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PRIMARY BONE TUMORS OF CATS

Incidence and Risk
Osteosarcomas
Cancer involving bones of cats is rare. An estimate of the incidence of all bone tumors in cats is 4.9 per
100,000. Anywhere between 67% to 90% of bone tumors in cats are histologically malignant, and tumors occur in long bones approximately twice as often as in axial skeleton sites. The hind limbs are affected nearly twice as often as the front limbs. Osteosarcomas account for 70% to 80% of all primary malignant cancer of cats. The disease in cats differs from that in dogs in that the primary lesions occur more often in hind limbs in cats, and the disease is far less metastatic than in dogs. Osteosarcoma generally affects older cats (mean 10.2 years), but the age range of reported cases is large (1-20 years). In one study, males outnumbered females, but the opposite was found in a second study. Osteosarcoma was reported to arise after a limb fracture was repaired with an intramedullary pin in one cat. A case of suspected radiation-induced osteosarcoma has been reported in a cat.

Multiple cartilaginous exostosis (MCE) is a disease that occurs after skeletal maturity in cats. This is in contrast to the disease in dogs, where exostoses develop before closure of growth plates. Also in contrast to the disease in dogs, the lesions seldom affect long bones, are rarely symmetric, and are probably of viral rather than familial origin. There does not appear to be any breed or sex predisposition, although early reports of this condition were in the Siamese . Affected cats range in age from 1.3 years to 8 years (mean 3.2 years). Virtually all cats with multiple cartilaginous exostosis will test positive for the FeLV virus. This disease is included in the discussion of primary bone tumors because of its aggressive natural behavior.

Pathology and Natural Behavior

Osteosarcoma of cats is composed of mesenchymal cells embedded in malignant osteoid. There may be a considerable amount of cartilage present, and osteoid may be scant. A feature of some feline OS cases is the presence of multinucleate giant cells, which may be numerous. Reactive host bone and remnants of host bone are often present in specimens. Tumors are seen to be invasive; however, some surrounding soft tissue may be compressed rather than infiltrated. There is often variation of the histologic appearance within the tumor, with some portions having more fibrosarcomatous appearance, others more cartilaginous, and so on. Some authors have described subtypes that resemble those seen in dogs: chondroblastic, fibroblastic, and telangiectatic, as well as the giant cell variant. These features, however, do not appear to confer any prognostic predictive balue. Osteosarcomas in cats can be of the juxtacortical type.

In cats with OS of a limb where there are no clinically detectable metastatic lesions, amputation alone may be curative. In one study of 15 cats, the median survival after amputation alone was 24 months. The metastatic potential is much less than for the same disease in dogs or humans.

Osteochondroma may occur singly in cats, but there is a form that is multicentric (osteochondromatosis). The lesions are composed of hard, irregular exostoses having a fibrous and cartilaginous cap. Endochondral ossification occurs from the cartilage cap, which extends to a variable thickness. This cap tends to blend with adjacent tissue, making its surgical removal difficult. Cats usually develop multiple sites of disease, and there is a potential for malignant transformation and metastasis. The presence of FeLV virus is also foreboding for these cats.

History and Clinical Sign

The most common signs of OS are deformity and lameness depending on the location of the lesion. The lesions may appear radiographically similar to the OS in dogs; however, some cats have lesions arising from the periosteal surface (juxta-cortical OS). It is very rare for cats to have metastatic osteosarcoma.

Cats with multiple cartilaginous exostosis have rather rapidly progressing, conspicuous, hard swellings over affected sites, causing pain and loss of function. Common sites for lesion development are the scapula, vertebrae, and mandible; however, any bone can become affected. Radiographically, the lesions are either sessile or pedunculated protuberances from bone surfaces with indistinct borders with the normal bone. There may be a loss of smooth contour with evidence of lysis, particularly if there is malignant transformation.

Diagnostic Workup

Both OS and MCE may be suspected by the radiographic appearance of the lesions and the FeLV status of the cat. Definitive diagnosis is made by histopathologic evaluation of properly collected biopsy tissue.

Therapy and Prognosis

For OS of a limb, amputation is recommended. No adjuvant therapy is known to be efficacious, and without adjuvant treatment the median survival of cats with OS is 2 years.

Cats with MCE have a guarded prognosis. Lesions may be removed surgically for palliation; however, local recurrences are common, and new, painful, debilitating lesions may occur. No reliably effective treatment is known for this condition in cats.


