Bone Fibrosarcoma

Bone fibrosarcoma is a malignant mesenchymal neoplasm arising from fibroblasts within the bone marrow, periosteum, or soft tissues adjacent to bone, characterized by the production of collagen fibers and a fascicular or herringbone histologic pattern. In veterinary surgical oncology, it is a primary bone tumor that is less common than osteosarcoma but shares similar clinical presentation and aggressive local behavior. It can be classified as central (medullary) or periosteal (surface) based on its origin. Central fibrosarcoma arises within the medullary cavity, while periosteal fibrosarcoma originates from the periosteum and may erode into the cortex. The tumor is locally invasive, with a high rate of recurrence after inadequate excision, but a lower metastatic potential compared to osteosarcoma. Surgical staging (Enneking system) is essential for planning, with most tumors presenting as Stage IIB (high-grade, extracompartmental) due to cortical destruction and soft tissue extension. Definitive treatment often requires wide or radical surgical excision, such as limb amputation or limb-sparing procedures, with adjunctive therapies considered for high-grade tumors.

The exact etiology of bone fibrosarcoma in animals is largely unknown, but several factors have been implicated. Chronic inflammation, previous trauma, and pre-existing bone lesions (e.g., bone infarcts, chronic osteomyelitis) may predispose to malignant transformation of fibroblasts. Genetic mutations, such as alterations in tumor suppressor genes (p53, Rb) and oncogenes (ras, myc), have been identified in some cases. Ionizing radiation exposure, although rare, can induce fibrosarcoma in bone. In dogs, there is no strong breed or sex predilection, but large and giant breeds are overrepresented, suggesting a possible role of rapid bone growth and remodeling. Viral etiologies have been proposed but not confirmed. In cats, fibrosarcoma can arise at sites of previous injection (feline injection-site sarcoma), which is a distinct entity but may involve bone if deeply invasive. The tumor arises from fibroblasts that undergo neoplastic transformation, leading to uncontrolled proliferation and production of collagen, which forms the tumor matrix.

Bone fibrosarcoma is a rare primary bone tumor in dogs and cats, accounting for approximately 5-10% of all primary bone tumors in dogs, making it the second or third most common primary bone malignancy after osteosarcoma and chondrosarcoma. It occurs most commonly in middle-aged to older dogs, with a median age of 7-9 years, but can occur in younger animals. Large and giant breeds, such as Golden Retrievers, Rottweilers, and Great Danes, are predisposed, likely due to their size and rapid bone growth. There is no consistent sex predilection, though some studies suggest a slight male predominance. In cats, fibrosarcoma is less common, but when it occurs, it is often associated with injection sites (feline injection-site sarcoma) and can involve underlying bone. The appendicular skeleton is most commonly affected, with the proximal humerus, distal radius, and proximal femur being frequent sites, similar to osteosarcoma. Axial skeletal involvement, such as the mandible, maxilla, and ribs, is also reported. The tumor is locally aggressive, with a high rate of recurrence after inadequate excision, but the metastatic rate is lower than osteosarcoma, with reported rates of 10-25% at the time of diagnosis or later.

Bone fibrosarcoma arises from fibroblasts within the bone marrow or periosteum. The neoplastic cells produce collagen and form interlacing fascicles, which can be seen histologically. The tumor grows expansively within the medullary cavity, causing bone lysis and cortical destruction. As it expands, it elevates the periosteum, leading to periosteal new bone formation, which may be seen radiographically as a sunburst or Codman's triangle, though less prominent than in osteosarcoma. The tumor can break through the cortex into the surrounding soft tissues, forming a palpable mass. Pain arises from periosteal stretching, cortical destruction, and pathologic fracture. The tumor is highly vascular, and angiogenesis is stimulated by vascular endothelial growth factor (VEGF). Local invasion is common, with tumor cells infiltrating along Haversian canals and into adjacent soft tissue. Metastasis occurs hematogenously, most commonly to the lungs, but also to regional lymph nodes and other bones. The metastatic potential is lower than osteosarcoma, but high-grade tumors with significant soft tissue extension have a higher risk. Systemic effects include paraneoplastic syndromes such as hypercalcemia, though less common than in osteosarcoma.

