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Keywords:

  • animal-type melanoma;
  • epithelioid blue naevus;
  • epithelioid melanocytoma;
  • melanoma;
  • pigmented sentinel lymph node biopsy

Summary

Background  Animal-type melanoma is a rare distinct melanoma subtype, characterized by proliferation of heavily pigmented epithelioid and spindled melanocytes that resembles the heavily pigmented melanomas seen in grey horses. While animal-type melanoma is generally considered to be more indolent than conventional melanoma, only a limited number of cases have been reported and, as such, the clinical characteristics of animal-type melanoma are incompletely understood.

Objectives  To characterize the clinical and histopathological features of animal-type melanoma, and determine any features that may predict outcome.

Patients/Methods  Data was extracted from a prospectively collected melanoma database (1994–2008), and a retrospective pathology database (1991–2008) for all patients with a diagnosis of both equivocal (8) and unequivocal (14) malignant animal-type melanoma. We reviewed the clinical and histopathological features, including the sentinel lymph node biopsy (SLNB) status.

Results  A total of 22 patients were identified, with a median age of 35 years. The median Breslow depth was 2·22 mm. A SLNB was performed in 17 patients, eight (47%) were positive. Younger age was associated with: (i) animal-type melanoma with features equivocal for malignancy (median age of 7 vs. 48 years, = 0·01), and (ii) a negative SLNB (median age 12 vs. 53 years, = 0·03). Four patients with unequivocal animal-type melanoma developed recurrent metastatic disease, with one patient death. No patient with an equivocal animal-type melanoma or negative SLNB developed recurrent disease; however, this did not reach statistical significance (= 0·13 and = 0·09, respectively).

Conclusions  Animal-type melanoma has a propensity for regional lymphatic metastasis and is rarely capable of disseminated metastatic disease and death. Animal-type melanoma appears to exhibit a spectrum of biological behaviour, with young patient age associated with more indolent disease.