Pathologic diagnosis

Abstract

Lentigo maligna melanoma (LMM) is one of the four traditional subtypes of melanoma along with superficial spreading, nodular, and acral lentiginous melanoma. The non-invasive intraepidermal counterpart is called lentigo maligna (LM) or melanoma in situ, lentigo maligna type. Histologically, LM is characterized by an increased density of predominantly solitary units of melanocytes along the dermal-epidermal junction and above it, typically in association with marked solar elastosis. Neoplastic melanocytes commonly extend into adnexal epithelium. Some lesions also feature junctional nests or pagetoid spread. The differential diagnosis between early or histopathologically subtle LM and benign melanocyte hyperplasia of sun-damaged skin can be challenging. Other entities that may be confused with LM include solar lentigo, pigmented actinic keratosis, junctional melanocytic nevus, or lichen planus-like keratosis. Immunohistochemical stains, such as the nuclear marker MITF or SOX10, may be a helpful ancillary diagnostic test in evaluating the density and growth pattern of melanocytes. Immunohistochemical stains may also help to identify associated invasive melanoma, especially if the invasive component is amelanotic. The traditional clinicopathologic classification of melanoma is being replaced by molecular classification that guides targeted therapy. Melanomas occurring on chronically sun-exposed skin, such as LM/LMM, show a high mutational burden and characteristic genomic signature different from melanomas of intermittently sun-exposed skin.