Soft Tissue Sarcomas

Soft tissue sarcomas (STS) are a heterogeneous group of malignant mesenchymal neoplasms arising from non-epithelial, extraskeletal tissues including fibrous connective tissue, adipose tissue, muscle, peripheral nerves, and vascular structures. In veterinary medicine, STS commonly present as firm, infiltrative, pseudoencapsulated masses in the subcutaneous and deep soft tissues. They are characterized by local invasiveness and a high rate of local recurrence after inadequate excision, but a relatively low metastatic potential (10-25% overall), with metastasis typically occurring to the lungs and, less commonly, regional lymph nodes. Histologic subtypes include fibrosarcoma, peripheral nerve sheath tumor (schwannoma, neurofibrosarcoma), perivascular wall tumors (hemangiopericytoma), liposarcoma, rhabdomyosarcoma, leiomyosarcoma, and undifferentiated sarcoma. The biologic behavior is influenced by histologic grade, tumor size, depth, and completeness of surgical margins. Surgical resection with wide margins is the mainstay of treatment, often combined with radiation therapy for marginal or incomplete excisions. The term 'soft tissue sarcoma' is a clinical classification based on similar biologic behavior and treatment strategies, rather than a single histopathologic entity.

The exact etiology of soft tissue sarcomas in dogs and cats is largely unknown, but several factors have been implicated. Ionizing radiation is a well-documented cause, with sarcomas developing in previously irradiated fields (radiation-induced sarcomas) after a latency period of several years. Chronic inflammation and trauma have been suggested as predisposing factors, particularly for vaccine-associated sarcomas in cats, where chronic inflammation at injection sites (e.g., rabies, feline leukemia virus vaccines) leads to malignant transformation of fibroblasts or myofibroblasts. Genetic mutations, such as alterations in tumor suppressor genes (p53, Rb) and activation of oncogenes (RAS, MYC), have been identified in some human and canine sarcomas, but specific hereditary syndromes are rare in veterinary patients. Chemical carcinogens, including certain herbicides and wood preservatives, have been epidemiologically linked to soft tissue sarcomas in dogs, though causality is not firmly established. In cats, feline sarcoma virus (FeSV) can cause multicentric fibrosarcomas in young cats, but this is rare and distinct from the more common injection-site sarcomas. Overall, most STS arise spontaneously without an identifiable cause.

Soft tissue sarcomas account for approximately 15% of all skin and subcutaneous tumors in dogs and 7% in cats. They are most commonly diagnosed in middle-aged to older animals, with a median age of 10 years in dogs and 11 years in cats. No strong sex predilection is reported, though some studies suggest a slight male predominance in dogs. Certain breeds appear to be overrepresented, including Golden Retrievers, Labrador Retrievers, Boxers, and Bernese Mountain Dogs, possibly due to genetic predisposition. In cats, domestic shorthair and longhair breeds are most common, but Siamese and other pointed breeds may have a higher risk for vaccine-associated sarcomas. Large-breed dogs may present with larger tumors at diagnosis, but size is not an independent prognostic factor when adjusted for grade. The incidence of vaccine-associated sarcomas in cats has decreased with the use of less reactive vaccines and altered vaccination protocols, but they remain a significant clinical challenge. Working dogs, such as those used in hunting or agility, may have a higher risk of trauma-related sarcomas, though evidence is anecdotal.

Soft tissue sarcomas arise from mesenchymal stem cells that undergo malignant transformation, leading to uncontrolled proliferation and local invasion. The tumor grows in an infiltrative pattern, extending along fascial planes, blood vessels, and nerves, which explains the high rate of local recurrence after incomplete excision. Despite a grossly encapsulated appearance, the pseudocapsule is composed of compressed tumor cells and reactive fibrous tissue, and microscopic tumor extensions often penetrate beyond the capsule. The tumor microenvironment, including fibroblasts, inflammatory cells, and extracellular matrix components, plays a role in tumor progression and metastasis. High-grade sarcomas exhibit increased cellular atypia, mitotic activity, and necrosis, and are more likely to metastasize hematogenously to the lungs. Lymphatic invasion is less common but can occur, particularly in certain subtypes. The metastatic potential is correlated with histologic grade: low-grade sarcomas have a <10% metastatic rate, intermediate-grade 10-20%, and high-grade 20-30%. Tumor hypoxia and angiogenesis, driven by vascular endothelial growth factor (VEGF), contribute to tumor growth and metastatic spread. Paraneoplastic syndromes, such as hypoglycemia (associated with insulin-like growth factor secretion) or hypercalcemia, are rare but can occur.

