Soft Tissue Sarcomas and Mast Cell Tumors

Chapter 28 Soft Tissue Sarcomas and Mast Cell Tumors

Joanne C. Graham

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

• Sarcomas have developed at sites of chronic tissue inflammation. Implants, particularly metallic orthopedic implants, are known to cause sarcoma development at the implant site. It is believed that this is the result of a foreign body reaction in the tissues rather than a direct carcinogenic effect of the implant. Most of these tumors are osteosarcomas, although fibrosarcomas and undifferentiated sarcomas have been reported.

• Both single and chronic traumatic episodes have been associated with intraocular sarcomas in the cat.

• Chronic inflammation caused by the parasite Spirocerca lupi has resulted in sarcoma development in the esophagus of infected dogs.

• Recently, certain soft tissue sarcomas in cats have been associated with the administration of vaccinations and medication (lufenuron). The exact incidence is unknown but may be greater than 1 in 10,000 vaccinated cats. The mechanism behind injection-associated sarcoma development is under investigation but may involve chronic inflammation induced by the injection combined with predisposing host factors.

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.

Myxosarcoma

Fibrosarcoma-like tumor with a mucinous matrix

• Rare in dogs and cats

• Infiltrative; no site predilection

• Metastasis is uncommon

Lymphangiosarcoma

Tumor of lymphatic endothelial vessels

• Rare in dogs and cats

• May be invasive

• Metastasis is rare

• Draining tracts on skin or edema are possible presenting signs

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.

• Soft tissue sarcomas are more common in older animals (mean age 9 years).

• Because of the widespread distribution of mesodermal tissues in the body, soft tissue sarcomas may occur in almost any anatomic location, including the abdomen.

• Tumors may become quite large before any clinical signs are apparent. They often are noticed first by the owner, either by observation or by handling the animal.

• Certain types of sarcoma may invade bone, causing lameness, or may obstruct lymphatics, causing edema.

• GI leiomyosarcomas may cause signs of GI obstruction (vomiting) or melena.

• Smooth muscle sarcomas of the bladder may cause hematuria, pollakiuria, or dysuria.

• Hemangiosarcomas may cause a variety of clinical signs, including collapse due to tumor rupture and hemorrhage.

• Injection-site sarcomas arise in injections sites.

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.

Diagnostic Imaging

Ultrasonography

• Use this technique to help determine the character, consistency, and extent of the primary mass.

• Ultrasound procedures are especially useful in the examination of abdominal masses and to examine the internal organs for intra-abdominal metastasis.

Tags: Saunders Manual of Small Animal Practice

Aug 27, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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