Oncology Update | Canine Melanoma Part III
Central Toronto Veterinary Referral Clinic
I have recently been writing a series of updates focusing on canine melanoma. To date I have discussed diagnosis, prognostic factors, staging and treatment. This month I will move on to radiation therapy and to treatment following local control.
Melanoma is an aggressive cancer. Local control is an essential part of the treatment plan. Surgery is the most common method of local control. However, melanoma is very radiation sensitive. This fact is important because when an animal has a tumour in a location not amenable to surgery, such as the caudal maxilla or tongue, radiation therapy (RT) can be considered for local control. Prior to consideration of radiation, a CT scan must be completed in order to allow for RT planning. RT is best applied with the use of a linear accelerator. Melanoma has an interesting biological response to RT with coarse fractionation (high dose of radiation, infrequently) being demonstrated to achieve local control comparable to surgery. In fact there is no statistical difference between local control with surgery versus RT, from a survival standpoint. When high doses of radiation are applied on an infrequent basis, there are few, if any side effects. Typically RT is applied once weekly for between 3-6 weeks. The long gap between each treatment allows for biological healing of normal tissues from the radiation, thus few side effects develop. When RT is used as the only therapy a median survival time of approximately 9 months can be expected, the same as is anticipated for surgery alone.
Melanoma is a highly metastatic cancer of the immune system. Though complete local control can be achieved with surgery or radiation, we know the tumour has likely spread microscopically to local lymph nodes and/or lungs. For this reason, recurrent metastatic disease is common and adjunctive care is necessary.
In the past chemotherapy with carboplatin was considered the best choice for adjunctive care. However, in reality chemotherapy is quite ineffective in providing adjunctive treatment for melanoma. In a study which compared outcomes of surgery or radiation alone with surgery or radiation followed by chemotherapy, there was no clear evidence that the addition of chemotherapy improved the median survival time. In fact the addition of chemotherapy still resulted in a median survival time of 9 months, no different than can be achieved with local control alone. Another study looked at the use of chemotherapy as the sole treatment for oral melanoma, when owners declined local control. That study found 25% of dogs had a response to the therapy but with only 5% having a complete response, the median survival time was low at 5 months. Given the data currently available, chemotherapy does not play a significant role in the treatment of melanoma.
A great deal of effort has been made to help extend survival times with melanoma. The key advancement came with the discovery of immune therapy techniques. Immune therapy uses the body’s immune system to help fight, control or destroy cancer. Immune therapy has become standard therapy in the treatment of melanoma and has significantly improved outcomes.
Next time the Melanoma Series will continue with a discussion of immune therapy, palliative options and prognosis.
Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.
Posted by: Michael Goldstein, DVM, Diplomate ACVIM
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