Intrinsic factors include age (middle-aged to older animals), breed (large and giant breeds), and genetic predisposition. Certain breeds, such as Golden Retrievers and Rottweilers, have a higher incidence, possibly due to inherited genetic mutations. Extrinsic factors include trauma, which may cause local tissue damage and inflammation, potentially leading to malignant transformation. Chronic osteomyelitis or bone infarcts can also predispose to fibrosarcoma. In cats, injection-site sarcomas, which can be fibrosarcomas, are associated with previous vaccinations or injections, particularly with adjuvanted vaccines, and have a higher risk in cats with a genetic predisposition (e.g., FeLV or FIV infection). Prior radiation therapy for other tumors can also induce fibrosarcoma. Excessive physical activity or repetitive microtrauma may contribute to the development of bone tumors, though this is not well established. Nutritional factors, such as high-calcium diets, have been suggested but not proven.

Clinical signs of bone fibrosarcoma are similar to other primary bone tumors. The most common presenting sign is progressive lameness, which may be intermittent initially and becomes persistent and severe over time. The lameness is often non-weight-bearing in advanced stages. Pain is localized to the affected bone and can be elicited by palpation or manipulation. A palpable mass may be present, especially if the tumor has broken through the cortex. The mass may be firm, warm, and painful. Swelling of the surrounding soft tissues is common. Pathologic fracture can occur, leading to acute, severe lameness and pain. Systemic signs such as lethargy, anorexia, and weight loss may be present in advanced disease or with metastasis. In axial skeleton involvement, signs depend on the location: mandibular tumors cause facial swelling, difficulty eating, and drooling; maxillary tumors cause nasal discharge, epistaxis, and facial deformity; rib tumors cause thoracic wall mass and respiratory distress; vertebral tumors cause spinal pain, paresis, or paralysis. Neurologic deficits may be present if the tumor compresses the spinal cord.

Differential diagnoses for bone fibrosarcoma include: 1) Osteosarcoma - the most common primary bone tumor, radiographically similar, but histologically distinct with osteoid production; more aggressive with higher metastatic rate. 2) Chondrosarcoma - arises from cartilage, often in flat bones (ribs, nasal cavity), radiographically shows stippled calcification, histologically has chondroid matrix. 3) Hemangiosarcoma - primary bone hemangiosarcoma is rare, but can cause lytic lesions; histologically shows vascular channels. 4) Multiple myeloma - a plasma cell tumor that can cause multiple lytic bone lesions, hypercalcemia, and monoclonal gammopathy. 5) Fungal osteomyelitis (e.g., blastomycosis, coccidioidomycosis) - causes lytic bone lesions and periosteal reaction, but is associated with systemic signs and can be diagnosed by serology or histopathology. 6) Bacterial osteomyelitis - usually secondary to trauma or surgery, with sequestrum formation and draining tracts. 7) Bone cyst - benign, often in young dogs, radiographically well-defined lytic lesion, but can be painful and cause pathologic fracture. 8) Metastatic bone tumors - from other primary sites (e.g., mammary, prostate), often multiple lesions. 9) Fibrous dysplasia - a benign fibro-osseous lesion, rare in animals. 10) Eosinophilic granuloma - a benign lesion that can cause bone lysis, but is more common in cats. Definitive diagnosis requires histopathology.

The diagnostic algorithm for bone fibrosarcoma begins with a thorough history and physical examination, including orthopedic and neurologic evaluation. If a bone tumor is suspected, three-view thoracic radiographs should be obtained to screen for pulmonary metastasis. Radiographs of the affected bone are essential to characterize the lesion and guide biopsy. Advanced imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), is recommended for surgical planning, especially for axial skeleton tumors, to assess the extent of bone and soft tissue involvement. A bone biopsy is the gold standard for diagnosis. This can be performed via needle biopsy (Jamshidi needle) or surgical biopsy. The biopsy should be taken from the center of the lesion, avoiding areas of necrosis or fracture. Histopathology confirms the diagnosis and provides tumor grade. If the tumor is high-grade, a staging workup including abdominal ultrasound and lymph node aspiration may be considered. For suspected metastatic disease, CT of the thorax is more sensitive than radiographs. In cases of pathologic fracture, the biopsy should be taken at the time of surgical stabilization or amputation. The diagnostic algorithm should be systematic to avoid delays in treatment.

Complete blood count (CBC) may show mild anemia of chronic disease, leukocytosis, or thrombocytosis. Serum biochemistry may reveal elevated alkaline phosphatase (ALP), especially in dogs with bone tumors, but this is not specific. Hypercalcemia may be present in some cases, though less common than in osteosarcoma. Serum protein electrophoresis may show a monoclonal gammopathy in multiple myeloma, which is a differential. Coagulation panel (PT, aPTT, platelet count) is important for surgical planning, as some tumors can cause consumptive coagulopathy. Urinalysis is routine. Synovial fluid analysis is not typically performed for bone tumors, but if the joint is involved, it may show inflammatory changes. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated but are nonspecific. Blood gas analysis is not routinely needed but may be indicated in critically ill patients. Preoperative laboratory tests are essential to assess organ function and anesthetic risk.

