Overview

Soft tissue sarcomas (STS) are a group of tumors that arise from mesenchymal (connective) tissues, such as muscle, fat, nerves, blood vessels and fibrous tissues. Despite coming from different types of tissue, these tumors all behave similarly and are therefore grouped together. They most commonly appear as a soft or firm lump growing under the skin of dogs. These tumors are typically locally invasive, meaning the tumor cells often extend microscopically beyond the visible or palpable edges of the mass. While many STS grow slowly and are non-painful, others grow more quickly and may become problematic due to their size, location, or invasive nature. Although a definitive cure is not always possible, early detection and treatment are essential for improving outcomes.

Cause

STS originate from connective tissues and include tumor types such as fibrosarcoma, peripheral nerve sheath tumor, myxosarcoma, among others. The exact cause of STS in dogs is often unknown. However, in some cases, the development of STS may be associated with previous trauma, radiation exposure, foreign bodies, and surgical implants.

Clinical Signs

STS appears as a lump or mass under the skin on your dog’s body, which can vary in its growth rate and texture. Some masses may feel firm and closely attached to the underlying structures, while others may be softer and more movable. Clinical signs can range from a slow-growing, painless mass to one that seems to grow rapidly. Some signs will depend on the tumor's location, such as difficulty walking if a mass obstructs limb movement or pain if the tumor compresses a nerve. Some tumors may develop open skin wounds that cause discomfort or infection.

Diagnosis

Initial evaluation often includes a fine needle aspirate (FNA), a minimally invasive test that collects cells with a small needle for evaluation under a microscope. While this test can help rule out other tumor types, it may be inconclusive for STS, as these tumors are very solid and tend to exfoliate cells poorly.

A definitive diagnosis of STS often requires a biopsy. An excisional biopsy (removal of the entire mass) may be recommended for small, movable tumors in easily accessible surgical locations. However, if the mass is large, an incisional biopsy (removal of a portion of the tumor) may be recommended for larger, fixed masses in parts of the body that may be difficult to operate on. 

You may be referred to a veterinary oncologist or surgeon to assist with diagnostics and treatment of STS, particularly for tumors in challenging locations (e.g., on the limbs, or in locations where wide surgical margins are difficult). 

Staging tests may be recommended to assess overall health and detect potential metastasis: 

• Blood and urine tests
• Chest x-rays (radiographs), abdominal ultrasound
• CT scan to assess the extent of the tumor and plan for surgery

Treatment

The mainstay of treatment is surgical excision, ideally with wide margins. Depending on the tumor size, location and grade, additional therapies may be recommended. 

• Surgery:
  • Requires aggressive surgical removal to reduce the risk of recurrence
  • Referral to a specialist may be needed
  • Incomplete removal increases the risk of local recurrence
• Radiation therapy
  • May be used before surgery to shrink a tumor, or as the primary treatment if surgery is not possible
  • May also be used after surgery for incompletely removed tumors to help prevent or delay recurrence
• Chemotherapy
  • May be considered for high-grade tumors (based on biopsy results) to manage the risk of metastasis
  • Response rates for STS vary, and it is often used in combination with other treatments

Outcome

The prognosis for dogs with STS varies depending on the tumor's grade, location and the completeness of surgical excision. Incomplete excision increases the likelihood of recurrence. The tumor's grade (histologic grade) determined after biopsy reflects the tumor’s aggressiveness. It is based on the cell’s appearance under a microscope and mitotic index (MI), which indicates how quickly tumor cells divide. A lower MI is associated with better outcomes compared to a high MI, which tends to behave more aggressively.

Certain types of STS and histologic grades are more likely to metastasize than others, with the lungs being the most frequent site.

• Low grade tumors: ~10% metastatic rate
• Intermediate grade: ~20%
• High grade: up to 40%

Low and intermediate-grade tumors often respond well to surgery and radiation therapy. High-grade and recurrent tumors are often more challenging to control. They usually have a poorer prognosis and may require a combination of treatments, including surgery, radiation therapy, and chemotherapy. 

After surgical removal, STS requires regular re-evaluation for local tumor recurrence or signs of metastatic disease. This often involves exams and radiographs at set intervals, such as every three months. 

Updated May 2025