Introduction

Malignancies of the tendon sheath are considered to be very rare in dogs, with only two previous cases reported in the literature.^1,2 In humans, malignant tumors of the tendon sheath are also considered rare.³ There are no previous reports of tendon sheath sarcomas of fibroblastic origin in dogs, nor are there any descriptions of tendon sheath tumors causing invasive osteolysis. In contrast, ∼85% of primary bone tumors are diagnosed as osteosarcoma.⁴ Osteosarcoma, in particular, has a poor long-term prognosis; therefore, differentiating it from other musculoskeletal malignancies is important for accurate treatment recommendations and prognostication.

This case below describes a previously unreported fibrosarcoma of the tendon sheath with associated bony lysis of the distal radius. Based on this case, this differential should be considered as a rare rule-out for an aggressive bone lesion of the distal limb.

Case Report

A 7 yr old, 39 kg female spayed golden retriever presented to The Ohio State University Veterinary Medical Center for further evaluation of an aggressive bone lesion involving the left distal radius. Before presentation, the dog was evaluated by the referring veterinarian for a 3 wk history of lameness of the left forelimb. On physical exam, the dog had severe weight-bearing lameness and accompanying swelling of the left forelimb at the level of the distal radius. The left superficial cervical lymph node was mildly enlarged on palpation, and mild generalized muscle atrophy was also reported, but no other pertinent physical exam abnormalities were detected. A complete blood count, serum biochemistry, and urinalysis were also performed and were all unremarkable. Radiographs taken of the affected forelimb by the referring veterinarian revealed lysis of the distal aspect of the radius with accompanying soft tissue swelling (Figure 1A).

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A fine-needle aspirate of the distal radial lesion was performed under sedation. Cytology revealed large, irregular to spindle-shaped cells with variable nuclear to cytoplasmic ratio and anisocytosis and anisokaryosis. Nuclei were round to slightly irregular and contained stippled chromatin with one to multiple variably sized, prominent nucleoli. Occasional binucleate and trinucleate cells were seen. The cytologic diagnosis was a malignant mesenchymal neoplasm, with osteosarcoma being the primary differential. Three-view thoracic radiographs and an abdominal ultrasound were performed for staging of this dog and did not reveal any abnormalities.

Computed tomography (CT)^a of the affected forelimb was then performed under light sedation (dexmedetomidine, 5 μg/kg^b intramuscularly; butorphanol, 0.2 mg/kg^c intramuscularly). Pre- and post-contrast images were obtained (iohexol, 370 mg/kg IV^d). The CT scan showed near-complete cortical bone destruction of the distal radius with a wide, bicortical, nondisplaced fracture of the distal metaphysis and a small amount of irregularly marginated periosteal proliferation surrounding the distal metaphysis and extending proximally along the distal diaphysis. Additionally, a moderately patchy, contrast-enhancing soft-tissue thickening surrounding the distal antebrachium was observed (Figures 1B, C). All other bone and soft-tissues structures visualized on the CT were within normal limits. Forelimb amputation was recommended because of the extent of the disease and the severity of the dog’s clinical signs.

The dog was placed under general anesthesia, and a left forequarter amputation was performed. A wound soaker catheter was placed, and the dog received ropivacaine^e (1.5 mg/kg q 6 hr) postoperatively. Carprofen^f (2.2mg/kg subcutaneous) was administered, and the dog was maintained on a fentanyl^g constant rate infusion (2 μg/kg/hr IV) for 24 hr following surgery. The dog recovered from surgery without complication and was discharged 3 days after amputation. She was sent home with cephalexin^h (23 mg/kg q 12 hr) tablets as well as carprofenⁱ (2 mg/kg q 12 hr) and gabapentin^j (7.5 mg/kg q 8 hr) for pain. Trazadone^k (4 mg/kg q 8–12 hr) was also sent home to be used as needed to minimize anxiety as the dog recovered from surgery.

Both the left forelimb and the left superficial cervical lymph node were submitted for histopathology. The left distal radial lesion was described as a mass expanding the periosseous connective tissue and infiltrating into the bone of the distal radius. The mass was composed of a monomorphic population of spindle cells with an ovoid to cylindrical nucleus, open chromatin with one to two distinct nucleoli, and scant eosinophilic cytoplasm. The neoplastic cells formed loose deposits of poorly organized eosinophilic collagen. The tumor was described as being highly invasive, penetrating through cortical bone, and eroding the subchondral bone of the distal radius. The mitotic count was 10 per 10 high-powered fields. Most notably, there was a distinct absence of osteoid deposition in all sections analyzed. As a result, a diagnosis of moderately to poorly differentiated extraosseous fibrosarcoma was favored, but an osteoid poor, fibroblastic osteosarcoma could not be ruled out. The lymph node was consistent with lymphofollicular hyperplasia with no evidence of metastatic disease.

Second opinion of the histopathology was sought because of the significant therapeutic and prognostic differences between fibroblastic osteosarcoma and fibrosarcoma. Multiple histopathologic sections of the distal left forelimb were sent to Texas A & M University for evaluation (Teaching Slides, Dr. Roy Pool). On review, it was noted that the periosseous portion of the mass had partially intact walls of a tendon sheath whose cavity was filled with a fibrous tissue–producing spindle-cell sarcoma (Figure 2A). This was determined to be compatible with fibrosarcoma of tendon sheath origin. The tendon sheath neoplasm appeared to penetrate the cranial cortex of the distal radius and had entered and advanced through the medullary cavity (Figure 2C). Within the medullary cavity, intersecting bands of fibrous tissue were observed causing resorption of the metaphyseal and epiphyseal spongiosa and invasion into the subchondral bone (Figure 2D). The tumor was not consistent with histopathologic changes seen with fibroblastic osteosarcoma, and the final diagnosis was a high-grade fibrosarcoma arising from the tendon sheath and secondarily invading the distal radius.

