Melan-A-Positive “Pseudomelanocytic Nests”: A Pitfall in the Histopathologic and Immunohistochemical Diagnosis of Pigmented Lesions on Sun-Damaged Skin

Abstract

We encountered recently 3 cases with a histopathologic diagnosis of melanoma in situ on sun-damaged skin (male = 2, female = 1; median age: 59 years; range: 52-60 years). The diagnosis was based mainly on the finding of actinic elastosis in the dermis and increased number of melanocytes in the epidermis and was confirmed by strong positivity for Melan-A in single cells and in small nests ("pseudomelanocytic nests"), located at the dermoepidermal junction. Indeed, examination of slides stained with hematoxylin and eosin revealed the presence of marked hyperpigmentation and small nests of partially pigmented cells at the dermoepidermal junction, positive for Melan-A. The histologic and especially the immunohistochemical features were indistinguishable from those of melanoma in situ on chronic sun-damaged skin. In addition, a variably dense lichenoid inflammation was present. Clinicopathologic correlation, however, showed, in all patients, the presence of a lichenoid dermatitis (phototoxic reaction, 1 case; lichen planus pigmentosus, 1 case; and pigmented lichenoid keratosis, 1 case). Our cases clearly show the histopathologic pitfalls represented by lichenoid reactions on chronic sun-damaged skin. Immunohistochemical investigations, especially if performed with Melan-A alone, may lead to confusing and potentially disastrous results. The unexpected staining pattern of Melan-A in cases like ours raises concern about the utility of this antibody in the setting of a lichenoid tissue reaction on chronic sun-damaged skin. It should be underlined that pigmented lesions represent a paradigmatic example of how immunohistochemical results should be interpreted carefully and always in conjunction with histologic and clinical features.

MART-1 Is a Reliable Melanocytic Marker in Lichen Planus-Like Keratosis: A Study on 70 Cases

“…Our findings provide further evidence to support the study conducted by DeMartini et al, 2 but they conflict with the studies of both Maize et al 1 and Beltraminelli et al 3 There are several potential reasons for this discrepancy. Although there were 4 cases in our cohort that had MART-1-positive nests, those proved to be of true melanocytic composition by virtue of positive MITF staining, and therefore were not ''pseudonests''.…”

Diagnostic Utility and Comparative Immunohistochemical Analysis of MITF-1 and SOX10 to Distinguish Melanoma In Situ and Actinic Keratosis

“…As a result, many dermatopathologists may use immunohistochemistry studies, most common being Melan-A, to help differentiate melanocytes from surrounding keratinocytes; however, it has been shown that this marker may overestimate the number of melanocytes present because of labeling melanocyte dendrites and possibly pigmented keratinocytes, thus giving false-positive results. [1][2][3] Actinic keratosis and lentigines on severely sun-damaged skin can be simulators of early melanoma in situ and may be difficult to exclude a superimposed melanoma in situ. This is an important diagnostic dilemma as they have very different prognoses, and the diagnosis of melanoma usually necessitates further surgery with wide local excision possibly resulting in significant morbidity, especially in lesions located on the head and neck.…”

Efficacy of Imiquimod Cream, 5%, for Lentigo Maligna After Complete Excision

“…42 Although it has been argued that there is a risk of missing minor residual changes if reliance is placed on hematoxylin-eosin stains only without additional immunohistochemical stains, the use of melan-A has been shown to contribute to errors in the diagnosis of melanocytic lesions in sun-damaged skin tumors. 43,44 Therefore, immunostaining was not relied on for diagnosis of LM in this study. Partial biopsy of lesions compounds these difficulties.…”