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An Immunohistochemical Analysis of Pseudomelanocytic Nests Mimicking Melanoma In Situ: Report of 2 Cases

Nicholson, Kimberly M MD; Gerami, Pedram MD

American Journal of Dermatopathology: August 2010 - Volume 32 - Issue 6 - pp 633-637
doi: 10.1097/DAD.0b013e3181d1537b
Extraordinary Case Report

Several case reports have discussed the difficulty in differentiating junctional pseudomelanocytic nests as a result of lichenoid inflammation from a true melanocytic neoplasm. Even immunohistochemistry can be misleading in these cases with both Melan-A and Mart-1 frequently resulting in false positivity as a result of nonspecific labeling of nonmelanocytic cells containing melanosomes. We present a series of 2 similar cases which were initially misdiagnosed as melanoma in situ likely as a result of Mart-1 positivity of the pseudomelanocytic nests. However, in our review, a significant lichenoid reaction was apparent at the dermal-epidermal junction. Staining with microphthalmia-associated transcription factor (MITF) showed a normal density of melanocytes along the dermal-epidermal junction and failed to uniformly label the Mart-1-positive pseudomelanocytic nests. In both patients, medications frequently resulting in fixed drug eruptions were identified, and a final diagnosis of fixed drug eruption was rendered in both cases. In light of these findings we suggest MITF is a more useful marker for evaluating lentiginous proliferations along the dermal-epidermal junction particularly when dealing with the differential diagnosis of lichenoid reaction with pseudomelanocytic nests versus melanoma in situ.

From the Department of Dermatology, Northwestern University Feinberg School of Medicine, Chicago, IL.

Dr. Gerami is supported in part by the IDP Foundation.

Disclosure statement: The authors have no conflict of interest to disclose.

Reprints: Pedram Gerami, MD, Department of Dermatology, 676 North Saint Clair Street, Suite 1600; Chicago, IL 60611 (e-mail: pedram.gerami@nmff.org).

© 2010 Lippincott Williams & Wilkins, Inc.