Melanocytic Lesions and Melanoma: Architecture, Measurement, and Mimics
Architecture before cytology
Melanocytic lesions are judged first on architecture: symmetry, circumscription, lateral borders, maturation with depth, and whether single melanocytes are consumed by the epidermis. Only then judge cytology. A lesion can show mild atypia and be melanoma if the architecture is wrong, or striking atypia in a Spitz nevus and be benign.
Concepts to master
Breslow thickness is measured from the top of the granular layer, or the base of an ulcer, to the deepest invasive melanocyte, perpendicular to the skin surface, recorded in millimeters to one decimal place; parakeratosis and crust are excluded. Mitoses are counted per mm squared. Ulceration upstages. Regression, satellites, and perineural invasion matter. Immunohistochemistry: S100 and SOX10 are sensitive, while Melan-A and HMB-45 are specific but can be lost in desmoplastic and spindle cell melanoma, so a negative Melan-A does not exclude melanoma. Know special-site variants, acral, genital, and nail, where benign junctional nesting and pagetoid scatter mimic melanoma in situ.
Traps
Nevoid melanoma is deceptively bland, with dermal mitoses and no maturation. Desmoplastic melanoma looks scar-like, is S100/SOX10 positive and Melan-A negative, and recurs locally. Dysplastic nevus versus melanoma in situ: examine the shoulder, confluence, and pagetoid cells above the basal layer. Pseudomelanoma after shave biopsy shows scar, fibrosis, and a lentiginous junctional component.
Rule of thumb
If the architecture is not right, cytology will not save it. A 1.1 mm lesion with a symmetric, well-circumscribed dermal nodule showing maturation and no mitoses is benign; a 0.6 mm lesion with asymmetric shoulders, pagetoid scatter, and dermal mitoses is melanoma, with Breslow measured from the granular layer to the deepest dermal melanocyte.