Soft Tissue Sarcoma Sts

Soft Tissue Sarcoma (STS)

By Dr. Alice Villalobos, October 2, 2010

Soft tissue sarcomas (STS) are invasive growths that are composed of fibrous connective tissue cells which have mutated into bizarre spindle cells. Unfortunately most STS's are difficult to remove entirely because the growth has tentacles that extend and invade into the normal appearing tissue or bone surrounding the mass. It is not unusual for a pet to have an incompletely or inadequately removed STS. Many tumors are removed without having enough normal tissue surrounding the mass to prevent recurrence. With residual cells present, they often grow back with a vengeance. We often consult with a board certified surgeon to remove STS for adequate tissue margins of 2-3 cm surrounding the mass.

Soft tissue sarcomas (STS) may grow anywhere but are seen most commonly in the skin and subcutaneous tissues. STS is often not suspected and often underestimated. All soft tissue sarcomas should be considered malignant. Unfortunately many were given kind names ending in "oma." STS's often kill senior and geriatric dogs because they are initially underestimated.

List of STS Cell Types

Hemagiopericytoma: This blood-filled tumor is likely to recur and grow quite large.
Fibrosarcoma: Likely to recur and be locally invasive. Cats may develop it from inflammation.
Schwannoma (nerve sheath tumor): Likely to recur and be locally invasive.
Neurofibrosarcoma: Likely to recur and be locally invasive.
Hemangiosarcoma: Highly metastatic (spreads to lungs & other organs in 4-8 months)
Mast Cell Tumor: Often recurs and can spread; the most fatal of all skin tumors in dogs.
Melanosaroma (malignant melanoma): Generally spreads to lungs in 10-14 months.

It may be necessary to use special "IH" stains to identify the cell type.
The IH stains are often important, if the result will influence therapy.

Behavior of STS Cell Types

Hemangiopericytoma and Schwannoma (nerve sheath tumor) neruofibrosarcoma may behave insidiously. If the mass is on a leg, wide surgical margins are nearly impossible without a graft.

Fibrosarcoma (FSA) is a brutal killer. It may fool the best doctor because it recurs even after the best planned surgeries. Be aware of the "firboma" growing on the muzzle of golden retrievers and Dobie's which may disfigure the muzzle. If not arrested when small, it is often fatal

Hemangiosarcomas generally grow internally and are fatal within a year. See the HSA handout. The skin form is not aggressive (caused by solar radiation) and can be treated with cryotherapy. The STS form appears under the skin, is more aggressive, and requires wide surgical margins.

Mast Cell Tumors behave variably and many recur. Please see the handout on MCT.

Melanosarcoma and squamous cell carcinoma (SCC) share similarities. It is important to run the IH stains to differentiate these two types of cancers because their biological behavior and treatment is quite different. Both may appear in the mouth an in nail beds mimicking osteomyelitis (bone or tooth root infections). SCC is solar induced on white dogs and is classified as a carcinoma. If they appear in the mouth, please read the handouts for oral tumors.

Whenever a toe or claw is festering in a senior dog, x-rays and a deep punch biopsy into the nail bed and abnormal bone using a 2mm punch instrument is warranted at the first visit. Antibiotics may control infection but it is best to be suspicious and get the diagnosis on the first visit.

The biopsy will usually find either squamous cell carcinoma or malignant melanoma of the digit. Cancer mimics many conditions! It is best to check the local lymph nodes for metastases. In the front limb, the prescapular node is more likely to contain metastatic cells from digital cancer than the axillary lymph node. The popliteal node drains hind leg digital tumors, however, check the inguinal nodes and use x-rays or ultrasound to check the sub lumbar nodes for metastasis.

Feline Vaccine Associated Sarcoma (FVAS)
Some cats are genetically prone to develop mutations of inflammatory tissue that may transition into STS (Feline Vaccine Associated Sarcoma). It has a high rate of fatality due to persistent recurrence within one year (>90%) after surgery or >50% after surgery and radiation. For clients who cannot afford radiation therapy, intralesional chemotherapy is an option worth pursuing.

Treatment Options for STS

Aggressive surgery followed by radiation therapy or intralesional chemotherapy (IC) is used to battle STS. MRI or CT scans are helpful before surgery. We recommend follow up therapy for STS following surgery even when fairly wide margins were achieved. Radiation therapy is considered the treatment of choice to "sterilize" the tumor bed. However, if one cannot afford the cost of radiation therapy, or if the pet is older or has co morbid conditions, risking complications, IV or IC may help slow recurrence and/or eradicate the residual cells. STS's are treated with carboplatin or adriamycin IV every 21 days for 4-6 treatments and tapered over the year. Metronomic (low dose) chemotherapy is also used to slow down blood vessel formation (anti angiogenesis) in the tumor.

IC is easier on older pets and less costly. We use a small dose of drug because it is targeted into the actual tumor bed or into the tumor. Some pets need complete sedation to avoid discomfort. The most common drugs that are used successfully in our practice for IC are bleomycin and neoplasene. In addition, we use (mixed with patient's serum): dexamethasone (for mast cell cancer), carboplatin (for sarcomas & melanosarcomas) and 5-F-U for SCC in dogs (never in cats).

Cryotherapy (freezing with liquid nitrogen) is also used to kill all cell types of STS's, if appropriate.

Dogs with melanosarcoma, can be given the special vaccine, Onceptâ„¢. It is most successful after complete removal of the primary mass. Masitinib is added to protocols for mast cell cancers.


Soft Tissue Sarcomas are often fatal in pets. This is because most pet owners do not recognize the mass because it is camouflaged in the fur. This results in many late diagnoses. By that time, the sarcoma cells may have invaded important structures. Another cause is when insufficient or conservative treatment was elected when the sarcoma was initially operable. When recurrence appears, the cancer is more invasive. For best results, we need to form a team with a vigilant pet owner and aggressive combination treatment at the first opportunity.

Pet owners need more education to recognize tumors and to be suspicious early on. They need to take their pets to their veterinarian during the earliest stages of their pet's cancer. Practitioners need to be more suspicious and perform more aggressive "chop" FNA's and tru cut biopsies during that first visit to get the diagnosis and refer consultation with an oncologist . We work within our client's budget to design a therapy program which maintains quality of life for the pet as we try to cure the cancer, slow its recurrences or arrest its growth. We use integrative medicine to support function and boost the immune system. We also conduct clinical trials using promising new agents which may help win the battle for more quality & life for our beloved cancer patients.

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