Bone cancer, also called osteosarcoma, is fairly common in dogs. Other types of cancer can form from bone, but osteosarcoma is by far the most common. The cause of osteosarcoma is unclear. However, it inevitably involves the uncontrolled growth of immature bone cells. Abnormal bone forms, usually but not always in the long leg bones, causing hard, swollen, painful masses. Osteosarcoma is malignant, meaning that it gets worse over time and ultimately is fatal. Bone cancer often spreads to other places, especially the lungs. Large and giant breeds, and middle-aged or older dogs, are especially prone to bone cancer. Most affected animals limp, and the cancerous area becomes hard, lumpy and extremely painful. Bone cancer in the jaw also causes problems with eating. If the cancer spreads to the lungs, the dog usually develops respiratory distress, including coughing, wheezing, shortness of breath and difficulty breathing. Any dog with prolonged lameness in one leg should be seen by a veterinarian.