“Occult” Melanocytes in Nail Matrix Melanoma

Abstract

To the Editor: Although there is little need these days to encourage the further use of immunohistochemistry in the study of nevomelanocytic lesions, there is one group of such lesions in which its use is essential to ensure the correct diagnosis—pigmented lesions of the nail bed/matrix. Longitudinal melanonychia (melanonychia striata in longitudinem) is a not uncommon clinical problem that may result from a melanotic macule or nevus of the nail bed/matrix or from an early stage of subungual melanoma. Nail streaks are also common in dark-skinned people adding to the diagnostic confusion in this area. The pathologic findings may also be difficult to interpret with the distinction between a melanotic macule and an early acral lentiginous melanoma problematic. A 2-mm biopsy from the advancing edge of a melanoma may be particularly difficult to diagnose as there is often no cell crowding and the melanocytes are difficult to discern from adjacent keratinocytes.1 This has given rise to the use of the term “acral atypical melanocytic hyperplasia” for these indeterminate lesions.2 Immunohistochemistry stains (such as the melan A or MART-1, HMB45, and some of the newer “cocktails” with a red chromogen) highlight the melanocytes distinctly.3 In early melanomas, the dendrites are plumper and longer than melanotic macules, and the nuclei of melanocytes are larger and hyperchromatic. Pagetoid spread of melanocytes is highlighted by these immunohistochemical stains (Fig. 1). Furthermore, the older (central) area of the lesion is much more likely to be diagnostic than the newer (advancing edge) part of the lesion. The presence of inflammatory cells in the dermis favors melanoma over a macule, but one of us (D.W.) has seen cases of matrical melanoma, in which the infiltrate was sparse or nonexistent.FIGURE 1: Nail matrix melanoma. A, Melanocytes are difficult to discern in this hematoxylin and eosin preparation (original magnification ×100). B, Melanocytes, some plump, are easily recognized in this same case using a “Melanoma Cocktail” preparation with a red chromogen (original magnification ×100).Dermatopathology of the nail matrix is not for the faint-hearted or inexperienced.