Jose A. Plaza - Pathology of Pigmented Skin Lesions

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The histomorphologic study of pigmented lesions of the skin constitutes the majority of our daily dermatopathology practice. In this book, we have compiled our experience in regard to pigmented lesions of the skin and have illustrated not only stereotypical examples of pigmented lesions but also unusual variants and a wide spectrum of variations that can be observed in such lesions. Our goal in this book is to illustrate thoroughly the most difficult topics in melanocytic tumors and to describe our view on how to diagnose these lesions. The opinions stated in this book represent our personal views on the topics. As with other topics in the field of pathology, the reader should consider the information provided and assimilate it to her/his own experience.

The histological diagnosis of melanocytic lesions is one of the most difficult areas in pathology, given the subjectivity and histologic variations that some of such entities may depict. In the last decade, there have been major advances in terms of diagnosis and prognosis of such lesions. It is well known that a number of these lesions cannot be precisely and reproducibly classified as either entirely benign or malignant just by using conventional histologic and immunohistochemical techniques. New understandings of molecular pathogenesis of melanocytic proliferations have revealed genetic differences between nevi and melanoma that can be used as targets for developing molecular diagnostic tests. FISH has emerged as a preferred molecular technique to interrogate chromosomal abnormalities, with proven utility as a diagnostic adjunct in lymphoid lesions and solid tumors and that has been recently validated for the diagnosis of melanocytic lesions. In this book, in addition to special mention to immunohistochemistry, we cover also the utility of adjunct molecular studies applied to the diagnosis of certain melanocytic lesions that are exceedingly difficult to diagnose on pure histomorphologic grounds.

We are in debt with our teachers, colleagues, and students who have guided and challenged us over the years. We also are thankful to many pathologists who have shared their challenging cases with us on consultation, and we have learned from them. The major sources of material used in book are from the dermatopathology divisions of Medical College of Wisconsin, University of Texas MD Anderson Center, and Miraca Life Sciences. And last, but not least, we are much in debt with our families who have supported us during all these years.

Ephelides (freckles) are common, uniformly pigmented, multiple macules (15 mm in size) mainly limited to body regions above the waist. These macules are more numerous on sun-exposed areas (face, shoulders, and upper back) and usually fade and become smaller in the winter season. Ephelides appear early in childhood and partly disappear with age and are closely related to pigmentary host characteristics such as fair skin and/or red hair. Only rarely ephelides are seen in individuals with dark skin. Ephelides may manifest an autosomal dominant pattern of inheritance (appearing in sequential generations). High levels of freckling may indicate a raised susceptibility to the development of melanoma. In contrast to solar lentigines, ephelides are not strongly associated with age [].

The epidermis appears normal in structure. The basal cells in the affected areas are more heavily pigmented with melanin than those in the surrounding skin, and there is usually sharp delimitation of the abnormal from the normal areas. There are normal and sometimes decreased numbers of melanocytes within epidermis; however, the melanosomes in those cells are larger than those in the surrounding skin and can sometimes be seen with the microscope as dark, large, intracytoplasmic granules (macromelanosomes). The adnexal structures are not involved. By electron microscopy studies, the melanocytes contain enlarged spherical granular melanosomes as opposed to the striated ellipsoid forms seen in normal skin [ac).

Fig. 1.1

Ephelides. Note the pigmented basal keratinocytes and the decreased number of melanocytes within epidermis (a). Higher magnification showing melanocytes with rare melanosomes (b). Observe the lack of melanocytes (c)

Differential Diagnosis

The clinical differential diagnosis of ephelides includes lentigo simplex and caf au lait spot. The diagnosis of caf au lait spot will rely on the identification of giant melanosomes and mild increased number of melanocytes. Lentigo simplex (as explained in detail below) will show elongated rete ridges with hyperpigmentation of basal keratinocytes (some authors accept also mild increased in numbers of melanocytes) (Fig. ac).