FELINE BONE TUMORS

Primary bone tumors are uncommon in cats, representing only 20 of 395 (5%) feline neoplasms in one retrospective study. Although osteosarcoma is the most frequently reported primary bone tumor in cats, squamous cell carcinoma (oral cavity, digits) is considered the most common feline neoplasm involving bone. Bone metastases are infrequently reported in cats.

As in the dog, most feline bone tumors are malignant. However, feline skeletal neoplasms have an apparently lower metastatic rate. Generally speaking, wide surgical excision or amputation is likely to result in a more favorable prognosis in cats than in dogs with malignant bone neoplasms. The most common bone tumors of cats are discussed in the sections that follow. A more complete section on tumors and tumor-like lesions of bone in cats is currently under construction.


FELINE OSTEOSARCOMA

In cats, osteosarcoma accounts for 70% of primary bone tumors. Older cats (mean, 10 years) are most frequently affected. Appendicular skeletal neoplasms are more prevalent than axial skeletal neoplasms, and a predilection for the metaphyses of long bones of the pelvic limb has been reported. Unlike the dog, lesions of the distal radius are uncommon. Although the radiographic features of feline osteosarcoma are variable, long bone lesions are predominantly lytic. A solitary - osteolytic lesion of the distal femur of an aged cat should be considered osteosarcoma until proved otherwise.

Osteosarcoma in cats behaves less aggressively than its canine counterpart. A metastatic rate of 16% was reported in 32 cats with osteosarcoma that underwent necropsy. Because tumor-free margins are readily achieved with limb amputation, cats with appendicular osteosarcoma have a better prognosis for long-term survival than cats with axial osteosarcoma. In one study, six of 12 (50%) cats with appendicular osteosarcoma that under-went limb amputation were alive 13 to 64 months postoperatively; median survival of cats that died was 49 months. In that same study, median survival of six cats with axial skeletal osteosarcoma that were treated with surgery, radiation or chemotherapy was only 5.5 months.


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 wellcircumscribed, 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 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 ha been reported in the treatment of at least three cats with chondrosarcoma. Two cats died of tumor-related causes at 7 and 52 weeks, and one cat was disease-free 18 weeks postoperatively.

FELINE FIBROSARCOMA

Primary fibrosarcoma of bone is rare in cats. More commonly, secondary bone invasion by fibrosarcoma originating in soft tissue is reported. After limb amputation, disease-free intervals of longer than 10 months have been reported in cats with appendicular fibrosarcoma.

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.

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