By CHRISTINE ANDERSON, DVM, MS, DACVIM (ONCOLOGY), DACVR (RADIATION ONCOLOGY) PITTSBURGH VETERINARY SPECIALTY AND EMERGENCY CENTER
Canine soft tissue sarcomas
Soft tissue sarcomas (STS) are a collection of tumors of connective tissues, most commonly located in the skin and subcutaneous tissues. They are grouped together because they appear similar microscopically to the pathologist and have similar biological behavior: potentially aggressive and invasive at the site of the primary tumor but with a less than 10-15% rate of spreading to other locations in the body for low and intermediate grade tumors. High grade tumors can have an up to 50% rate of metastasis. STS include fibrosarcomas, peripheral nerve sheath tumors, hemangiopericytomas, myxosarcomas and liposarcomas. There are other types of sarcomas that occur in dogs that are not included in the category of STS because they either have a higher metastatic rate or typically arise in other locations in the body. STS usually occur in middle age to older dogs. No specific breed or sex is predisposed to STS, but they are more common in large breed dogs.
Typically, a mass is noted by either the owner while petting their dog or by a veterinarian during a routine physical exam. The mass is usually firm and attached to the underlying tissue rather than mobile in the skin. No mass can be confirmed to be non-cancerous on exam alone. The only method to diagnose a skin or SQ mass is to obtain a tissue sample either with a fine needle aspirate (FNA) or biopsy. While a FNA can be done in seconds with the pet awake, they may be non-diagnostic for STS up to 40% of the time. A biopsy, where a larger tissue sample is obtained, may be required. When STS metastasize, they spread to regional lymph nodes or the lungs, so careful palpation of the lymph node, with FNA and cytology of the lymph node when possible, and chest x-rays should be performed. Depending on location, abdominal ultrasound may also be recommended. Advanced imaging of the primary tumor with CT or MRI can be very helpful in planning the surgical approach as the true extent of the tumor may not be evident on physical exam.
Because of the low metastatic rate, treatment is aimed at removing or controlling the primary tumor. The ideal treatment, when possible, is complete surgical excision which can be curative. Because these tumors are invasive into the surrounding tissues, in order to remove the entire tumor the surgeon will attempt to remove the mass with adequate tissue around the mass, known as a margin. For this reason, it is ideal that the veterinarian is aware that the mass is a sarcoma prior to surgery to best plan the surgical approach. After surgery, the entire surgical specimen should be submitted to a pathologist to determine if the mass has been completely excised and to determine the grade (ranging from 1- 3, or low, intermediate, and high). In some cases, due to the size or location of the tumor, complete surgical removal is not possible and tumor cells are left behind. In these cases additional therapy may be warranted to prevent or delay tumor regrowth.
The best outcomes for incompletely excised STS are achieved with radiation therapy (RT). Eighty-five percent of incompletely removed STS are controlled long term with RT. The one year local control rate with radiation alone (no surgery prior to treatment) is approximately 50%. Definitive radiation therapy is typically administered as 18-20 daily treatments (Monday-Friday) over 3 to 4 weeks. Patients need to be anesthetized for each treatment to ensure they remain still in the correct position, but very short acting anesthesia drugs are used so patients are usually fully awake within an hour of their treatment. Side effects from radiation usually develop in the last week of treatment and include hair loss, redness and ulceration of the skin at the radiation site, like a bad sunburn. The pain can be managed with topical and oral pain medications, but some yemporary discomfort is likely. Given the hope and expectation of long term tumor control, this short term discomfort is considered acceptable. Veterinary patients do not experience nausea and lethargy from radiation therapy as some humans do.
Cisplatin (chemotherapy) beads
Another option is to place beads containing the chemotherapy drug cisplatin under the skin at the surgical site after surgical removal of the mass to try to kill any remaining cancer cells in the area. According to unpublished data, 90% of patients have tumor control at one year, and 70% have tumor control two years after bead placement. Most animals tolerate this procedure very well, but inflammation at the bead sites can occur.
Low dose continuous chemotherapy/metronomic chemotherapy
Another possible treatment option following surgery is low dose continuous oral chemotherapy known as metronomic chemotherapy. Understanding of how metronomic chemotherapy works to treat cancer is evolving. There is less data regarding the efficacy of this treatment, but in one study of dogs with STS, the median time to tumor regrowth was 13.5 months compared to 7 months in dogs treated with surgery alone. The metronomic chemotherapy protocol involves oral chemotherapy given at home, and a non-steroidal anti-inflammatory medication (NSAID). While chemotherapy can cause lethargy, vomiting or diarrhea, and low white blood cell counts, these side effects are rare in patients on metronomic chemotherapy. Patients on metronomic chemotherapy protocols are seen for regular recheck appointments, including physical exams and blood work to monitor changes in physical condition.
Soft tissue sarcomas can be effectively cured in many patients, especially with early diagnosis and effective treatment. Many skin and subcutaneous masses are non-cancerous, but this cannot be determined on physical exam alone. “Lumps and bumps” on your pets should always be evaluated by a veterinarian with FNA and cytology, but masses that are increasing in size are particularly concerning.
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