Chapter 28 Soft Tissue Sarcomas and Mast Cell Tumors
SOFT TISSUE SARCOMAS
Soft tissue sarcomas are tumors that arise from mesodermal tissue. They make up 14% to 17% of all malignancies in the dog and approximately 7% to 9% in the cat. These tumors are non-epithelial and extraskeletal and may arise from fibrous tissue, adipose tissue, muscle, synovial tissue, and from blood and lymph vessels. Schwannomas, nerve sheath tumors, or neurofibrosarcomas arise from primitive ectodermal tissues but are included in the soft tissue sarcoma category because of similarities in location, clinical presentation, and clinical behavior. Soft tissue sarcomas are classified histologically according to the specific tissue of origin. However, some tumors are so undifferentiated that this classification is difficult. These tumors are appropriately named undifferentiated sarcomas.
Etiology
The etiology of most soft tissue sarcomas remains unknown. Several causes and predisposing factors have been suggested. These include genetic predisposition, viral agents, chemical carcinogens, ionizing radiation, foreign body implantation, trauma, parasites, and injections of vaccines or medication.
Genetic Predisposition
This is suspected to play a role in tumor development because certain breeds of dogs have a higher incidence of sarcomas. These breeds include boxers, German shepherds, Great Danes, Saint Bernards, golden retrievers, basset hounds, and flat-coated retrievers.
Viral Agents
Viruses have been implicated as causes of sarcoma development in rodents, poultry, non-human primates, and cats. Feline sarcoma viruses (FeSVs) are replication-defective variants of the feline leukemia virus (FeLV). These retroviruses together induce formation of multicentric fibrosarcomas in young cats (see Chapter 8). In contrast, solitary fibrosarcomas found in older cats usually are not associated with FeSV.
Chemical Carcinogens
Chemical carcinogens and environmental contaminants have been shown to induce sarcomas in rodents and humans. Although this has not been documented in dogs and cats, it likely occurs.
Ionizing Radiation
X-rays, gamma rays, and particulate radiation have been shown to cause sarcoma development. In dogs, sarcomas have been reported to occur at treatment sites after orthovoltage radiotherapy of acanthomatous epulides.
Chronic Tissue Inflammation
Biologic Behavior
In general, soft tissue sarcomas are locally invasive and infiltrative along fascial planes, resulting in poorly defined tumor margins. The metastatic rate varies according to tumor grade, with low-grade tumors often slow to metastasize. Metastasis is usually via hematogenous spread to the lungs and liver. Regional lymph node metastasis is uncommon. A brief description of various soft tissue sarcomas follows.
Hemangiopericytoma
Tumor of pericytes, spindle-shaped, contractive cells that surround precapillary arterioles
Schwannoma, Neurofibrosarcoma, or Peripheral Nerve Sheath Tumor
Tumor of the nerve sheath or Schwann cell
Malignant Fibrous Histiocytoma
Tumor containing a mixture of fibroblast-like cells and histiocyte-like cells
Feline Injection-Site Sarcomas
Most are fibrosarcomas, but the category also includes rhabdomyosarcomas, leiomyosarcomas, chondrosarcomas, osteosarcomas, malignant fibrous histiocytomas, and undifferentiated sarcomas
Clinical Signs
Clinical signs depend on the location, size, and degree of invasiveness of the tumor as well as on the presence and degree of metastatic disease.
Diagnosis
The goals of diagnosis are to identify the histologic type and grade of the primary tumor, to delineate the extent of the tumor, and to determine whether metastatic disease is present. Although history, physical examination, laboratory evaluation, and diagnostic imaging provide valuable information, the only way to obtain a definitive diagnosis is through biopsy and histopathologic evaluation of the tumor. Histologic grade is useful in predicting tumor behavior and prognosis for dogs with soft tissue sarcomas.
History
The history helps determine how long the mass has been present and the rate of growth. Also important is information regarding exposure to carcinogens, recent vaccination, and past traumatic incidents. Concurrent systemic disorders may be present if metastatic disease or paraneoplastic syndromes have developed.
Physical Examination
Perform a thorough examination to determine the number of masses and their physical characteristics.
Clinical Pathology
Evaluate a complete blood count (CBC), platelet count, serum biochemical profile, and urinalysis in animals with soft tissue sarcomas. Although results are often unremarkable, in some cases these may suggest the presence of metastatic disease, paraneoplastic syndromes, or other concurrent disease. The values obtained also provide a baseline for future therapy. Abnormalities may include the following:
Diagnostic Imaging
Radiography

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