Radiography: Bone fibrosarcoma typically appears as a lytic, destructive lesion with a wide zone of transition, cortical thinning, and periosteal reaction. The periosteal reaction may be mild to moderate, with a sunburst or Codman's triangle pattern, but is often less aggressive than osteosarcoma. Pathologic fracture may be present. In the appendicular skeleton, the lesion is often located in the metaphysis or diaphysis. In the axial skeleton, radiographs may show bone lysis and soft tissue swelling. Thoracic radiographs are essential to detect pulmonary metastasis, which appears as nodular interstitial patterns. Ultrasonography: Not typically used for bone tumors, but can be used to evaluate soft tissue extension and to guide biopsy. CT: Provides detailed assessment of bone destruction, soft tissue mass, and tumor margins. It is essential for surgical planning, especially for limb-sparing procedures and axial skeleton tumors. 3D reconstructions help in planning resection. MRI: Superior for evaluating soft tissue and bone marrow involvement, especially in the spine and pelvis. It can delineate the tumor extent and relationship to neurovascular structures. Arthroscopy: Not used for bone tumors, but may be used to evaluate joint involvement. Angiography/Fluoroscopy: May be used for vascular mapping in limb-sparing surgery.

Cytology: Fine-needle aspiration of the bone lesion may yield spindle cells with moderate to marked anisocytosis and anisokaryosis, but cytology is often nondiagnostic for bone tumors due to the dense fibrous stroma. It is more useful for evaluating regional lymph nodes for metastasis. Histopathology: A biopsy is essential for diagnosis. The tumor is composed of interlacing fascicles of spindle-shaped fibroblasts with variable collagen production. The cells have elongated nuclei, moderate to marked pleomorphism, and mitotic figures. The tumor is graded based on cellularity, pleomorphism, mitotic index, and necrosis. Low-grade tumors have well-differentiated fibroblasts and low mitotic count, while high-grade tumors have marked pleomorphism and high mitotic activity. Immunohistochemistry can be used to differentiate fibrosarcoma from other sarcomas: vimentin is positive, while cytokeratin, S100, and desmin are negative. Special stains such as Masson's trichrome can highlight collagen. Surgical margins should be evaluated for tumor-free margins.

Treatment of bone fibrosarcoma is primarily surgical. The goal is wide or radical excision to achieve clean margins. For appendicular tumors, limb amputation is the most common surgical treatment, especially for large, aggressive tumors. Limb-sparing surgery (e.g., segmental bone resection and allograft or endoprosthesis placement) may be considered for distal radial tumors in dogs, but is associated with higher complication rates. For axial tumors, surgical excision depends on the location: mandibulectomy or maxillectomy for jaw tumors, rib resection for rib tumors, and hemilaminectomy or vertebrectomy for vertebral tumors. In cases where complete excision is not possible, debulking surgery may be performed, but recurrence is likely. Adjunctive therapies include radiation therapy, which can be used for local control in incompletely excised tumors or for palliation. Chemotherapy (e.g., doxorubicin, carboplatin) may be considered for high-grade tumors or metastatic disease, but its efficacy is not well established. Pain management is essential, including analgesics and possibly bisphosphonates to reduce bone pain. Postoperative care includes pain control, antibiotics, and physical rehabilitation.

The prognosis for bone fibrosarcoma is guarded to fair. The median survival time for dogs with appendicular fibrosarcoma treated with amputation alone is approximately 1-2 years, with a 1-year survival rate of 60-70% and a 2-year survival rate of 30-40%. The metastatic rate is lower than osteosarcoma, but local recurrence is common if margins are incomplete. Negative prognostic indicators include high histologic grade, large tumor size, soft tissue extension, and incomplete surgical margins. Axial tumors have a worse prognosis due to difficulty in achieving complete excision. Cats with injection-site fibrosarcoma have a high recurrence rate (up to 50-70%) even with aggressive surgery, and the prognosis is guarded. Early detection and wide surgical excision improve the prognosis. The presence of metastasis at diagnosis is a poor prognostic indicator, with survival times of only a few months.

