myxoid tumor. In myxoid tumors, the spindled cells often have
a somewhat more random arrangement rather than in tumors
with a more collagenous or mixed background. However, the
tumor cells and the wiry collagen seen in myxoid cases are
identical to those seen in ordinar y spindle cell lipomas.
Ironically, the presence of adipocytes was the least
helpful feature. In the low-fat tumors, they were scattered
single adipocytes or very small clusters of adipocytes.
Admittedly, it is possible that additional sampling could have
detected rare adipocytes in the fat-free tumors, but these
tumors appear to have been adequately sampled, with at least
one section submitted per centimeter of tumor.
In selected cases, immunohistochemical stains may be
helpful. The spindle cell component of spindle cell lipomas
shows strong immunoreactivity for CD34.
This needs to be
interpreted in the appropriate histologic context, however, as
expression of CD34 is relatively nonspeciﬁc and occurs in
a variety of mesenchymal neoplasms.
The potential diagnostic difﬁculty caused by the low-fat
and fat-free spindle cell lipoma is highlighted by the broad
differential diagnosis considered by contributing pathologists,
dermatologists, and dermatopathologists. Although most cases
were referred with a suggested benign diagnosis, the diagnosis
of spindle cell lipoma was considered in just three cases.
The most common benign entities in the differential diagnosis
were benign nerve sheath tumors including neuroﬁbroma.
Neuroﬁbroma is composed of spindled cells with comma-
shaped nuclei and the stromal collagen is more delicate in
neuroﬁbroma. Although some admixed CD34-positive cells
are frequently seen in neuroﬁbroma,
ical stains for S100 protein highlight the majority of the
Superﬁcial angiomyxoma (cutaneous myxoma) was also
considered in the differential diagnosis of myxoid fat-free
spindle cell lipomas. The tumor cells in superﬁcial angiomyx-
oma are always randomly arranged and have a spindled to
Superﬁcial angiomyxoma lacks ropey
collagen, and the vasculature is typically more delicate. The
presence of scattered neutrophils and/or entrapped epithelial
structures may be helpful clues in the diagnosis of superﬁcial
In the more collagenous tumors, solitar y ﬁbrous tumor
was also a consideration. Solitary ﬁbrous tumor most
frequently presents as a pleural mass, but rare cutaneous
lesions have been reported.
Solitary ﬁbrous tumors may
rarely have associated adipocytes, which could further cloud
In this series, two of the cases had focal areas of
mat-like collagen similar to solitary ﬁbrous tumor. However,
these cases also had zones with a more organized growth
pattern with nuclear palisading rather than the patternless
pattern of solitary ﬁbrous tumor. Admittedly, the distinction
could be very difﬁcult if not impossible in selected cases, and
some authors have suggested that spindle cell lipoma and
solitary ﬁbrous tumors are part of a related group of CD34-
positive spindle cell tumors.
We believe that distinction is
possible in the majority of cases given the clinical presentation
and presence in areas of typical features of spindle cell lipoma.
A low-grade sarcoma, speciﬁcally low-grade myxoﬁ-
brosarcoma and low-grade ﬁbromyxoid sarcoma, was
considered in the differential diagnosis in a few cases.
Although both of these tumors may present as superﬁcial
lesions, neither typically involves the upper trunk or head and
Low-grade myxoﬁbrosarcoma tends to be
purely myxoid and has more diffuse nuclear atypia. Low-grade
ﬁbromyxoid sarcoma has a variably myxoid to collagenous
stroma and often has associated collagen rosettes.
Superﬁcial low-grade ﬁbromyxoid sarcoma usually presents
in younger patients, most frequently on the extremities.
low-grade ﬁbromyxoid sarcoma and myxoﬁbrosarcoma
usually lack strong CD34 expression, but CD34-positive cells
may be seen in these entities.
In summar y, low-fat and fat-free spindle cell lipoma are
uncommon and frequently mistaken for other entities. Cases
such as these underscore an important but easily forgotten
principle in the diagnosis of adipocytic neoplasms—that the
key to their diagnosis lies in the nonlipogenic, rather than
lipogenic, component. Examples of this include the arborizing
capillary network of myxoid liposarcoma, the irregular ﬁbrous
septa containing enlarged hyperchromatic cells in atypical
lipomatous tumor/well-differentiated liposarcoma, and, as we
have emphasized here, the parallel arrays of bland spindled
cells and stromal background in spindle cell lipoma. Careful
attention to these histologic features and clinical correlation
should allow for the distinction of rare fat-free spindle cell
lipomas from their various benign and malignant morphologic
We thank Dr. Sharon W. Weiss for facilitating the study of
cases from the Emory University Soft Tissue Consultation
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Am J Dermatopathol
Volume 29, Number 5, October 2007 Low-Fat and Fat-Free Spindle Cell Lipoma