PART TWO
Diagnostic Cytology
Fig. 18.1 Hibernoma. (A) Histologic section. Adipocytes with
multivacuolated cytoplasm (H&E x LP). (B) Smear. Round cells with small
cytoplasmic vacuoles and small, round nuclei and delicate branching
capillaries present (H&E x MP).
The most common sites include deep soft tissue of the extremi-
ties, retroperitoneum, paratesticular area, mediastinum, and
subcutaneous tissue. These lesions range from small to very
large in size and are usually asymptomatic. Histologically, they
can be subdivided into lipoma-like, sclerosing, inflammatory,
and spindle cell. The most common type is the lipoma-like vari-
ant, which is characterized by a proliferation of adipocytes with
varying cell sizes. Histological sections show adipocytes and
stromal cells with focal nuclear atypia associated with hyper-
chromasia. Mono- or multivacuolated lipoblasts might be also
seen. FNAB smears usually show the presence of lipoblasts scat-
tered throughout the aspirate smears with occasional large cells
containing multilobulated nuclei.33,34 It is very difficult for FNAB
to distinguish subcutaneous fat, lipoma, or well-differentiated
liposarcoma, as all of these lesions may contain areas of rela-
tively normal-appearing adipose tissue. For these reasons, deep-
seated soft tissue lesions that appear predominantly fatty are
probably best evaluated by incisional or excisional biopsy.
Sclerosing well-differentiated liposarcoma is more com-
monly seen in the retroperitoneum and paratesticular area. It is
characterized by the presence of scattered bizarre stromal cells
with hyperchromatic nuclei associated with multivacuolated
lipoblasts. The cells are present in a dense collagenous stroma.
These tumors are characterized cytologically by lipoblasts, atyp-
ical fibroblasts, multinucleated cells, adipocytes, and delicate,
dispersed collagen.34,35
Inflammatory well-differentiated liposarcoma is rare and
occurs most often in the retroperitoneum. At a histological
level, it is characterized by an inflammatory infiltrate associated
with an atypical adipocytic cell population and bizarre multi-
nucleated stromal cells. The inflammatory infiltrate presents
as lymphoplasmacytic aggregates. The FNAB of inflammatory
well-differentiated liposarcoma shows numerous small lym-
phoid cells and plasma cells, admixed with scattered atypical
cells. These atypical cells display multiple or hyperlobated
nuclei with coarse chromatin and abundant ill-defined cyto-
plasm.36
Key features of atypical lipomatous tumor/well
differentiated liposarcoma
• Scattered lipoblasts;
• Occasional large cells containing multilobulated
nuclei with coarse chromatin and abundant
ill-defined cytoplasm;
• Scattered bizarre stromal cells with hyperchromatic
nuclei (sclerosing well-differentiated liposarcoma);
and
• Dispersed inflammatory cells, predominantly
lymphoid cells and plasma cells (inflammatory
well-differentiated liposarcoma).
Dedifferentiated Liposarcoma
Dedifferentiated liposarcoma is defined as the transition from
a well-differentiated liposarcoma to non-lipogenic sarcoma of
variable histological grade, usually measuring at least several
millimeters in diameter. The dedifferentiation occurs in 10%
of well-differentiated liposarcomas, a risk apparently higher in
deep-seated lesions. There is no sex predilection. It is much more
common to arise
de novo
than as a recurrence. The most com-
mon site is the retroperitoneum followed by the extremities. It
is extremely rare in subcutaneous tissue. The clinical presenta-
tion is usually of a large painless mass, which radiologically has
a fatty and non-fatty component. Histologically, it is character-
ized by the presence of a sarcoma, usually high grade, in close
association with a well-differentiated liposarcoma component.
The most common histological appearances of the non-lipo-
genic sarcoma are a pleomorphic sarcoma and intermediate to
high-grade myxofibrosarcoma. There are also cases of low-grade
dedifferentiation characterized by a uniform population of spin-
dle cells with mild nuclear atypia as well as lesions with heterolo-
gous differentiation such as muscle or chondroid differentiation.
The FNAB of dedifferentiated liposarcoma shows a polymorphous
population of round or spindle cells, including multinucleated
forms, with occasional lipoblasts. Myxoid matrix and arboriz-
ing vascular structures are rarely seen.33,37 Therefore, the clue for
the correct diagnosis resides in the identification of lipoblasts,
which can be very difficult. Dedifferentiated liposarcomas have
the same cytogenetic abnormalities as seen in well-differentiated
liposarcomas, such as ring or giant marker chromosomes.38,39
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