Vaccine-associated Sarcoma in Cats
What are Soft Tissue Sarcomas?
Soft tissue sarcomas are tumors originating from structural and connective tissues. They are most often evident as masses in the skin or subcutaneous tissues. Although seemingly diverse in their tissues of origin, these tumors share similar biologic behavior and are therefore considered together. Examples of soft tissue sarcomas include fibrosarcoma, malignant fibrous histiocytoma, and myxosarcoma.
Soft tissue sarcomas are locally invasive tumors and readily invade adjacent normal issues with finger-like projections. While they are locally invasive, spread to other places in the body (called metastasis) is less common; the overall metastatic rate is between 11-23%.
Diagnostic tests are recommended for all cats diagnosed with soft tissue sarcomas to determine which sites in the body are involved. These tests include:
- Complete blood count (CBC)
- Serum biochemistry panel
- Urinalysis (U/A)
- Lymph node aspiration
- Thoracic radiographs
- +/- Abdominal sonogram (indicated for tumors on the caudal portion of the trunk or proximal hindlimbs)
There are several treatment options for soft tissue sarcomas. Ultimately the decision to pursue a particular treatment is based on the tumor location, stage, clients' wishes and consultation with a veterinarian or veterinary oncologist. The following is a list of commonly discussed treatment options:
Option 1 - Surgery Complete excision with adequately wide margins generally involves resection of approximately 3cm margins laterally and removal of one intact fascial plane deep to the tumor. When these margins can be achieved, surgery is generally considered the treatment of choice. Although in cats, this can be very difficult unless the tumor is very small or located on a distal extremity. Because of this difficulty, one study of vaccine-associated sarcomas reported a median time to recurrence of 3 months.
Option 2 - Surgery + radiation therapy This involves the combination of surgery to remove the obvious bulk of the tumor (macroscopic disease) + radiation therapy to destroy microscopic disease. If multimodality therapy is a possibility, we recommend that cats be evaluated before any surgery (i.e., with gross disease present) by both a radiation oncologist and a surgeon so that an ideal treatment plan can be formulated. Because of the need for very wide excision (relative to body size) to effectively treat these tumors, many cats are optimally treated with radiation therapy prior to surgical removal. In our hospital, radiation therapy consists of 16-19 treatments given on a Mon-Friday schedule. Potential side effects of radiation include an early, reversible reaction to rapidly dividing tissues (skin, hair) that develops midway through treatment and is generally very mild in cats. There is also the potential, although unlikely, for late irreversible side effects to slowly dividing tissues (bone, muscle, nerves, underlying organs). A CT scan is performed prior to radiation therapy to aid the radiation oncologist in planning treatment for the individual animal in a way that optimizes the effect against the tumor and minimizes the impact on adjacent normal tissues. With this combination, the risk of recurrence is decreased and delayed with median control times of approximately 19-20 months. In a study reported by NCSU, the impact of surgical margins when surgery was performed after radiation therapy was demonstrated. In cats where clean margins were achieved following radiation therapy, the median disease control was approximately 3 years, compared to approximately 10 months for cats with dirty margins.
Option 3 - Chemotherapy While generally used for tumors with a higher likelihood of metastasis, chemotherapy can be considered for cats with vaccine-associated sarcomas as another tool for targeting cancer cells. The drug of choice for treating soft tissue sarcomas is doxorubicin. It is given at 3-week intervals for a total of 4-6 treatments. Chemotherapy drugs work by targeting fast-growing cells; cancer cells divide faster and abnormally compared to normal cells. Because of this, chemotherapy preferentially destroys cancer and spares normal tissues. There is the potential for some normal tissues with rapid growth rates (intestinal lining, bone marrow, hair cells) to be transiently damaged by chemotherapy. However, our goal in treating cancer in animals is quality of life, so the dosages are lower than those used in people. We do not normally see significant toxicity with chemotherapy. There is always some risk when a drug is given for the first time, and the oncologist will discuss what types of side effects may develop and what to watch for. If the pet has no problems the first time they receive a drug, they should have no problems with subsequent treatments of that drug. If the pet experiences any side effects, we address them as needed and lower the dosage of the causative drug for future treatments to prevent recurrent problems. A unique side effect of doxorubicin is its ability to weaken the kidneys with multiple doses. Periodic blood tests will be done to monitor this.
Option 4 - Palliative radiation therapy If a tumor cannot be removed surgically, cannot be treated with the combination of surgery + radiation therapy, or if distant metastasis is identified, palliative radiation therapy can be considered. Palliative radiation therapy consists of two consecutive twice-daily doses of radiation therapy and is aimed at alleviating pain, inflammation, and swelling associated with the tumor. We recommend that the treatment be repeated in one month if no disease or symptom progression is noted in hopes of extending the duration of response.
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