Abstract:

Background: Melanoma on the buttocks is rare, with some prominent dermatopathologists reluctant to render such a diagnosis. Materials & Methods: The files of a large academic pathology department were queried for cases of melanoma described as arising on the buttocks. Twenty-one cases of primary lesions in patients without prior history of invasive melanoma were identified from 1993-2015. In each case, hematoxylin and eosin-stained slides were reviewed by two board certified dermatopathologists to confirm the diagnosis and record the histopathologic features. Results: Overall, there were 5 male and 16 female patients with a mean age of 46 years (range: 1891 years; median: 48 years). The tumors were located on the right buttock in 10 patients (48%), left buttock in 8 patients (38%), and midline or unspecified in 3 patients (14%). The mean tumor thickness was 1.89 mm (range: 0.245.3 mm; median 0.9 mm). Mitotic figures were identified in 15/21 cases (71%); the mean mitotic figure count was 2.24/mm2 (range: 012/mm2; median: 2). Ulceration was not present in any of the cases. Three cases contained a benign nevus component (14%) and six cases contained a dysplastic nevus component (29%), thus melanoma arose in conjunction with a nevus in 43% of our cases. Sixteen cases were categorized as superficial spreading (76%) and five cases were nodular (24%). Gene expression signature analysis was performed on 17/21 cases (81%). The average score was 0.88 (range: -11.8-8.4; median: 3.88). Scores between -16.1 and -2.1 are considered likely benign, scores between 0.0 and 11.1 are reported as likely malignant, and scores between -2 and -0.1 are indeterminate. Key message: Melanoma on the buttocks is an uncommon yet indisputable entity that dermatopathologists and dermatologists alike should be aware of.

Financial Disclosure:
No current or relevant financial relationships exist.

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Wendi Wohltmann, MD