Soft Tissue Sarcoma in Dogs and Cats
Spindle cell sarcomas are also known as soft tissue sarcomas (STS) and in general, this term acts as an umbrella for a variety of tumors including (fibrosarcoma-connective tissue, chondrosarcoma-cartilage, liposarcoma-fat tissue, hemangiosarcoma-blood vessel cells, nerve sheath tumor, hemangiopericytoma-cells supporting blood vessels, malignant fibrous histiocytoma). These tumors range in their behavior based upon their cell of origin (see above) as well as their grade, which correlates to the aggressiveness of the tumor. Most tend to be low to moderate grade but exceptions exist. These tumors often grow slowly and grow passively along tissue planes. The rate of metastasis (spread to other organs) depends on the grade of the tumor with low and intermediate grade tumors metastasizing in <20% of dogs whereas high-grade tumors metastasize in 40-50% of dogs.
STS are often first noted as a lump by the owners that often grow slowly over time. Most often the mass feels fixed to the underlying tissue and its borders are indistinct. The best description of these tumors is an “octopus” in which the head of the octopus is the area of the tumor you can feel but the tumor has tentacles (tendrils) that can go much deeper (up to 2 inches) out into the surrounding normal tissue. The tendrils of the tumor are actually microscopic cells that are the most active part of the tumor and lead to re-growth when not surgically removed. Patients are staged with chest x-rays, blood work (CBC, serum chemistry), and abdominal ultrasound (if necessary). These tests allow us to determine if the cancer has spread to other areas in the body, which is dependent on the type/grade of the tumor.
Treatment for STS involves surgery that may be incomplete because of the local microscopic invasiveness; however, in cases where a tumor is on a limb, surgery can be curative with an amputation. In cases where the tumor is incompletely removed, we will often recommend either a second surgery (if feasible) or radiation therapy. Studies have shown that following up with radiation therapy can provide excellent local control (the majority of dogs have survival beyond 4-5 years). In cases where a tumor is a high grade or it is the type of tumor that has a great chance to spread, we often recommend chemotherapy in addition to the local therapy. Most often this entails adriamycin (IV) given once every 3 weeks for 5 treatments. At the end of the chemotherapy, dogs are restaged and continuously monitored for local recurrence. Dogs diagnosed with high-grade tumors have an average prognosis of 1.5 years due to the more aggressive features associated with these tumors.
STS that occurs in the oral cavity (mouth) tend to be more aggressive with higher rates of local recurrence. Treatment for these tumors often involves bone removal and minor reconstructive surgery followed by radiation. With aggressive therapy, 50% of dogs remain tumor-free 1.5-2 years later. STS that occurs internally on organs (liver, spleen, etc), tend to be more aggressive, high-grade tumors with a rate of metastasis that is approximately 50%. Removal of the tumors, when possible, is performed and these cases are almost always followed with chemotherapy. The average survival time is 6-12 months following the appropriate therapy.
Palliative therapy can be used in lieu of aggressive/curative-intent treatment options in cases where the tumor is not considered to be operable. These treatment options include radiation therapy, chemotherapy, medical therapy with metronomic chemotherapy (see below), and pain medications as deemed necessary. Approximately 1/3 of dogs have a favorable response to therapy and 1/3 of dogs will have tumor stabilization. The average survival time for patients treated with solely palliative therapy is typically between 6 months but mostly based on the presenting circumstances and response to therapy.
Metronomic therapy is sometimes used in addition to the “standard therapy” and consists of giving an NSAID (such as Deramaxx or Rimadyl) along with a low daily dose of a chemotherapy drug called Cytoxan. This type of therapy is administered orally and is designed to kill the growing blood vessels that feed tumors. Tumors produce growth factors that cause the growth of nearby blood vessels (angiogenesis) which then feed the tumor cells. These blood vessels are sensitive to chemotherapy at very low dosages. A recent study evaluated metronomic chemotherapy to delay soft tissue sarcoma recurrence if the tumors are incompletely removed. The study successfully demonstrated that dogs treated with metronomic chemotherapy had a significantly longer disease-free interval than dogs that did not receive chemotherapy (albeit this is only one study and not considered to be the “gold-standard”). In this series of patients, it was found that incompletely excised tumors recurred following surgery alone in close to 100% of the cases. The average time to recurrence is 8-9 months. The use of metronomic chemo for its immunomodulatory and anti-angiogenic effects delayed recurrence to close to 1.5 years and not all tumors will recur with this type of therapy.