Most cases of cutaneous melanoma diagnosed on haired skin are benign; however, cutaneous melanoma associated with a mucocutaneous junction, a nail bed, or the oral cavity is often malignant and has aggressive clinical behavior. Melanomas arising from haired skin can occasionally be malignant with an aggressive clinical course (≈12% of melanomas from haired skin)4; thus, all pigmented or partially pigmented skin masses that are removed should be submitted for histopathologic evaluation to determine the surgical margins, MI, and any additional features of malignancy.
Dogs with benign cutaneous melanoma are typically younger (median age, 8.1 years) than those with malignant melanoma (median age, 11.6 years) at diagnosis.4 A better prognosis for cutaneous melanoma is generally associated with an MI <3, <20% of cells displaying nuclear atypia, a higher degree of pigmentation (>50% of cells), containment within the dermis, no ulceration of overlying skin, and a Ki67 score of <15% (out of 500 counted cells).5 The median 2-year survival rate in dogs with cutaneous melanoma is 84%, unless the tumor is associated with a digit (median 2-year survival rate, 56%).2 In another study, ulceration of cutaneous masses was associated with a significantly shorter survival rate, although a separate study did not find ulceration to be a prognostic factor.2,3,6
The prognosis for stage I cutaneous melanoma can be quite good if the MI is <3 but guarded if the MI is higher. The metastatic rate for malignant mucosal melanoma, which spreads most commonly to the lungs and regional lymph nodes, is approximately 60% (see Take-Home Messages).3,7,8
An update provided by the owner indicated that Buster was doing well 56 months postdiagnosis, with no evidence of metastasis or spread of the original tumor, despite the high MI. No additional therapy was performed during this 56-month period.