Intrinsic factors include age (older animals), breed (Golden Retrievers, Boxers, etc.), and genetic predisposition. Immunosuppression, whether due to chronic disease or iatrogenic (e.g., corticosteroid use), may increase the risk of tumor development. Extrinsic factors include prior radiation therapy, chronic inflammation (e.g., injection sites in cats, wounds, burns), and trauma. In cats, the administration of vaccines, particularly adjuvanted vaccines, is a well-established risk factor for injection-site sarcomas. The risk increases with the number of vaccinations and the frequency of administration. Other injectable medications, such as long-acting corticosteroids or insulin, have also been implicated. Environmental carcinogens, such as exposure to certain chemicals, may play a role, but evidence is limited. Obesity and nutritional factors have not been strongly linked to STS. Prior surgical procedures, especially those with significant tissue trauma, may rarely lead to sarcoma development at the surgical site.

Soft tissue sarcomas typically present as a solitary, firm, painless, slowly growing mass in the subcutaneous tissue or deeper soft tissues. They are often mobile over underlying structures but may be fixed if deeply invasive. The overlying skin is usually intact, but ulceration can occur in advanced cases or after trauma. The mass may be present for months to years before diagnosis. Clinical signs are related to the location and size of the tumor. For example, tumors on the limbs may cause lameness or limb swelling, while tumors in the oral cavity may cause dysphagia, drooling, or halitosis. Tumors in the abdominal cavity may lead to vomiting, anorexia, or abdominal distension. Neurologic signs can occur if the tumor arises from peripheral nerves or compresses the spinal cord. Systemic signs such as weight loss, lethargy, and fever are uncommon but may occur with large or metastatic tumors. On palpation, the mass is often poorly circumscribed, and the overlying skin may be movable. Pain is not a consistent feature, but deep tumors may be painful on palpation.

Differential diagnoses for soft tissue sarcomas include: 1) Lipoma - benign, soft, well-circumscribed, slow-growing, often in subcutaneous tissue; cytology shows mature adipocytes. 2) Mast cell tumor - can be firm, may have erythema, edema, or ulceration; cytology shows mast cells with metachromatic granules; may have systemic signs due to histamine release. 3) Histiocytic sarcoma - often aggressive, may be multicentric; cytology shows atypical histiocytes; immunohistochemistry positive for CD18, CD11c. 4) Fibroma - benign, well-circumscribed, slow-growing; histopathology shows mature fibroblasts. 5) Peripheral nerve sheath tumor - often associated with nerve, may cause pain or neurologic deficits; histopathology shows Antoni A and B patterns. 6) Myxosarcoma - soft, gelatinous, often in subcutaneous tissue; histopathology shows myxoid stroma. 7) Osteosarcoma - primary bone tumor, but can arise in soft tissue (extraskeletal); radiographs may show mineralization; histopathology shows osteoid production. 8) Hemangiosarcoma - highly malignant, often in spleen or right atrium, but can be subcutaneous; may cause bleeding and anemia; histopathology shows vascular channels. 9) Granulomatous inflammation (e.g., foreign body reaction, fungal infection) - may mimic a mass; cytology shows inflammatory cells, and culture or histopathology is diagnostic. 10) Abscess - painful, fluctuant, often with fever; cytology shows degenerate neutrophils and bacteria. Definitive diagnosis requires histopathology.

