Background:

Canine oral melanoma is highly aggressive, with an infi ltrative and metastatic behavior. The staging scheme for dogs with oral melanoma is primarily based on size, with stage I = < 2 cm diameter tumor, stage II = 2 cm to < 4 cm diameter tumor, stage III = 4 cm or greater tumor and/or lymph node metastasis and stage IV = distant metastasis. Surgery and radiation therapy are commonly used for local treatment of oral melanoma. Surgery must be aggressive and wide excision, such as partial mandibulectomy or maxillectomy, can be declined by owners. Median survival times for dogs with oral melanoma treated with surgery and chemotherapy is approximately seventeen, five and three months with stage I, II and III disease, respectively. Radiation therapy plays a role in the local treatment of canine melanoma when the tumor is not surgically resectable, the tumor has been removed with incomplete margins and/or the melanoma has metastasized to local lymph nodes without further distant metastasis. Case A dog with stage III oral melanoma was treated with radiation therapy and chemotherapy. The protocol consisted of three 8 gy radiation fractions (days 0, 7 and 21) delivered by an orthovoltage unit. Energy of 120 kV, 15 mA e 2 mm aluminum filter were used. Collimator size was 6 x 8 cm and source to skin distance was 30 cm. Dose rate was 187 cgy/minute delivered at 1 cm tissue depth, with the animal positioned in left recumbency. Treatment field included visible tumor plus a three cm margin. Lead sheets of 2 mm thickness were used to protect normal tissues around tumor. The dog was anesthetized with propofol (5 mg/kg EV) for correct position every radiation fraction. The chemotherapy consisted of four cycles of carboplatin (300 mg/m2 intravenously) administered every 21 days. The radiation therapy was well tolerated, and the only acute reaction observed in the irradiated field was epilation. The tumor had a partial remission of about 90% of the lesion, which was stable for six months.

Discussion:

The reported dog had a mandibular melanoma greater than 4 cm diameter with no evidence of regional or distant metastasis, and was diagnosed as having stage III disease. The animal was referred for radiation therapy because of non-acceptance of the owner to carry out the hemimandibulectomy, believing that the animal would have decreased quality of life to have a short survival even with surgery and chemotherapy. Radiation therapy was delivered with palliative intention to reduce tumor size and animal discomfort. With radiation therapy and chemotherapy, survival time was six months, exceeding the median survival for patients with stage III treated with wide surgical excision and chemotherapy (that would be three months), without showing side effects that diminish its quality of life. Systemic chemotherapy was used in the reported case with the purposes of acting as a radiopotentiation agent and delaying development of metastasis. Carboplatin has been used as radiopotentiation agent because it interferes with DNA synthesis. In the reported case, chemotherapy was well tolerated. Common radiation side effects include stomatitis, glossitis, skin epilation, erythema and desquamation. In the reported dog, treatment was very well tolerated, and only skin epilation was observed. Radiation therapy can be considered as an alternative option for oral melanoma when wide surgical resection is declined by owners.