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While waiting for second opinion histopathology, chemotherapy (Carboplatin^l 300 mg/m² IV q 21 days) was initiated 1 mo after surgery. A total of four doses of carboplatin were administered without complication. Oral maropitant^m (∼2 mg/kg per os) was prescribed for 4 days following each treatment. After completion of the prescribed chemotherapy protocol, restaging with thoracic radiographs was routinely performed every 3 mo without evidence of gross metastatic disease. Approximately 1 yr after amputation, the dog was restaged with repeat thoracic radiographs and an abdominal ultrasound. All diagnostics were unremarkable and the dog continues to have no gross evidence of metastatic disease >1 yr after amputation.

Discussion

This is the first described case of fibrosarcoma of the tendon sheath. Overall, primary tumors of the tendon sheath are a rarely diagnosed entity in veterinary species. Whereas most cases have been reported in horses and cats, few case reports describing tendon sheath tumors exist in dogs.^5–8 To the authors’ knowledge, only two previous case reports could be found describing neoplasms originating from the tendon sheath in canines.^1,2 One case report describes a giant-cell tumor of the tendon sheath (GCTTS), diagnosed via cytology and histopathology, in the proximal metatarsus of a dog.¹ This dog was reported as disease-free 8 mo after surgery and radiation. In humans, this benign tumor is also referred to as a tenosynovial giant-cell tumor or localized nodular synovitis and is the second most common tumor of the human hand.⁹ Although less information is known about the biologic behavior in dogs, in humans, GCTTSs are considered benign but have the potential to recur after surgical excision.¹⁰ Interestingly, true malignant tumors of the tendon sheath are rare in human medicine.³ In dogs, the only case report in the literature of a malignant neoplasm invading the tendon sheath was a synovial sarcoma.²

Differential diagnoses for this case included fibroblastic osteosarcoma as well as primary bone fibrosarcoma. In this case, a primary, extraosseous sarcoma originating in the fibrous layer of the tendon sheath was favored for multiple reasons. First, no osteoid was identified in the histopathologic sections. In the case of fibroblastic osteosarcoma, early in the disease, osteoid may be sparse and difficult to locate in histopathologic sections. However, as the disease progresses, additional osteoid deposition occurs.¹¹ Second, the soft-tissue swelling associated with the lesion is more organized and discrete than what is normally seen with primary bone tumors. Radiographs of the lesion show a notable, fusiform soft-tissue swelling on the cranial aspect of the radius, which tapers at both ends and is most pronounced over the cortices of the radial metaphysis. This soft-tissue lesion is likely consistent with an enlarged tendon sheath of an extensor tendon as it passes over the radius toward the carpal joint. If this lesion had been caused by a primary bone neoplasm originating in the metaphysis of the radius, it would have been expected to form a more radially destructive pattern and not form a fusiform soft-tissue mass of the cranial cortical surface of the radius.

In people, malignant tumors of the tendon sheath are considered very rare, and in some cases, their true existence is debated.^3,12,13 Some 30 reports in the literature describe a malignant variety of GCTTS arising from the tendon sheath.² Nearly half of these reported cases were originally diagnosed as benign GCTTS that developed features of malignancy upon recurrence.¹⁴ Another rare sarcoma of uncertain lineage in humans is myxoinflammatory fibroblastic sarcoma. This tumor is often associated with the tendon sheath and typically occurs in the distal extremities, most commonly the hands and fingers¹⁵ Myxoinflammatory fibroblastic sarcoma is often misdiagnosed because of its composition of inflammatory cells and the location mimicking other benign tumors of the hand.¹⁶ Bony lysis is not a feature typically apparent with either of the aforementioned tumor types in people. Furthermore, none of these malignancies have been described in dogs.

Direct conclusions about canine tendon sheath fibrosarcoma behavior and prognosis cannot be gleaned from a single case report. However, there is information in the literature about canine fibrosarcomas originating from other anatomic locations, as well as the biologic behavior of fibrosarcoma in humans, that can potentially be guiding. In dogs, fibrosarcomas are often well-differentiated tumors.¹⁷ They are usually locally invasive and are prone to local recurrence. Metastasis is considered less common.^17,18 A high mitotic index has been associated with higher-grade, more aggressive tumors in fibrosarcomas of the skin in canines.¹⁸ One well-described, histopathologically paradoxical form of fibrosarcoma seen in the dog is oral fibrosarcomas.¹⁹ These tumors are often diagnosed as low-grade tumors histopathologically but biologically function as high-grade malignancies that often recur and may metastasize in later stages in ∼10–14% of dogs.¹⁹ Comparatively, in humans, ∼80% of fibrosarcomas diagnosed in adults are high-grade tumors.²⁰ Lung and bone metastasis is also more common in people.²⁰

The benefit of chemotherapy in the treatment of high-grade soft-tissue sarcomas is controversial in veterinary oncology. In a retrospective case series of 39 dogs, Selting et al. found that the addition of doxorubicin chemotherapy did not improve the survival of a dog with high-grade soft-tissue sarcomas.²¹ The addition of carboplatin to the treatment of the dog in this case was made after discussions with the owner about cost versus potential benefit. Ultimately, the decision was made to start carboplatin before the final histopathology results in the chance that pathologists felt this tumor was a fibroblastic osteosarcoma rather than a fibrosarcoma.

Conclusion

This case report describes the first reported tendon sheath fibrosarcoma in a dog. In this case, the tumor was very locally aggressive with invasion into the distal radius. Tendon sheath tumors with secondary bone invasion should be considered as a differential diagnosis for osteosarcoma, particularly for cases with unusual imaging and/or histopathologic features.