The diagnostic workup for a suspected soft tissue sarcoma should proceed as follows: 1) Complete history and physical examination, including thorough palpation of the mass and regional lymph nodes. 2) Fine-needle aspiration (FNA) of the mass for cytology. While cytology may not always be definitive for sarcoma, it can help rule out other differentials such as mast cell tumor or lipoma. 3) If cytology is suggestive of sarcoma or non-diagnostic, perform a biopsy (incisional or core needle) for histopathology. Biopsy should be planned to avoid compromising future surgical excision; the biopsy tract should be excised en bloc with the tumor. 4) Preoperative staging: thoracic radiographs (three views) to evaluate for pulmonary metastasis; abdominal ultrasound if the tumor is on the caudal half of the body or if abdominal involvement is suspected; lymph node aspiration if enlarged or if the tumor is high-grade. 5) Advanced imaging (CT or MRI) may be indicated for deep tumors or those in complex anatomical locations (e.g., head, neck, extremities) to assess tumor extent and plan surgical margins. 6) Preoperative blood work: CBC, serum biochemistry, urinalysis, and coagulation profile to assess overall health and surgical risk. 7) Surgical excision with wide margins (3 cm lateral and one fascial plane deep) is the treatment of choice. The excised tissue should be submitted for histopathology to assess margins and grade. 8) If margins are incomplete or the tumor is high-grade, consider adjuvant radiation therapy or chemotherapy.

Complete blood count (CBC) is often within normal limits, but may show mild anemia of chronic disease or neutrophilia in cases of inflammation or necrosis. Serum biochemistry may reveal elevated alkaline phosphatase (ALP) or alanine aminotransferase (ALT) if there is hepatic involvement or paraneoplastic syndrome. Hypoglycemia can occur in rare cases of insulin-like growth factor secretion. Hypercalcemia may be seen in some sarcomas, though more common in other tumors. Urinalysis is typically unremarkable. Coagulation profile (PT, aPTT, platelet count) is important for surgical planning, especially if the tumor is large or invasive. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated, but are non-specific. Synovial fluid analysis is not typically performed unless the tumor involves a joint. For vaccine-associated sarcomas in cats, feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) testing may be recommended, though not directly related to the sarcoma.

Radiography: Soft tissue sarcomas appear as soft tissue opacities on radiographs. They may be well-defined or ill-defined, and may contain mineralization in some subtypes (e.g., extraskeletal osteosarcoma). Thoracic radiographs are essential for staging to detect pulmonary metastases, which typically appear as well-defined nodules. Abdominal radiographs may show organ displacement or a soft tissue mass. Ultrasonography: Useful for evaluating the internal architecture of the mass, its relationship to surrounding structures, and for guiding FNA or biopsy. It can also detect abdominal metastases. Computed Tomography (CT): Provides detailed cross-sectional images, allowing accurate assessment of tumor size, depth, and involvement of adjacent structures. CT is particularly valuable for surgical planning in complex areas such as the head, neck, and extremities. It can also be used for radiation therapy planning. Magnetic Resonance Imaging (MRI): Offers superior soft tissue contrast, making it ideal for evaluating tumors in the central nervous system, peripheral nerves, and musculoskeletal system. MRI can delineate tumor margins more accurately than CT, which is crucial for achieving complete excision. Angiography or fluoroscopy may be used to assess vascular invasion, but is rarely needed. Positron Emission Tomography (PET) is not widely available in veterinary medicine but can be used for staging in some referral centers.