Postoperative follow-up is crucial for monitoring recurrence and metastasis. For the first 2 years, thoracic radiographs should be performed every 3 months to screen for pulmonary metastasis. After 2 years, every 6 months is recommended. Physical examination should be performed monthly for the first 3 months, then every 3 months for the first year, and every 6 months thereafter. If limb-sparing surgery was performed, radiographs of the surgical site should be taken at 4, 8, and 12 weeks postoperatively to assess bone healing and implant stability. Activity restriction is recommended for 6-8 weeks after amputation, with gradual return to normal activity. Physical rehabilitation, including passive range of motion exercises and controlled leash walks, is important. Suture removal is typically 10-14 days postoperatively. Long-term monitoring for complications such as infection, implant failure, or tumor recurrence is essential.

Pearls: 1) Always obtain a biopsy before definitive surgery to confirm the diagnosis and grade, as treatment and prognosis differ from osteosarcoma. 2) Use a Jamshidi needle for bone biopsy to obtain a core sample; avoid crush artifact. 3) For limb-sparing surgery, ensure that the tumor is confined to the bone and that at least 2 cm of normal bone can be resected proximal to the tumor. 4) In cats, be aware of injection-site sarcomas; wide surgical excision with 2-3 cm margins is recommended. 5) Consider radiation therapy for incompletely excised tumors to improve local control. Pitfalls: 1) Do not perform a biopsy through a separate incision that will contaminate the surgical field; plan the biopsy site to be excised en bloc with the tumor. 2) Avoid curettage or intralesional biopsy, which can seed tumor cells. 3) Do not delay surgery for a pathologic fracture; stabilize the fracture and biopsy at the same time. 4) Do not rely on radiographs alone to determine tumor extent; use CT or MRI for surgical planning. 5) Do not underestimate the risk of metastasis; monitor closely.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery, then every 8 hours for 24 hours postoperatively. For patients with open fractures or contaminated surgery, continue for 48-72 hours. Analgesia: Preoperative: Opioid (e.g., hydromorphone 0.05-0.1 mg/kg IV, or methadone 0.1-0.3 mg/kg IV) and a NSAID (e.g., carprofen 4.4 mg/kg SC or PO q24h, or meloxicam 0.2 mg/kg SC or PO q24h). Intraoperative: Fentanyl CRI at 5-10 mcg/kg/hr IV, or lidocaine CRI at 25-50 mcg/kg/min IV. Postoperative: Continue opioids (e.g., buprenorphine 0.01-0.02 mg/kg IV q8-12h) for 24-48 hours, then transition to oral opioids (e.g., tramadol 2-5 mg/kg PO q8-12h) and NSAIDs for 5-7 days. Local anesthetic blocks: For limb amputations, perform a brachial plexus block or epidural with bupivacaine (1-2 mg/kg) for intraoperative and postoperative analgesia. Muscle relaxants: Not routinely used. Chondroprotectants: Not indicated for bone tumors. Chemotherapy: For high-grade tumors or metastasis, consider doxorubicin (30 mg/m² IV every 3 weeks) or carboplatin (300 mg/m² IV every 3 weeks), with monitoring for myelosuppression and cardiotoxicity. Bisphosphonates: Pamidronate 1-2 mg/kg IV diluted in saline over 2 hours, every 3-4 weeks, for bone pain palliation. Antiemetics: For chemotherapy-induced nausea, maropitant 1 mg/kg SC q24h. Gastroprotectants: If NSAIDs are used long-term, consider omeprazole 1 mg/kg PO q24h.

Literature on bone fibrosarcoma in veterinary medicine is limited due to its rarity. A retrospective study by Ehrhart et al. (2001) evaluated 30 dogs with appendicular fibrosarcoma treated with amputation; median survival was 1.5 years, with a 1-year survival rate of 70% and a 2-year survival rate of 40%. Tumor grade and soft tissue extension were significant prognostic factors. Another study by Straw et al. (1996) reported that limb-sparing surgery for fibrosarcoma had a higher complication rate than for osteosarcoma, with a 30% infection rate and 20% implant failure. In cats, a study by Hendrick et al. (1994) on injection-site sarcomas, which are often fibrosarcomas, showed that wide surgical excision with 2-3 cm margins reduced recurrence from 70% to 30%. Radiation therapy has been shown to improve local control in incompletely excised tumors, with a study by Forrest et al. (2000) reporting a median progression-free interval of 1 year. Chemotherapy has not been shown to significantly improve survival in fibrosarcoma, but a study by Batschinski et al. (2014) suggested that doxorubicin may be beneficial in high-grade tumors. Overall, the evidence supports aggressive surgical excision as the primary treatment, with adjunctive therapies for high-risk cases.