10-year-old, castrated male, border collie cross dog (Canis familiaris).One-month history of a mass on the ventral abdomen. The mass had doubled in size over the last few weeks. It was freely moveable within the subcutaneous layers of the skin and measured approximately 3 x 3 cm at the time of removal.

Gross Description:  

Received a 3 x 2.5 cm ellipse of haired skin containing a firm, pale 2 cm diameter subcutaneous mass.

Histopathologic Description:

Haired skin: Beneath the superficial dermis is an unencapsulated, well demarcated, moderately cellular, focally infiltrative neoplastic mass. Neoplastic cells are arranged in thin streams dissecting between large thick bundles of well defined eosinophilic homogenous (hyalinized) birefringent material (collagen) within a fine fibrovascular stroma. In one location, neoplastic cells form interlacing bundles and streams with small embedded collagen bundles. Cells are spindloid with indistinct cell borders, a scant amount of finely fibrillar eosinophilic cytoplasm with an elongate central nucleus with finely stippled chromatin and indistinct nucleoli. There is three-fold anisocytosis and anisokaryosis. The mitotic rate is low with an average of 0.1 mitotic figures per 400x HPF. Neoplastic cells extend to one margin of the biopsy. Small areas of hemorrhage are scattered throughout the tumor and around the tumor base. Small numbers of lymphocytes and plasma cells surround small vessels around the margins of the tumor.

Morphologic Diagnosis:  

Keloid fibrosarcoma.


Keloidal fibrosarcoma

Contributor Comment:  

Keloid fibrosarcomas are an uncommon variant of fibrosarcomas, distinct from the other forms of fibrosarcoma and other collagen-rich masses due to the presence of thick bands of hyalinized collagen.(3) These bands of hyalinized collagen can also be seen in cytologic preparations.(2) Keloidal tumors are infrequently reported in dogs and have not been reported in other domestic animal species. They appear somewhat similar histologically to keloids, hypertrophic scars and keloid dermatofibromas in humans.(2) The one published retrospective study of keloidal tumors in dogs suggests that the presence of macrophages within the tumors may indicate that these tumors are reactive inflammatory lesions, rather than true neoplasms.(3)

Differentiation of keloid fibromasarcomas from keloidal fibromas is based on the presence of portions of the tumors that are composed largely of thickly packed neoplastic cells with only a small amount of fibrovascular stroma and small numbers of hyalinized fibers.(3) These areas are reportedly more common in the deeper margins of the tumors as is the case in this example. It was suggested that keloidal fibrosarcomas may represent a malignant transformation of keloid fibromas; however, no difference in prognosis has been demonstrated between keloid fibromas or keloid fibrosarcomas.

The neoplastic cells in keloidal tumors of dogs are vimentin positive and smooth muscle actin negative and are interpreted to be fibroblasts in contradistinction to human keloidal tumors which are comprised of myofibroblasts. (3)

JPC Diagnosis:  

Haired skin and subcutis: Fibrosarcoma, low grade (keloidal).

Conference Comment:  

The large bundles of hyalinized collagen are a distinctive histologic feature in this case, and in dogs are generally limited to keloidal fibromas and fibrosarcomas and mast cell tumors with keloidal change. (1) Differentiation between keloidal fibromas and keloidal fibrosarcomas is based on the presence of interlacing fascicles of neoplastic cells and/or infiltration in the malignant variant. Since keloidal fibrosarcomas may arise from keloidal fibromas, the malignant characteristics may be present only in a small portion of the tumor.(3) Keloidal fibrosarcomas also may contain cellular atypia and have an increased mitotic rate.(1) The differential diagnosis includes dermatofibroma and peripheral nerve sheath tumor (PNST). In dermatofibromas, and occasionally in PNSTs, the spindle cells are separated by enlarged, but non-hyalinized, collagen bundles. Additionally, dermatofibromas usually contain inflammation and overlying epithelial hyperplasia, while PNSTs have a neural pattern, demonstrate variation in cellularity, and frequently contain a myxomatous matrix.(1)

Conference participants also noted the presence of small capillaries within the large, hyalinized collagen fibers and areas of microhemorrhage; these changes have been described previously.(3)


1. Gross TL, Ihrke PJ, Walder EJ, Affolter VK: Skin Diseases of the Dog and Cat: Clinical and Histopathologic Diagnosis, 2nd ed., pp. 721-722. Blackwell Publishing, Ames, IA, 2005
2. Little LK, Goldschmidt M: Cytologic appearance of a keloidal fibrosarcoma in a dog. Vet Clin Pathol 36:364-367, 2007
3. Mikaelian I, Gross TL: Keloidal fibromas and fibrosaromas in the dog. Vet Pathol 39:149-153, 2002

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2-1. Fibrosarcoma (keloidal), haired skin and subcutis

2-2. Fibrosarcoma (keloidal), haired skin and subcutis

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