Cytology: Fine-needle aspiration of soft tissue sarcomas often yields low to moderate cellularity with spindle-shaped cells arranged in clusters or individually. The cells may show mild to moderate anisocytosis and anisokaryosis, with oval to elongated nuclei and variable amounts of cytoplasm. However, cytology is often non-diagnostic for sarcoma due to the firm, cohesive nature of the tumor, and a biopsy is usually required for definitive diagnosis. Histopathology: The tumor is composed of spindle cells with variable collagen production, arranged in interlacing bundles or herringbone patterns. The histologic subtype is determined by the cell of origin and differentiation. Key features include cellular pleomorphism, mitotic index, necrosis, and invasion into surrounding tissues. Grading is based on the degree of differentiation, mitotic count (per 10 high-power fields), and percentage of tumor necrosis. Low-grade sarcomas are well-differentiated, have low mitotic counts (<5 per 10 HPF), and minimal necrosis. High-grade sarcomas are poorly differentiated, have high mitotic counts (>10 per 10 HPF), and extensive necrosis. Immunohistochemistry (IHC) can be used to determine the cell of origin: vimentin (mesenchymal), S-100 (neural), desmin and actin (muscle), factor VIII-related antigen (vascular). Surgical margins should be evaluated by inking the margins and assessing for tumor cells at the margin. A margin is considered clean if tumor cells are >1 mm from the inked edge.

The primary treatment for soft tissue sarcomas is surgical excision with wide margins. The goal is to achieve complete histologic margins, which requires a lateral margin of at least 3 cm and one fascial plane deep. For tumors on the limbs, this may necessitate amputation if the tumor is large or involves critical structures. For tumors on the trunk, wide excision with primary closure or reconstructive surgery (e.g., skin flaps, skin grafts) may be required. In cases where wide margins are not achievable, marginal excision followed by adjuvant radiation therapy is recommended. Radiation therapy is highly effective in controlling local disease, with local control rates of >85% at 1 year when combined with surgery. Chemotherapy (e.g., doxorubicin, ifosfamide) may be considered for high-grade sarcomas or those with metastasis, but its efficacy is limited. For vaccine-associated sarcomas in cats, aggressive surgical excision with wide margins (5 cm lateral and two fascial planes deep) is recommended, often followed by radiation therapy. Inoperable tumors may be treated with radiation therapy alone, but local control is inferior to surgery plus radiation. Palliative options include debulking surgery, pain management, and metronomic chemotherapy (e.g., cyclophosphamide and piroxicam).

The prognosis for soft tissue sarcomas is variable and depends on histologic grade, tumor size, depth, and completeness of excision. Low-grade sarcomas with clean margins have an excellent prognosis, with local recurrence rates of <10% and metastatic rates of <10%. Intermediate-grade sarcomas have a local recurrence rate of 15-20% and metastatic rate of 10-20%. High-grade sarcomas have a poor prognosis, with local recurrence rates of 30-50% and metastatic rates of 20-30%. Incomplete excision is associated with a high rate of local recurrence (up to 70%) if no adjuvant therapy is given. However, with radiation therapy, local control can be achieved in >85% of cases. The median survival time for dogs with soft tissue sarcomas treated with surgery alone is >2 years, but for high-grade sarcomas, it is <1 year. Cats with vaccine-associated sarcomas have a median survival time of 2-3 years with aggressive treatment, but recurrence is common. Negative prognostic indicators include high histologic grade, large tumor size (>5 cm), deep location, incomplete margins, and presence of metastasis at diagnosis.

Postoperative follow-up is essential to monitor for local recurrence and metastasis. Patients should be examined every 3 months for the first 2 years, then every 6 months thereafter. Thoracic radiographs should be repeated every 3-6 months for high-grade tumors, and every 6-12 months for low-grade tumors. Local recurrence is assessed by palpation and, if suspected, confirmed by FNA or biopsy. Surgical site healing should be monitored for complications such as seroma, infection, or dehiscence. Sutures or staples are typically removed 10-14 days postoperatively. If radiation therapy is administered, the skin should be monitored for radiation dermatitis, and the patient may require additional supportive care. Physical rehabilitation may be recommended for patients undergoing limb amputation or extensive reconstructive surgery. Long-term monitoring for metastatic disease is important, as metastasis can occur years after initial treatment.