OTHER PRIMARY BONE TUMORS OF CATS

Fibrosarcoma is the second most common primary bone tumor of cats. Chondrosarcoma is reported to be next in terms of frequency, and hemangiosarcomas rarely involve bones of cats. Little is known about the biologic behavior of these rare lesions; however, metastases have been seen in cats with chondrosarcoma and hemangiosarcoma.

 

FELINE CHONDROSARCOMA

Chondrosarcoma is rare in cats, representing only four of 103 (3.9%) skeletal neoplasms in three previous series. The scapula is the most frequently reported site. Limb amputation has been reported in the treatment of at least three cats with chondrosarcoma. Two cats died of tumorrelated causes at 7 and 52 weeks, and one cat was disease-free 18 weeks postoperatively.


FELINE PAROSTEAL (JUXTACORTICAL) OSTEOSARCOMA

Parosteal osteosarcoma is considered a distinct entity from central osteosarcoma, as it arises from tissue (presumably periosteum) outside the cortex or medullary cavity. Parosteal osteosarcoma represented eight of 55 (14%) osteosarcomas reported in four large series of feline skeletal neoplasms. This neoplasm is distinguished from central osteosarcoma by its gross, radiographic, and histologic appearance. In cats, parosteal osteosarcoma typically appears as a painless growth on the surface of the skull or long bones.   Radiographs   show a well-circumscribed, osteoproductive lesion. A radiolucent line of demarcation between the tumor and underlying cortical bone may be present. Histologically, the neoplasm is composed of fibrous, osseous and cartilaginous tissue elements. Distant metastases are uncommon, and amputation or wide surgical excision is often curative. Although the prognosis for humans with parosteal osteosarcoma is much better than for patients with central osteosarcoma, data are insufficient to conclude that the prognosis for cats with parosteal osteosarcoma differs from that of central osteosarcoma.

FELINE SOLITARY OSTEOCHONDROMA AND OSTEOCHONDROMATOSIS

An osteochondroma is a trabecular bone exostosis covered by a cartilaginous cap. Solitary osteochondromas in cats may involve the axial skeleton or the physeal regions of long bones. These lesions are well-circumscribed and often amenable to local excision. Although local recurrence or malignant transformation is possible, prognosis for cats with solitary osteochondroma is good after surgical excision.

Osteochondromatosis (multiple cartilaginous exostoses) is considered a disease entity distinct from solitary osteochondroma. Feline osteochondromatosis is characterized by multiple exostotic lesions that involve the cranium, scapula, ribs, vertebrae, or pelvis; appendicular skeletal lesions are rare. Relentless growth distinguishes feline osteochondromatosis from its canine counterpart, which is nonprogressive after skeletal maturity. As the disease progresses in cats, lesions acquire increasingly more sarcoma-like features. The role of retrovirus infection in cats with osteochondromatosis remains unclear. Affected cats frequently test positive for feline leukemia virus, and survival beyond 1 year after diagnosis is uncommon.

BENIGN SKELETAL NEOPLASMS AND NON-NEOPLASTIC TUMOR-LIKE LESIONS OF BONE

Benign tumors represent a very small percentage of primary bone tumors reported in dogs and cats. Osteoma, osteoid osteoma, ossifying fibroma, enchondroma, chondroma, and osteochondroma  are among the histopathological types reported. Scant data are available concerning the prevalence and clinicopathological features of these neoplasms. They are generally slow growing and do not metastasize. Complete surgical excision is curative. However, in some cases tumor location may make this impossible.

An appreciation for the radiographic features and biological behavior of these non-neoplastic lesions of bone is essential because these conditions may mimic bone tumors. In some instances, radiographic and clinical findings may not be sufficient to distinguish these conditions from skeletal neoplasms. In such cases, biopsy and histopathological examination may be necessary. Alternatively, sequential radiographic evaluation may be used to distinguish progressive lesions from static or self-limiting non-neoplastic processes. Important conditions include osteomyelitis (fungal, bacterial), simple bone cyst, aneurysmal bone cyst, bone infarction, craniomandibular osteopathy, hypertrophic osteopathy, and hypervitaminosis A.

Giant Cell Tumor of Bone

At least six giant cell tumors of bone (osteoclastomas) have been reported in cats, although detailed clinical information was available in only four of these. Most tumors originated on the pelvic limb metaphyses and caused lameness and locomotor difficulty, one tumor origi nated in the rib cage. The characteristic radiographic findings were the intramedullary origin of the tumors, and the marked lysis of the cortex and trabeculae in the metaphyses that resulted in pathologic fractures in three of the cats reported. One of the cats described had metastases to the lung and kidneys. Only one cat was treated by amputation and remained free of metastatic disease one year postoperatively.

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