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An 18-month-old neutered male Dobermann/German Shepherd Dog-cross presented with a well-circumscribed, alopecic, pink mass, approximately 2 cm in diameter, located on the right shoulder. The mass was first noticed five days previously. Fine needle aspiration of both masses is performed and smears are prepared (Wright’s, ×100).
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What is your diagnosis?
The young age of this animal, the rapid appearance, anterior location and cytological appearance are compatible with a cutaneous histiocytoma, a benign dermal tumour of dogs.
The cells are round, discrete cells with well-defined borders.
They exhibit moderate anisocytosis and anisokaryosis.
Discrete cytoplasmic vacuolation and occasional multinucleation may be noted (not shown).
Nuclei are round to ovoid.
Nucleoli may vary from inconspicuous to prominent, and may be single or multiple.
Lymphocytic infiltration is commonly observed but was not observed in this case.
This tumour generally regresses spontaneously within a few months such that no therapy is needed unless the mass causes itching or bleeding and discomfort to the animal, or if it does not appear to regress within the anticipated time following observation.
A differential diagnosis would include a poorly differentiated mast cell tumour, which may have a similar cytological appearance, especially if a modified Wright’s stain or Diff-Quik is used – these may not stain the small number of cytoplasmic metachromatic granules present in poorly differentiated mast cells.
However, close examination of a poorly differentiated mast cell tumour specimen, with Wright’s or Giemsa staining, will reveal granules in at least a small number of cells.
In addition, the cytoplasm will have a different texture, appearing finely granular and more basophilic regardless of whether a modified Wright’s or Giemsa stain is used.
Note: Lymphocytes in a regressing tumour and/or presence of neutrophils and bacteria in an ulcerated histiocytoma may sometimes outnumber tumour cells. In these cases it may be difficult to determine if the aspirate represents chronic and active inflammation or pyogranulomatous inflammation, or an inflamed and/or regressing histiocytoma.
Histiocytomas with more atypical features are uncommon, but they may be difficult to differentiate from systemic histiocytosis, which may also have cutaneous lesions; these often occur in young dogs.
