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Mesenchymal Tumours and Other Neoplasms
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9.2 Histiocytic Tumours
VetBooks.ir Histiocytic sarcoma (HS)

Malignant tumour of histiocytic origin deriving from interstitial dendritic cells (DCs).

Clinical features
• Most commonly described in dogs, very rare in cats.
• Localized forms are reported in several organs, including the skin.
• The disease is considered disseminated when it spreads beyond the local draining
lymph node and involves distant sites. This disease was previously known as malig-
nant histiocytosis.
• Solitary or multiple, variably sized, cutaneous or subcutaneous masses. In dogs,
most tumour masses are located on extremities, especially in the periarticular
regions (e.g. elbow and stifle); in cats, they have been reported on the extremities
and abdomen.
• Localized histiocytic sarcoma has a better prognosis than the disseminated form, if treat-
ed early with wide surgical excision. The prognosis of the disseminated form is poor.
• Over-represented canine breeds: Bernese Mountain Dog, Rottweiler, Golden
Retriever, Flat-coated Retriever and Miniature Schnauzer.

Cytological features
• Cellularity is variable, often high.
• Background: clear to lightly basophilic and variably haemodiluted.
• Neoplastic cells are pleomorphic. They are usually round to oval, slightly elongated and
occasionally with small cytoplasmic tails. They exfoliate individually.
• Nuclei are round to oval, often large, with coarse, granular or clumped chromatin. They are
eccentric to paracentral. Multiple prominent and irregularly shaped nucleoli are frequently
seen.
• The cytoplasm is lightly or occasionally moderately basophilic, and moderate in amount. It
may contain clear intracytoplasmic vacuoles or small phagosomes.
• Neoplastic cells can display erythrophagocytosis.
• Cytological features of atypia are often but not always prominent. Anisocytosis and anisokar-
yosis are variable to marked and the N:C ratio is variable. Binucleation and/or multinuclea-
tion and frequent mitotic figures are often seen. In some cases, the pleomorphism is less
prominent and anisokaryosis and anisocytosis are modest.