Clinical Rounds: Battling a Labrador's oral malignant melanoma

After a referring veterinarian diagnosed this deadly neoplasm in a senior Labrador retriever, this team of experts stepped in to help extend the dog's life with targeted treatment. What can you learn from their approach to help your patients?

OVERVIEW

Oral malignant melanoma is the most common tumor of the oral cavity in dogs.1-4 Predisposed breeds include Scottish terriers, golden retrievers, poodles, and dachshunds.2-4 The metastatic rate is high, with more than 80% of patients developing metastasis.5 Common metastatic sites include draining lymph nodes and the lungs.5 About 40% of lymph nodes that are normal on palpation have evidence of metastasis on fine-needle aspiration.6 The median survival time is eight to 10 months, but it can vary depending on the tumor's location and size and the presence of bone lysis.

CASE PRESENTATION

A 12-year-old castrated male black Labrador retriever was presented to the University of Tennessee Veterinary Medical Center for staging and treatment of a right maxillary oral malignant melanoma. The owners noted that the patient was having difficulty eating and that they saw drops of blood around the food bowl about one month before presentation. Three weeks before presentation, the referring veterinarian performed a debulking surgery, removing as much of the gross disease as was possible without a maxillectomy, and submitted a biopsy sample, which confirmed a diagnosis of oral malignant melanoma.

Physical examination and tumor staging

On physical examination, multiple, soft subcutaneous masses were noted on the dog's trunk, which were aspirated as lipomas. An awake oral examination revealed a pink, raised, multi-lobulated mass on the right hard palate, extending from the first premolar caudally to the third premolar. The mass extended medially to about 0.5 cm beyond midline. All of the peripheral lymph nodes palpated normally.

Staging tests consisted of a complete blood cell count, a serum chemistry profile, a urinalysis, three-view thoracic radiography, two-view abdominal radiography, abdominal ultrasonography, and cytologic examination of fine-needle aspirates of the right and left mandibular and prescapular lymph nodes.

All of the test results were normal, with no obvious evidence of metastasis. A computed tomography (CT) scan from the tip of the nares caudally to the area of the second cervical vertebral body was performed to evaluate the extent of the tumor and assess the lymph nodes in the head and cranial neck.

The CT scan confirmed that the tumor extended from the first premolar caudally to the third premolar and that it crossed midline on the hard palate. The CT scan also revealed lysis of the right hard palate and portions of the right maxilla. The tumor extended through the hard palate and into the right nasal cavity, causing almost complete occlusion of the right nasal passage. The lymph nodes appeared normal in size and contrast enhancement on the CT scan.

Treatment

A surgery consult confirmed that the tumor was too large to be completely excised, and hypofractionated radiation therapy was recommended. Treatment consisted of four weekly 8 Gy fractions to the primary tumor site and draining lymph nodes. We also recommended a canine melanoma vaccine (Oncept—Merial) once every other week for four treatments to control potential metastatic disease.

The patient finished radiation therapy and the four initial treatments with the melanoma vaccine without any serious clinical effects. The local tumor responded well to radiation therapy, with a reduction in size of about 50%.

Follow-up

One month after finishing the melanoma vaccine (seven weeks after radiation therapy) the patient presented for follow-up three-view thoracic radiography. The owners reported that the patient was doing well at home but noted halitosis.

A physical examination revealed an enlarged right mandibular lymph node and infected necrotic tissue associated with the right hard palate, which was the original tumor location. Thoracic radiographs showed a soft tissue nodule in the right middle lung lobe, and aspirates of the right mandibular lymph node revealed neoplastic cells consistent with malignant melanoma. The patient was sedated, and the necrotic tissue was débrided and cleaned. Amoxicillin trihydrate-clavulanate potassium was prescribed, and we recommended the patient return the next week for chemotherapy if the infection was under control.

The following week, the infection appeared to be well-controlled, so we initiated carboplatin chemotherapy at a dose of 250 mg/m2. While the recommended dose of carboplatin is 275 to 300 mg/m2, the oncologists at the University of Tennessee have seen a high number of cases develop marked neutropenias and sepsis when carboplatin is initiated at this dose. Therefore, we treat patients at a dose of 250 to 275 mg/m2. The patient tolerated the carboplatin therapy well and received a second treatment three weeks later.

When the dog returned for restaging three weeks after the second carboplatin therapy, thoracic radiographs revealed multiple soft tissue nodules throughout the lung lobes. Since the patient continued to do well at home, the owner opted to stop therapy at that point and monitor the patient's quality of life.

The patient was humanely euthanized because of poor quality of life three months later, nine months after diagnosis.