Diagnosis: Melanoma In Situ of the Lentigo Maligna Type
• Melanoma in situ of the lentigo maligna type accounts for 10% of melanomas and typically occurs on chronic, sun-exposed areas of elderly adults. It manifests clinically as a slowly expanding, unevenly pigmented macule with irregular borders.
• The key histologic findings of lentigo maligna include a broad, contiguous proliferation of basal melanocytes, epidermal atrophy, and pronounced solar elastosis in the dermis. The cells exhibit variable degrees of cytologic atypia. The basal layer of follicular infundibula is often involved. Pagetoid melanocytes in the upper portion of the epidermis may be seen but are less common than in the superficial spreading type of melanoma.
• Pronounced nesting of melanocytes should raise suspicion for invasion of the dermis. Mitotic figures in the dermis are indicative of a vertical growth phase melanoma.
• Immunohistochemistry with melanocytic markers (i.e., HMB-45 or melan-A [MART-1]) is often used to highlight the contiguous growth of atypical melanocytes.
• Chronic, sun-damaged skin may show noncontiguous, lentiginous proliferation of epidermal melanocytes in the absence of a clinical lesion; clinicopathologic correlation is critical in the diagnosis of lentigo maligna.
