Osteosarcoma is the most common bone cancer in dogs, arising from bone-forming cells (osteoblasts) that begin to grow and divide uncontrollably. In the appendicular (limb) form, the tumour typically develops near the growth plates of the long bones — most often the radius (near the wrist) or the proximal humerus (shoulder area) in the forelimbs, and the distal femur or proximal tibia (around the knee) in the hind limbs. The disease is strongly associated with large and giant breeds such as Rottweilers, Great Danes, and Irish Wolfhounds. The tumour causes progressive bone destruction and pain, often presenting as worsening lameness. Critically, at the time of diagnosis, the overwhelming majority of dogs (roughly 90–95%) already have microscopic cancer cells that have spread to the lungs, even though these are too small to detect on X-rays. This is why treatment plans address both the primary tumour and this invisible distant disease.