Musculoskeletal Cancer, Cats Are No Dogs!
Introduction Musculoskeletal Tumours
Musculoskeletal tumours are much less frequent in felines compared to dogs. They can be broadly divided into those affecting bone and cartilage, and those affecting muscles and soft tissues around bone and joints. There is some overlap, however, since soft tissue tumours may invade and metastasize to bone and cartilage.1
Feline bone tumors are uncommon, with a reported incidence of 3.1–4.9 per 100,000 cases and an average age of occurrence of 8–10 years. Although OS is generally a tumor of the older cat and mean age of diagnosis was 10.7 years, there is a wide range of distribution, and therefore, OS cannot completely be ruled out in the young cat. Osteosarcoma (OS) is the most common bone tumor in the cat, accounting for 70–80% of all primary bone tumors. OS are generally defined as tumors composed of malignant mesenchymal spindle cells that produce an extracellular matrix of bone or osteoid.2
Other diagnoses include juxtacortical osteosarcoma, chondrosarcoma, ossifying fibroma, chondroma, osteochondroma and osteochondromatosis (multiple cartilaginous exostosis).
Other bone lesions may mimic primary tumours such as aneurysmal bone cysts and fibrous dysplasia. Tumors may also invade bone from adjacent soft tissue and these include, most commonly, squamous cell carcinoma, but also adenocarcinoma, fibrosarcoma, malignant lymphoma, rhabdomyosarcoma, haemangiosarcoma, melanoma, meningioma and reticulum cell tumour.1
Location and Prognosis
Feline OS can be located in appendicular, axial, and extraskeletal sites. Occurrence of skeletal OS in appendicular and axial sites seems evenly distributed, and extraskeletal OS (EOS) account for up to 40% of all feline OS. Subcutaneous EOS is most common, with sporadic reports of occurrence in the mammary gland, the eye, and other locations. Feline EOS have been linked to injection sites and seem to carry a poorer prognosis than skeletal OS. Feline OS behave differently compared with canine OS. Whereas an incidence of metastasis of up to 80–90% is described for dogs, only 5–10% of feline OS are reported to metastasize.
Because of this low metastatic rate, the therapy of choice has been wide surgical excision of the tumor. Feline appendicular OS is often amenable for wide surgical excision and showed longer survival rates compared with feline axial and EOS. Isolated case reports of feline OS treated with adjunctive chemotherapy and radiotherapy promise longer survival times (ST).
Although OS have previously been divided into morphologic subtypes such as osteoblastic, fibroblastic, chondroblastic, telangiectatic, and mixed types, significant correlation between the subtypes and survival statistics in humans and dogs is lacking. In contrast to subtyping, histologic grading is an effective prognostic factor in human and canine OS. Histologic grading system was also a significant prognostic indicator for ST, DFI, and RFI in feline OS.
There is one study that reviewed medical records of 62 cats (1994–2004) with OS diagnosed by histologic examination and compared histologic variables of 22 feline OS with twenty-two dogs with skeletal tumors of identical location and subtype corresponding. Osseous (40) were more common than EOS (22) tumors. The maxilla, mandible, digits, scapula, proximal humerus, and tibia were the most frequently affected skeletal sites. Most EOS occurred in the interscapular area, flank region, and mammary glands.
Histologically, feline OS had moderate to abundant pleomorphism, had a low mitotic index, produced small to moderate amounts of tumor matrix, but were relatively cellular. Most tumors had a moderate amount of necrosis, a moderate amount of MNGC, and little whirl formation. Almost all OS (82.3%) had signs of tumor cell invasion within blood vessels. No significant differences in any histologic variables were observed among extraskeletal, axial, and appendicular OS. The most common feline OS subtypes were osteoblastic and mixed osteoblastic/fibroblastic. Tumor grade and histologic variables of feline skeletal OS showed no significant differences compared with the corresponding canine skeletal OS, although the number of mitoses in feline OS was almost half the number of mitoses in canine OS.
Mean metastatic rate was 10%, and 44% of feline OS had local recurrence. Recurrence frequency was influenced by marginal excision and the high number of axial and EOS.
Among the clinical variables that were evaluated as prognostic indicators using multivariate analysis, only complete surgical resection had a significant effect on ST, DFI, and RFI. Although the recurrence rate in feline OS appears to be high (44%) according to our study, cats that had surgery had increased ST and time to recurrence compared with animals that had no resection at all. It is possible that surgery in combination with adjunctive chemotherapy can further prolong ST and recurrence-free periods for feline OS. Although some case reports of feline OS treated with surgery and adjunctive chemotherapy have shown long ST and recurrence-free periods, there are currently no blinded studies that evaluate the role of chemotherapy and radiation in the management of feline OS.2
In conclusion, feline OS has variation in histologic appearance that is not related to tumor localization. Histologic characteristics of feline OS are similar to those observed in canine skeletal OS. Histologic grade and mitotic index seem to be the most important prognostic factors for survival.
1. Helm J, Morris J. Musculoskeletal neoplasia: an important differential for lumps or lameness in the cat. J Feline Med Surg. 2012;14(1):43–54.
2. Dimopoulou M, Kirpensteijn J, Moens H, Kik M. Histologic prognosticators in feline osteosarcoma: a comparison with phenotypically similar canine osteosarcoma. Vet Surg. 2008;37(5):466–471.