Melanoma is a malignant neoplasm of melanocytes, the melanin-producing cells of skin. It is less common than cancers of the squamous/basal epithelium, but carries a much higher mortality. It is often related to UV radiation exposure and most commonly affects the skin, but can also involve the uvea, oral, and anogenital mucosa. More favourable prognosis is conveyed by thinner tumour depth, few mitoses, tumour infiltrating lymphocyte response, lack of ulceration, negative regional lymph node status, and absence of regression.

The predictive value of tumour regression is controversial. In the past, it was thought to correlate with aggressiveness; however, more recent studies suggest that regression is not associated with sentinel lymph node status. Histological regression occurs in 10-35% of cases and is characterized by lymphocytes mixed with pigment-laden macrophages underlying an atrophic epidermis with flattened rete ridges. Melanoma often incites significant immune response by the body; a small retrospective review of cases by Martin et al. showed a higher rate of regression in subsequent primary melanoma.

This case shows > 75% regression and melanoma in situ at the margins. The lesion is highlighted with Melan-A and SOX-10 (not shown).

See related content for references:
1) Botella-Estrada, R. et al. JAMA Dermatol. 2014; 150(8): 828-35
2) Martin, JM., et al. Am J Dermatopathol. 2014 Nov;36(11):e183-8