Page 147 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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Perivascular wall tumours (PWTs)
VetBooks.ir Group of tumours that arise from cells of the perivascular wall and adventitia.

Clinical features
• Relatively common mesenchymal neoplasms affecting dogs, in particular large breeds,
middle-aged to older subjects. Very rare in cats.
• They belong to the class of soft tissue sarcomas.
• Recent studies have shown that PWTs represent a spectrum of tumours arising from
various cells of the perivascular wall and adventitia, such as pericytes and myopericytes.
These tumours include haemagiopericytoma, myopericytoma, myoma, angioleiomyoma,
angiomyofibroblastoma, angiofibroma and glomus tumours.
• They often appear as solitary lesions, with predilection for limbs and joints. Gross appear-
ance is variable, often rubbery; macroscopically it may be confused for lipoma.
• PWTs are locally infiltrative with a relatively low metastatic risk.
• Good prognosis if associated with early diagnosis, small tumour size (< 5 cm), cutaneous/
subcutaneous localization with no deeper involvement and clean surgical margins.

Cytological features
• Cellularity is variable, generally higher than most mesenchymal tumours.
• Background: clear, often containing variable numbers of red blood cells.
• Cells appear individualized or form bundles adherent to the surface of capillaries. Perivas-
cular or whorling arrangements can be observed. Cells may be associated with pink amor-
phous collagenous stroma.
• Cell morphology varies between wispy, plump-oval, stellate and veiled.
• Nuclei are generally medium sized and round, with granular chromatin and occasionally visible
nucleoli.
• The cytoplasm is moderate in amount, moderately basophilic, often containing distinct,
clear, round intracytoplasmic vacuoles and rarely small eosinophilic granules. Cells margins
are often fringed.
• Cytological features of atypia are variable.
• Binucleated (insect-head cells) and multinucleated elements (crown cells) may be seen and are
considered characteristic.
• Variable numbers of small lymphocytes have been found in approximately 10% of cases.

Differential diagnoses
• Peripheral nerve sheath tumours (PNSTs), including schwannoma
• Soft tissue sarcoma of other origin
• Round cell tumour (including plasma cell tumour and cutaneous histiocytoma)

Pearls and Pitfalls
Crown cells in PWTs are so called because they are characterized by having the nuclei arranged
in a circle at the periphery of the cytoplasm, resembling a crown or wreath.