Pearls: 1) Always perform a thorough physical examination and palpate the mass and regional lymph nodes. 2) Use a biopsy technique that does not compromise future surgical excision; the biopsy tract should be excised en bloc. 3) Plan surgical margins based on histologic grade and tumor location; for high-grade tumors, wider margins are necessary. 4) Use a marking pen to outline the intended excision margins before surgery. 5) During surgery, dissect along fascial planes and include the deep fascia in the excision. 6) Submit the entire excised mass for histopathology, and ink the margins to allow accurate assessment. 7) Consider adjuvant radiation therapy for incomplete margins or high-grade tumors. 8) For cats with vaccine-associated sarcomas, excise the tumor with 5 cm lateral margins and two fascial planes deep, and consider referral to a surgical oncologist. Pitfalls: 1) Inadequate surgical margins due to underestimating tumor extent. 2) Failure to obtain a biopsy before surgery, leading to misdiagnosis. 3) Not staging the patient for metastasis before surgery. 4) Closing the surgical site under tension, leading to dehiscence. 5) Not using drains when dead space is present, leading to seroma formation. 6) Ignoring the importance of histologic grade in treatment planning. 7) Performing a marginal excision without adjuvant therapy for a high-grade sarcoma. 8) Not monitoring for local recurrence or metastasis postoperatively.

Perioperative antimicrobial prophylaxis: Cefazolin (22 mg/kg IV) administered 30 minutes before incision, repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely indicated unless contamination occurred. Analgesia: Preoperative: Opioid (e.g., hydromorphone 0.05-0.1 mg/kg IV, or methadone 0.1-0.3 mg/kg IV) and a non-steroidal anti-inflammatory drug (NSAID) (e.g., carprofen 2.2 mg/kg PO q12h, or meloxicam 0.1 mg/kg PO q24h). Intraoperative: Fentanyl CRI (5-10 mcg/kg/hr IV) or lidocaine CRI (25-50 mcg/kg/min IV) for multimodal analgesia. Postoperative: Continue opioids (e.g., buprenorphine 0.01-0.02 mg/kg IV q6-8h) for 24-48 hours, then transition to oral NSAIDs for 3-5 days. Local anesthetic blocks: Incisional line block with bupivacaine (1-2 mg/kg) or lidocaine (2 mg/kg) at the surgical site. For limb tumors, a brachial plexus block or epidural may be used. Chemotherapy: Doxorubicin (30 mg/m² IV every 3 weeks) for high-grade sarcomas or metastatic disease. Ifosfamide (375 mg/m² IV every 3 weeks) may be used in combination, but requires aggressive hydration and mesna. Metronomic chemotherapy: Cyclophosphamide (10 mg/m² PO q24h) and piroxicam (0.3 mg/kg PO q24h) for palliative treatment. Antiemetics: Maropitant (1 mg/kg IV or PO q24h) if chemotherapy-induced nausea. Gastroprotectants: Omeprazole (1 mg/kg PO q12h) if NSAIDs are used long-term.

Several landmark studies have shaped the management of soft tissue sarcomas in veterinary patients. A seminal study by Kuntz et al. (1997) evaluated prognostic factors in dogs with soft tissue sarcomas and found that histologic grade, tumor size, and completeness of excision were significant predictors of local recurrence and metastasis. Another study by Dernell et al. (1998) demonstrated that marginal excision combined with radiation therapy provided local control rates of >85% in dogs with soft tissue sarcomas, comparable to wide excision alone. For cats, a study by Hershey et al. (2000) on vaccine-associated sarcomas showed that aggressive surgical excision with 5 cm margins and two fascial planes deep significantly reduced local recurrence. A more recent study by Ehrhart et al. (2013) evaluated the use of CT for surgical planning and found that it improved the ability to achieve clean margins. The Veterinary Society of Surgical Oncology (VSSO) has published consensus guidelines on the management of soft tissue sarcomas, recommending wide surgical excision as the primary treatment, with radiation therapy for incomplete margins or high-grade tumors. Chemotherapy is not routinely recommended for low-grade sarcomas but may be considered for high-grade or metastatic disease. Overall, the evidence supports a multimodal approach for high-risk tumors, with surgery and radiation therapy being the cornerstones of treatment.