Soft Tissue Sarcomas in Dogs
Soft tissue sarcomas (STS) make up a large category of tumors that arise from connective tissue. This category includes tumors of fibrous tissue, fat, smooth muscle, nerves, and lymphatic vessels. The diagnosis of soft tissue sarcoma includes fibrosarcomas, malignant peripheral nerve sheath tumors, histiocytomas, myxosarcomas, liposarcomas, lymphangiosarcomas, and undifferentiated sarcomas. STS comprise ~15% of all skin/subcutaneous (under the skin) tumors. These tumors are typically very invasive to the surrounding tissue but generally have a low risk of spreading (metastasis).
- Tumor location plays an important role in the behavior of STS. STS of the oral cavity or areas that are difficult to do surgery in (such as the axilla or “armpit”) are more aggressive than tumors of the trunk or extremities.
- STS of the trunk and extremities behave similarly in that they are invasive into the surrounding tissue, often extending far beyond gross margins—where you can feel or see the tumor. Microscopic tumor cells can extend up to 5cm beyond the visual tumor.
- When biopsied, STS are given a grade by a pathologist based on features seen under the microscope, including mitotic index (amount of actively dividing cells), percent of tumor necrosis (amount of dying cells), and degree of differentiation (how similar the cells are to normal cells). Based on these factors, a tumor is given a grade (1, 2, 3, OR low, intermediate, high). Tumor grade is highly predictive of how the tumor is going to behave, with higher grade tumors having a higher chance to spread and re-grow locally. Metastasis (spread to a distant organ) occurs in ~10% of low grade tumors, ~20% of intermediate grade tumors, and up to 50% of high grade tumors.
- Wide surgical excision is always the treatment of choice when feasible. The completeness of this surgical removal is determined by the pathologist by microscopically examining all surgical margins.
- Recurrence of the tumor is 10 times more likely in patients with histologically incomplete surgical margins than patients undergoing wide surgical resection with adequate margins.
What role does radiation therapy play in the treatment of STS?
- Radiation therapy is very effective at preventing tumor recurrence when performed following incomplete resection of low to intermediate grade STS. Local control of 5 years or longer can be achieved in many cases. Radiation therapy alone (without surgery) is less effective for treating tumors that have not been surgically removed.
- Radiation therapy can also be used in a palliative (pain relief) setting for tumors that are not amenable to surgical excision or have failed other forms of therapy.
What role does chemotherapy play in the treatment of STS?
- Intravenous chemotherapy may be of benefit in 2 situations:
- Used prior to surgery to help shrink a very large intermediate or high grade STS
- Following complete resection of a high grade STS to prevent metastasis
- The prognosis often depends upon the grade and the location of the tumor. For low grade tumors that are able to resected with clean wide margins, the prognosis is excellent with most dogs living disease free for over 5 years.
- If a complete, wide surgical excision is not possible; radiation therapy is very effective in the treatment of these tumors.
- The importance of angiogenesis (blood vessel growth) for the development of the primary tumor and distant metastatic disease is well understood for all cancers including STS. Using drugs to slow down or stop this blood vessel growth is called metronomic chemotherapy and involves low dose oral chemotherapy in conjunction with a non-steroidal anti-inflammatory agent. It is designed to effectively delay tumor re-growth (in incompletely excised tumors) and possibly prevent or delay metastasis (spread).