Keloidal fibrosarcoma in a Labrador Retriever

This is a case from a 5-year-old entire male Labrador with a slowly enlarging cutaneous mass on its flank. Below are photographs of the fine needle aspirates and histopathological sections from the mass.

Fig1a. Fine needle aspirate: the predominant cell population consists of mildly pleomorphic plump spindle cells interspersed with small amounts of fine eosinophilic matrix. There is a small amount of blood in the background.
Fig 1b. Fine needle aspirate: on higher magnification, the spindle cells surround a discrete elongate/ovoid, pale eosinophilic and somewhat glassy structure consistent with a large collagen fibre (red arrow).
Fig 2. Histopathology (H&E) section: the subcutis is infiltrated by a highly cellular mesenchymal neoplasm.
Fig 3. On higher magnification, the mesenchymal neoplasm is composed of interlacing streams of plump spindle cells (white arrows) interspersed with large, hyalinised collagen fibres (black arrows).
Fig. 4. Routine H&E section: a close up view of the large, hyalinised collagen fibres – these are consistent with the large collagen fibres seen cytologically in the fine needle aspirate smears.

Final Diagnosis

Keloidal fibrosarcoma (low grade)


This is a malignant neoplasm of fibroblasts and is an unusual variant of fibrosarcoma. The term keloidal refers to the deposition of large hyalinised collagen bundles and this is a distinctive histopathological feature of keloidal neoplasms. Malignancy in this case is based on the presence of infiltrative behaviour and regions of high cellularity. Features that are supportive of its low grade malignant nature include the low mitotic index, identification as a specific histological subtype and absence of necrosis. As with all soft tissue sarcomas, infiltrative behaviour is typical. Based on the low grade nature of the tumour in this case, the risk of metastasis is considered low and complete excisison should be curative. Keloidal fibrosarcomas are less common than their benign counterpart (keloidal fibroma) in dogs and it has been suggested that keloidal fibrosarcomas arise from malignant transformation of keloidal fibromas.

On a comparative note, although distinct from the keloidal fibroma/fibrosarcoma in the dog, keloid scars in humans most commonly occur in young adults as a result of local trauma and usually develop on the head/neck region (particularly the ears). The lesions essentially consist of fibrous scar tissue associated with large hyalinised collagen fibres similar to those described above. Surgical excision is often difficult and the recurrence rate is high. The pathogenesis is not certain, however a genetic predisposition to excessive growth factors and consequent excessive scar tissue production is suspected.