Parakeratosis and pagetoid melanocytosis in the evaluation of dysplastic nevi and melanoma.

BACKGROUND: It is our experience that parakeratosis with pagetosis is common in early melanoma when there is no history of trauma in the anatomical site. In lesions where the differential diagnosis includes dysplastic nevus (DN) and melanoma, we hypothesize that parakeratosis may be a marker for cases in which immunohistochemistry (IHC) may identify occult pagetosis. METHODS: We performed a retrospective case-control study on cases with a histologic differential diagnosis of DN versus melanoma, including 423 cases with parakeratosis and 125 cases without parakeratosis. IHC staining (Mart-1 and/or Sox-10) was performed in all cases. The frequency of pagetosis and diagnostic upgrades in the cases versus the controls was calculated. RESULTS: The presence of parakeratosis was significantly associated with pagetosis (p < 0.0001). Diagnostic upgrades were more frequent in the cases with parakeratosis versus controls without parakeratosis (p = 0.0029). In the favored moderate DN group, 56% of cases were upgraded compared to 30% of the controls (p = 0.0017). In the favored mild DN and severe DN groups, there were more case upgrades compared to the controls (p = 0.1386, p = 0.2738). CONCLUSIONS: Parakeratosis may be a useful marker to identify lesions with occult pagetosis for which IHC would be appropriate and may result in a diagnostic upgrade.