IntroductionCanine osteosarcoma is an aggressive primary bone tumor with a high metastatic rate and poor long-term prognosis. Therapeutic resistance remains a major cause of treatment failure. In human osteosarcoma, sex hormones influence tumor survival and proliferation, and emerging evidence suggests a similar role in canine osteosarcoma. Tamoxifen, a selective estrogen receptor modulator, inhibits estrogen receptor signaling and also exhibits estrogen-independent effects, including suppressio