Page 191 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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10.3 Cutaneous Histiocytoma (Dog)

VetBooks.ir Benign tumour of histiocytic origin, deriving from Langherans cells (LCs).

Clinical features
• Common cutaneous neoplasm of the dog, accounting for about 12–14% of all skin
masses. Not reported in cats.
• Age: it mostly occurs in dogs less than 4 years of age, though histiocytoma may pre-
sent in dogs of all ages.
• Variably sized, solitary, dome-shaped, exophytic, often red, cutaneous masses with
complete or partial alopecia and often ulcerated. Often referred to as ‘button’ tumour.
Multiple nodules have been reported, especially in Shar Pei. Rare cases of solitary
cutaneous histiocytoma with lymph node metastasis have been reported and have
variable outcomes.
• Head (pinnae), neck and extremities are the most common locations but histiocyto-
ma can occur in any region of the body.
• Spontaneous regression within a few months is common. Recurrence rate at the surgical
excision site is only rarely observed.
• Over-represented canine breeds: Boxer and Dachshund.

Cytological features
• Cellularity is variable, often high.
• Background: often basophilic and variably haemodiluted.
• Neoplastic cells are round to oval, occasionally with small cytoplasmic projections and vari-
ably distinct cytoplasmic borders. They exfoliate individually.
• Nuclei are round, oval or reniform. They are often indented, paracentral to eccentric and
have finely stippled chromatin. Single or multiple round nucleoli are frequently seen.
• The cytoplasm is moderate in amount, lightly basophilic, often lighter than the surround-
ing background. It may contain rare clear vacuoles and at times it may be wispy and paler
at the outer edge of the cells.
• Cytological pleomorphism is variable to moderate. Anisocytosis and anisokaryosis are mild
to moderate; binucleated cells and mitotic figures can be present.
• Small lymphocytes are often observed in the regressing phase of the tumour and may
become the predominant cell type.

Differential diagnoses
• Large cell lymphoma
• Plasma cell tumour
• Small cell lymphoma (in regressing forms)