OSTEOSARCOMA

DESCRIPTION
Osteosarcoma is a common cancer of the bone, usually affecting larger dog breeds. It is usually seen in middle aged or elderly dogs but can also occur in the young. It is highly inherited especially in certain breeds such as rottweilers and mastiffs. Osteosarcoma can develop in any bone but the limbs are the most common bones affected. Osteosarcoma begins deep within the bone and becomes progressively more painful as it grows outward and the bone is destroyed from the inside out. The lameness goes from being intermittent in the early stages to constant over a 1 to 3 month time span. Obvious swelling becomes evident as the tumour grows and normal bone is replaced by tumorous bone. The soft tissue over the affected bone will also swell and appear painful. Tumorous bone is not as strong as normal bone and can break with minor injury. This type of broken bone is called a pathologic fracture and may be the finding that confirms the diagnosis of bone tumour. Pathologic fractures will not heal and there is no point in putting on casts or attempting surgical stabilization. At this point the only way to stop the pain is through amputation.

DIAGNOSIS
Any ongoing lameness in a large breed dog should be followed up with an xray. Pain killers can be quite successful in masking the pain of an osteosarcoma. During the early stages of osteosarcoma, many people frequently believe that their dog has just “sprained” its leg. If they put their dog on pain killers without having an xray taken by a veterinarian, they can postpone the diagnosis by weeks, which unfortunately delays early detection and treatment. Radiographs of osteosarcomas usually show a very typical appearance, the affected area of bone looks “eaten away” and have a “sunburst” pattern to it. Sometimes a pathologic fracture may be seen in the damaged bone. Osteosarcomas do not cross the joint space as arthritis does. In most cases, radiography is all that is needed to make the diagnosis but sometimes there are ambiguities. If the veterinarian can not tell for certain by an xray that the dog has osteosarcoma, they may do a biopsy of the bone and send the sample to the lab for a definitive diagnosis. This can be a painful procedure as the affected bone is very sensitive.

TREATMENT
Amputation of the affected bone is recommended for any tumour involving bone. When the malignant structure has been removed, it is submitted for biopsy and the diagnosis confirmed at that time. Biopsy before amputation is felt to simply add a painful procedure to the patient and, if possible, is reserved for tissue already amputated. Treatment of osteosarcoma involves two aspects: Treating the pain caused by the bone tumour and fighting the spread of cancer. Treating the pain: Dogs are often euthanized due to the pain in the affected bone. Treating the pain successfully will allow a dog to live comfortably. The pain can be controlled by several methods. · Amputation: Amputation is the removal of the affected limb, which resolves the pain completely. Many people are hesitant to have this procedure, as losing a leg is very handicapping in a person, however, most pets function extremely well on three legs, running and playing like any other dog. · Limb Sparing: In some cases a procedure called “limb sparing” can be done in order to avoid amputation and to save the leg. The tumorous bone is removed and is replaced by a bone graft from a bone bank. The joint nearest the tumour is fused (i.e., fixed in one position and cannot be flexed or extended.) Limb sparing cannot be done if more than 50% of the bone is involved by tumour or if neighbouring muscle is involved. Limb sparing works best for tumours of the distal radius (forearm bone). · Radiotherapy: In the situation of a dog that can not undergo amputation and is not eligible for a limb spare technique, a procedure called radiotherapy can be performed. Radiation therapy does not remove the cancer, but it does significantly dull the bone pain caused by the tumour. Radiation doses can be applied to the tumour in two to three doses, sometime being given on two consecutive days. Improved limb function is usually evident within the first 3 weeks and typically lasts 4 months...) When pain returns, radiation can be re-administered for further pain relief if deemed appropriate based on the stage of the cancer at that time. · Drugs: There is not one specific drug to control the pain of osteosarcoma, but frequently combinations of narcotics, gabapentin, bisphosphonates and NSAIDs can significantly improve the dog’s quality of life. The average survival time for dogs who do not receive chemotherapy for osteosarcoma is 4 to 5 months from the time of diagnosis regardless of whether or not they have amputation. Do you want your dog's last 4 to 5 months to be painful or comfortable? Treating the Cancer: Osteosarcoma is unfortunately a fast spreading tumour. By the time the tumour is found in the limb, it is considered to have already spread. Osteosarcoma spreads to the lung in a malignant process called metastasis. Prognosis is substantially worse if the tumour spread is actually visible on chest radiographs, so if chemotherapy is being contemplated, it is important to have chest radiographs taken. The cancer can also spread to other bones in the body. Chemotherapy is used to slow down the disease within the body where it will ultimately prove fatal. Three different chemotherapies are frequently used as single agents for the treatment of osteosarcoma and are given once every 3 weeks for four to five treatments. They are carboplatin, cisplatin and doxorubicin. Sometimes a combination of carboplatin with doxorubicin may be used to treat osteosarcoma. Prognosis: With the use of chemotherapy, the median survival time of osteosarcoma is 12 months. Some dogs will live for up to two years and unfortunately others as short as 6 months. Young dogs with osteosarcoma tend to have shorter survival times and more aggressive disease than older dogs with osteosarcoma. Elevations of alkaline phosphatase, one of the enzymes screened on a basic blood panel, bode poorly for dogs with osteosarcoma. These dogs have approximately 50% of the survival times of dogs without an elevation of this enzyme. The presence of tumour in lymph nodes local to the leg being amputated is also carries a poorer prognosis. Dogs with affected lymph nodes live significantly shorter (59 days) compared to 316 days average survival time in dogs where the tumour was not evident in local lymph nodes at the time of amputation.

Other cancer types.