This video features Dr. Jared Gardner and his residents examining various melanocytic lesion unknowns, providing a comprehensive guide to differentiating benign nevi from malignant melanomas. The session covers key diagnostic challenges, morphological features, and common pitfalls for lesions including desmoplastic melanoma, metastatic melanoma, pigmented spindle cell (Reed) nevus, congenital pattern nevus, nevoid melanoma, recurrent nevus, lentigo, lentiginous nevus, dysplastic nevus, Spitz nevus, and blue nevus. Dr. Gardner emphasizes the critical role of clinical context, architectural patterns, specific cellular features, and the judicious use of immunostains (Sox10, S100, MART1/HMB45) for accurate diagnosis, while also highlighting the inherent subjectivity and ongoing controversies in dermatopathology.
Summarizing Melanocytic Lesions: Key Insights for Diagnosis
Often found on sun-damaged skin of the head and neck, characterized by spindle cells within a collagenous/myxoid background, often with lymphocytes.
Crucially, about half of cases lack an in situ melanoma component.
Immunostains: S100 or Sox10 are positive; MART1/Melan-A and HMB45 are typically negative in pure desmoplastic forms.
Behavior: Pure desmoplastic melanoma (90%+ desmoplastic component) has a better prognosis than other melanomas of similar depth but is locally aggressive and loves to invade nerves.
Presence of multiple dermal nodules without epidermal connection strongly suggests metastasis (satellite, intransit, or distant).
Superficial mets often bulge against the epidermis, usually lacking an in situ component, though epidermotropism (melanoma cells growing into the epidermis) can occur, mimicking a primary lesion.
Diagnosis: Requires careful evaluation of cellular atypia, mitotic activity, and crucial clinical history.
Characterized by plump, elongated, spindled to oval melanocytes forming nests that can be horizontally "bridging" or vertically oriented ("brown bunches of bananas").
Often highly pigmented and can show pagetoid spread in the center, but importantly, lesions end sharply at the periphery.
Distinction: Favored over dysplastic nevus by delicate collagen (not dense fibroplasia) and its typically small, superficial, circumscribed nature.
15:35 Congenital Pattern Nevus: Embedded in Structures
Melanocytes infiltrate and track around adnexal structures (hair follicles, erector pili, nerves, vessels). Perineural growth is a benign finding.
Key features include infiltration of erector pili muscle, compressed multi-nucleation (pseudo-atypia), and sometimes Meissner's corpuscles (tactoid bodies).
Can exhibit epidermal overgrowth (mimicking seborrheic keratosis) or pseudo-vascular changes.
Melanomas that morphologically resemble benign nevi, often with nesting and apparent maturation at low power, but reveal obvious atypical cells at high power.
Clues: "Puffy shirt sign" (nests packed tightly together) and concerning cytology (big nucleoli, mitoses, ugliness) especially in deeper parts. Always start at low power for overall architecture.
25:20 Recurrent Nevus ("Pseudomelanoma"): Over Scar Tissue
A benign nevus recurring over a biopsy or excision scar can mimic melanoma with pagetoid spread and atypical features.
Key: Atypical melanocytes are confined directly over the dermal scar and do not extend into adjacent normal skin. Clinical history of prior removal is vital.
Lentigo: Characterized by hyperpigmented keratinocytes and an increased number of solitary melanocytes along elongated rete ridges. Distinguishing simplex (younger, no solar elastosis) from solar (sun-damaged, "hockey stick" or "dirty puppy dog feet" rete) is often based on solar elastosis. Pigmented dermal cells are usually melanophages.
Lentiginous Nevus: Similar to lentigo but includes small, often vague nests of melanocytes at the tips of elongated rete. Melanin pigment can obscure melanocytes, making it tricky for beginners.
Key takeaway: Melanocytes remain largely confined to the rete ridges, primarily below an imaginary line drawn across the top of the dermal papillae, differentiating it from melanoma.
Classically described as thin, "east-to-west" lesions with elongated rete, bridging of nests, pink fibroplasia (lamellar/concentric fibrosis), and "shouldering" (junctional component extending beyond dermal). Cytoplasm can be grayish.
Grading: Traditionally mild, moderate, severe, though WHO now recommends low-grade and high-grade due to subjectivity.
Important Distinction: Features of dysplastic nevus help differentiate it from regular nevi, not melanoma. Melanoma typically shows confluent inter-rete growth or pagetoid spread beyond the rete tips.
Pitfall: Avoid diagnosing dysplastic nevus on sun-damaged skin of older individuals, especially on the face or scalp, as these are frequently melanomas mimicking dysplastic features (e.g., lentigo maligna).
57:49 Spitz Nevus (Epithelioid and Spindle Cell Nevus): Challenging Diagnosis
Characterized by abundant, often pink/gray cytoplasm, large nuclei with prominent central nucleoli, and plump spindle/epithelioid cells. Junctional nests can be large with characteristic "clefting" artifact.
Features: Often wedge-shaped architecture, abrupt lateral borders, and presence of Kamino bodies (reassuring but not definitive for benignity). Pagetoid spread is acceptable.
Challenges: Benign Spitz nevi can appear very atypical, and melanomas can mimic Spitz features. Diagnosis is extremely difficult, especially in older patients, often requiring molecular testing and expert consultation.
Concerning features for malignancy: Bizarre pleomorphism, numerous/atypical dermal mitoses (especially deep), and trickling growth to the periphery.
Composed of abundant melanin pigment (in spindle melanocytes and melanophages) within a dense sclerotic collagenous background.
Features: Often shows collagen trapping at its edges, which can mimic dermatofibroma. Hypopigmented blue nevi (lacking melanin) are a particular mimic of dermatofibroma.
Cellular Blue Nevus: A benign variant that can look scary due to its deep, cellular, fascicular, and sometimes bulging growth, which breaks the normal maturation rule (more cellular deeply). Scattered mitoses can be seen but significant atypia is concerning for melanoma.