Clinical Rounds: Battling a Labrador's oral malignant melanoma - Veterinary Medicine
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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?



Nathan D. Lee, DVM, DACVR (radiation oncology)

Nathan D. Lee, DVM, DACVR (radiation oncology)
Oral malignant melanomas that are incompletely excised or are too large for surgical resection can benefit from radiation therapy. Typically, oral malignant melanomas are treated with hypofractionated protocols or palliative radiation therapy because of the poor overall survival time associated with oral melanomas and reported low alpha-beta ratio of melanomas.31-35 Patients that have favorable prognostic factors (see below) and a reported longer median survival time may benefit more from a fractionated radiation protocol, which would decrease the chances of late side effects.

The response rate of oral melanomas to radiation therapy is good; one report showed over an 80% response rate with, 51% of patients achieving a complete response to therapy.31 That same study, which is the largest study to date looking at dogs treated with external beam radiation therapy for oral melanomas, also showed that there was no difference in response rates or overall survival times among different radiation protocols, mainly because most patients die of metastatic disease.31

Three factors have been shown to effect median survival times in patients undergoing radiation treatments for oral melanomas:

1. The tumor's location (caudally located tumors have a poorer prognosis)

2. Bone lysis on imaging (those with bone lysis have a worse prognosis)

3. Tumor volume (dogs with macroscopic disease have a poorer prognosis).31

Patients that have none of these factors have a median survival time of 21 months. Patients with one of the factors have a median survival time of 11 months.31 The presence of two factors decreases the median survival time to five months, while the median survival time is only three months in patients with all three factors.31

Hypofractionated radiation therapy side effects are minimal. Most patients may experience mild mucositis around the tumor area, depending on tumor location and size. Partial to complete alopecia can develop in the radiation fields, especially in some breeds (e.g. poodles, Shih Tzus). The fur will grow back, but it can take up to six months and usually regrows in a different color. Late side effects to hypofractionated radiation therapy (occurring six months to several years after treatment) include osteonecrosis, muscle fibrosis, and secondary tumor formation. It is extremely rare to see late effects in patients with oral malignant melanoma because of the high metastatic rate of this disease, but if a patient did have favorable prognostic factors, late side effects may be more of a concern.

At the University of Tennessee we typically treat oral malignant melanomas with 8 Gy fractions spaced one week apart. We do not routinely use platinum-based chemotherapies as radiation sensitizers in these patients since there has been no significant difference in overall survival times reported.32,33 We do routinely recommend the melanoma vaccine be started immediately upon diagnosis and, anecdotally, have not seen any serious side effects associated with a combination of the melanoma vaccine and hypofractionated radiation therapy.


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