only in tumors located in the back, neck, or shoulders.
They are characterized by the presence of a variable number of
cells with enlarged nuclei, including floret cells, in close associa-
tion with adipocytes and collagen bundles. The FNAB shows a
population of floret cells and pleomorphic cells admixed with
adipocytes and spindle cells. The floret cells show multiple
enlarged hyperchromatic nuclei arranged in a circle or semicir-
cle.26-29 No atypical lipoblasts or capillary network is present.
The diagnosis must be given with care and correlated with
the clinical presentation because pleomorphic lipomas can be
misdiagnosed as an anaplastic carcinoma or liposarcoma.
Key features of spindle cell lipoma
• A population of adipocytes, spindle cells, and collagen
bundles;
• Spindle cells with mild nuclear enlargement, focal
nuclear irregularities, and rare intranuclear inclusions;
and
• No lipoblasts.
Key features of pleomorphic lipoma
• Floret cells and pleomorphic cells admixed with
adipocytes and spindle cells;
• Floret cells with multiple enlarged hyperchromatic
nuclei arranged in a circle or semicircle; and
• No atypical lipoblasts or capillary network.
Hibernoma
Hibernoma is a benign tumor composed of brown fat cells
with granular, multivacuolated cytoplasm. It is more common
around the third and fourth decade of life and there is slight
male predominance. The more frequent sites of involvement
are the subcutaneous tissue in the thigh, trunk, upper extrem-
ity, and head and neck. Hibernomas present as brown fat cells
with pale to variably eosinophilic cytoplasm (Fig. 18.1A). Multi-
vacuolated lipoblast-like cells can be seen, but mitotic figures
or atypia are unusual. Hibernomas are near-diploid or pseudo-
diploid at a genetic level and are associated with involvement
of 11q13 in structural rearrangements.20 The FNAB findings
included round, brown, fat-like cells with uniform, small cyto-
plasmic vacuoles and small, round nuclei. Delicate branching
capillaries are usually seen (Fig. 18.1B).30-32
Key features of hibernomas
• Round cells with small cytoplasmic vacuoles and
small, round nuclei; and
• Delicate branching capillaries present.
Atypical Lipomatous tumor/Well-Differentiated
Liposarcoma
Atypical lipomatous tumors have been recognized by the 2002
WHOclassificationassynonymouswithwell-differentiatedliposa-
rcoma. The prognosis is related to the resectability of the lesion.
They represent 40% of all liposarcomas with a peak incidence in
the sixth decade of life and they equally affect men and women.
473
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