Page 145 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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Keloidal fibroma and fibrosarcoma
VetBooks.ir Tumours of mesenchymal origin characterized by deposition of a large amount of hyalinized
collagen.

Clinical features
• Very uncommon skin neoplasms in dogs. Not reported in cats, except for a single
case of vaccine-associated fibrosarcoma with keloidal differentiation.
• Male dogs appear over-represented.
• Masses may develop at any site.
• They can be spontaneous or secondary to trauma or inflammation.
• Most tumours present as single, rarely multiple, nodular or plaque-like lesions. They
are often poorly demarcated and arise in the dermis and/or subcutis.
• The vast majority of keloidal tumours are benign (keloidal fibroma). However, keloi-
dal fibroma may undergo malignant transformation into keloidal fibrosarcoma. The
differentiation between these two forms is based on histological findings.
• Over-represented canine breeds: shorthaired dogs, including Rhodesian Ridgeback.

Cytological features

• Cellularity is variable, from low to high.
• Background: clear and variably haemodiluted.
• Presence of numerous elongated fragments of pink, glassy bundles of collagen fibrils.
• Aspirates yield variable numbers of mesenchymal cells that are slender, spindle shaped,
often arranged in large aggregates and embedded in collagenous material.
• Nuclei are oval, with finely stippled to lacy chromatin. Nuclear features of atypia may be
observed in malignant forms.
• The cytoplasm forms one or two tails with poorly demarcated margins. It is variably baso-
philic and occasionally contains pink granules.
• Mast cells, macrophages/histiocytes and eosinophils may be occasionally seen.

Differential diagnoses
• Mast cell tumour with numerous collagen fibrils
• Collagenous hamartoma
• Reactive fibroplasia/scar tissue
• Soft tissue sarcoma of other origin

Pearls and Pitfalls
Hyalinized collagen in keloidal fibroma/sarcoma is bright magenta, glassy, lacking of distinct
linear fibrils, and is more striking than the amorphous, wispy, eosinophilic to magenta matrix
frequently associated with other soft tissue sarcomas. In mast cell tumours, the collagen flame
figures are less eosinophilic and more fibrillar.