The appropriate group of people to review this topic is Dermatopathology Specialists.
Abstract
This lecture, delivered by Dr. Phillip McKee, details the diagnostic criteria and potential pitfalls associated with Nevoid Melanoma (NM). NM is defined purely morphologically as a melanoma mimicking a benign nevus, emphasizing the ease of misdiagnosis. NM often presents as a new verrucoid or warty lesion, typically on the scalp of elderly patients. Key diagnostic features include a solid, blue, expansive proliferation of melanocytes filling the papillary dermis, characterized microscopically by a complete lack of cellular maturation with depth (or "reverse maturation"). Further clues involve prominent junctional and in situ components, large, confluent nests obliterating dermal collagen, and atypical nuclear morphology (vesicular nuclei with prominent nucleoli). The use of immunohistochemistry (specifically Cyclin D1 over Ki-67) is discussed as a confirmatory measure to identify brisk, deep proliferation, although it is often deemed unnecessary given clear architectural and cytologic findings.
Nevoid Melanoma: Pathological Diagnostic Criteria
0:00 Defining Nevoid Melanoma (NM): NM is a morphological term used to describe melanoma that closely resembles a benign nevus. The term is intended strictly to draw attention to the diagnostic difficulty and does not imply unique prognostic or therapeutic features.
1:29 Prototypical Presentation: NM typically develops in the elderly population, often presenting clinically as a new verrucous or warty lesion, frequently arising on the scalp. Clinically, a new melanocytic warty lesion on the head of a patient in their 70s should be highly suspicious for melanoma.
It is "absolutely solid blue," indicating that the proliferating cells completely fill and expand the papillary dermis without intervening spaces.
3:31 Epidermal Changes: The papillary dermis is "stuffed full of cells." The overlying epidermis is stretched and attenuated (fenestrated acanthosis).
3:50 Lack of Maturation: A critical feature is the lack of maturation: the blue cells at the bottom are cytologically identical to the blue cells at the top. This is the reverse of a banal nevus, where cells typically mature (become smaller, darker, or Schwannian-like/Type C) with increasing depth.
4:17 Junctional/In Situ Component: Evidence of an in situ component is present, including junctional activity, pagetoid spread, and tremendous thinning/attenuation of the epidermis overlying the population.
5:09 Expanding Nests: The presence of large, expanding nests that obliterate the normal dermal connective tissue is a major clue that distinguishes NM from benign nevi, which should not exhibit such expansive growth.
5:39 Cytologic Atypia: Nuclei are described as slightly clearer, "vesicular," and possessing prominent nucleoli. Melanocytes at the base may resemble Type A nevus cells, contrasting sharply with expected deep maturation.
7:12 Mitotic Activity: Mitotic figures are expected to be present, though they may require examination of deeper sections.
7:24 Immunohistochemistry (IHC) Utility: IHC is generally not required for diagnosis but can be confirmatory.
HMB-45: Expected to stain the infiltrate diffusely.
Ki-67/Cyclin D1: Proliferation markers. Cyclin D1 is preferred as it avoids staining lymphocytes, providing a clearer assessment of melanocytic proliferation. Brisk positivity or proliferation seen deep within the lesion is highly diagnostic of malignancy, as banal nevi should show minimal or superficial proliferation only.