Canine Osteosarcoma: Early Chemotherapy Treatment can Improve Survival Times

CANINE OSTEOSARCOMA: Written by LILLIE DAVIS, DVM, DACVIM (ONCOLOGY)

Osteosarcoma is the most common primary bone tumor in dogs and they represent up to 85% of malignancies originating in the skeleton. It classically occurs in large and giant breeds and the front limbs are affected more often than hind limbs. More than 75% of osteosarcoma develop in the long bones of the skeleton. It can also be located in the bones other than those of the limbs (mandible, maxilla, spine, ribs, pelvis, nasal cavity, etc.), however this is much less common.

The exact cause of osteosarcoma is generally unknown but may be associated with genetic, environmental, and/or physical factors (weight, implants, or trauma). These tumors are locally invasive, cause severe pain, and have a very high metastatic potential. If left untreated survival time may be as short as 4-6 months on average. The treatment of choice for local disease is surgical resection of the tumor. This is an excellent pain reliever, but does not generally extend survival time due to the high frequency of metastatic disease (spread of the cancer elsewhere), with patients still having a prognosis of 4-6 months.

Metastasis of osteosarcoma through the circulatory system is extremely common. Although the lung is the most common reported site, it can occasionally spread to bones or other soft tissues. Although most (~90%) of dogs do not have grossly visible metastasis on thoracic radiographs at the time of diagnosis, the greater than 90% will eventually develop metastatic disease regardless of promptness of amputation. Chemotherapy after amputation, which used to delay the time to development of metastatic lesions, may extend survival time to approximately 8-10 months on average.

Successful treatment includes control of the primary tumor as well as micrometastatic disease that is presumed to be present. The standard treatment for osteosarcoma of a leg bone is amputation of that leg. Even large and giant breeds can function well after limb amputation and most owners are pleased with their dog's mobility and quality of life after this surgery. This is recommended as the first line of treatment. Following amputation, chemotherapy is recommended to decrease the chance of early failure due to lung metastasis. Without chemotherapy, amputation is considered palliative. In this case, most dogs develop lung tumors within 4 to 6 months. It usually is another month or two before there are signs associated with the lung tumors (coughing, difficulty breathing, etc.) Once tumors develop in the lungs there are very few options for treatment.

The chemotherapy protocol that has been shown to be most effective at treating osteosarcoma is an injectable drug called Carboplatin. This drug is given every 3 weeks for 4 treatments. The median survival time with surgery followed by chemotherapy is approximately 8-10 months.

A new study published in JAVMA (2021, Marconato et al) revealed that dogs with osteosarcoma who get chemotherapy within 5 days of amputation had a significantly higher median survival time of 445 days (14.8 months) versus 239 days for dogs who received chemotherapy >5 days after amputation. Median time to tumor progression for dogs who received chemotherapy less than or equal to 5 days was also significantly higher at 375 days versus 202 days for dogs who received chemotherapy >5 days after amputation.

Therefore, I recommend canines with osteosarcoma who undergo limb amputation and adjuvant chemotherapy are given their first dose of chemotherapy prior to discharge from their surgical appointment. There are some cases where chemotherapy has been given the same day of surgery with no adverse effects on healing.