Problems of Differential Diagnosis in Melanoma Arising from Blue Naevus.

Melanoma arising from blue naevus is an exceedingly rare variant of melanoma. Most of the cases are located in head and neck area, with scalp the most affected site. This type of melanoma develops on blue naevus or resembles blue naevus, most frequently cellular blue naevus. We present the case of a 52 years old female diagnosed with a pigmented melanoma of the scalp, with maximum tumoral thickness of 6.8 mm and invasion of the hypodermis, highly resembling cellular blue naevus. All criteria of malignancy were present: prominent nuclear pleomorphism, infiltrative pattern, tumoral necrosis, high mitotic rate (11 mitoses/mm2) with atypical mitosis and high Ki67 index. No lymph node or distant metastases were identified at presentation. Because of the rarity of cases, histopathological diagnosis can be difficult, differential diagnosis with blue naevi, atypical blue naevi or metastatic melanoma being the most challenging problems. Prognosis is given by tumoral thickness, frequently being more than 4 mm.