Page 151 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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Lipoma
VetBooks.ir Benign tumour of the adipose tissue.

Clinical features
• Common neoplasm, representing 9% of all skin and subcutaneous tumours in dogs.
Less common in cats with an overall estimated incidence of 3–5%.
• More common in adult/old animals.
• Single or multiple, variable size, soft, often freely moveable, subcutaneous masses,
most commonly observed on the trunk, gluteal region and proximal limbs.
• Generally, it is a slow-growing tumour with good prognosis after surgical excision.
• Two uncommon forms have been described in dogs and cats: infiltrative and inter-
muscular lipomas.
• Infiltrative lipoma: so called as it invades adjacent tissues such as muscle, fascia,
nerve and bone. In spite of the local aggressive growth pattern, it is still con-
sidered a benign tumour.
• Intermuscular lipoma: it usually occurs in the intermuscular area of the thoracic
(axilla) and pelvic (gluteus) limbs of dogs. It is slow growing and can occasionally
cause lameness.
• Both infiltrative and intermuscular lipomas are cytologically similar to conven-
tional lipomas and subcutaneous adipose tissue.
• Their differentiation is based on clinical presentation, imaging and histopathology.

Cytological features
• Cellularity is generally low.
• Background: clear, often containing variably sized clear lipid vacuoles and variably haemodiluted.
• Adipocytes can be seen individually or in aggregates.
• Cells are large, containing a single clear lipid vacuole occupying most of the cytoplasm.
• The nucleus is small, round to oval, hyperchromic and peripherally placed.
• Supporting stromal cells, collagen, capillaries and/or cartilage may be seen occasionally. When
these components are abundant, the terms fibrolipoma, angiolipoma, angiofibrolipoma and
chondrolipoma can be used.
Variants
A new histological variant of lipoma has recently been described:

• Spindle cell lipoma:
• Rare in dogs, not reported in cats.
• It resembles an undifferentiated soft tissue sarcoma; differentiation is based on histo-
pathological, histochemical and immunohistochemical findings.
• Vacuolated spindle-shaped and plump cells are seen and are admixed with collagen,
myxoid matrix and low numbers of mature adipocytes.