Feline osteosarcoma (OSA) is a rare tumor in cats. Ninety (62%) of feline OSAs detailed in this study arose from the skeleton, and 55 (38%) arose from extraskeletal sites. Fifty OSAs originated in the appendicular skeleton, and 40 OSAs originated in the axial skeleton. Extraskeletal OSA sites included subcutaneous sites (n=44), with an apparent prevalence for sites commonly used for vaccination. Other locations included ocular/orbital (n=4), oral (n=3), intestinal/omental (n=3), and mammary sites (n=1). Survival data was available for 74 cases. When considered as a group, cats with either appendicular (mean, 11.8 mos) or extraskeletal (mean, 12.67 mos) OSA survived longer than those with axial (mean, 6.07 mos) OSA. Regardless of the type of feline OSA, aggressive surgical excision with or without ancillary therapy appeared to be the most effective form of treatment.

Extraskeletal osteosarcomas (EOSs) are rare tumors that arise in various soft-tissue sites (e.g., gastrointestinal tract, subcutaneous tissue, spleen, liver, skin, kidney, urinary bladder, muscle, thyroid gland, eye, and mammary glands). Soft-tissue osteosarcomas (STOs) occur in older dogs with no sex predilection; beagles and rottweilers are at higher risk. Mammary gland osteosarcomas (MGOs) occur in older females; mixed-breed dogs, German shepherd dogs, and miniature poodles are at higher risk. The median survival time for cases with STO was 26 days, and the major cause of death was local recurrence (92%). The median survival time for cases with MGO was 90 days, and the major cause of death was pulmonary metastasis (62.5%).