Body composition measurements (BCM), obtained via computed tomography (CT), have been used as predictors of survival, tumour recurrence, and post-surgical infections in human oncology. There are no reports on using BCM to predict outcomes of dogs with cancer. Elevated BCM is hypothesised to place extra stress on bones weakened by cancer. Pathologic fracture following stereotactic body radiation therapy for canine appendicular osteosarcoma (OSA) frequently results in limb amputation or euthanasia. Additional tools are needed to better predict the risk of fracture development. Our objectives were to determine if any relationships could be identified between BCM and the occurrence of a pathologic fracture and/or survival time in dogs with naturally occurring OSA. Forty-seven dogs with a confirmed OSA and whole-body CT pre-SBRT were included. Several BCM were evaluated, including abdominal volume, visceral adipose tissue volume, whole-body volume, whole-body adipose tissue volume, normalised cross-sectional area of the epaxial muscles at the mid-body of the 13th thoracic vertebra, and attenuations of adipose tissue and epaxial muscles. No BCMs were correlated with survival time. The volume of the entire body (cm) was significantly positively associated with development of a fracture. No other BCM were correlated with the development of a fracture. The volume of the abdomen (cm) among our patient subset was positively correlated with the volume of the entire body, and the volume of visceral adipose tissue (cm) was positively correlated with the total body volume of adipose tissue (cm). Additional research is needed to verify whether these findings are replicable in larger sample sizes and in prospective settings.