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Dermatopathology: Practical & Conceptual April - June 2007
4. New Heights: Most “large dysplastic nevi” are really small congenital nevi!
Yaqin Zhang, M.D.
A. Bernard Ackerman, M.D.
superficial congenital nevus
In actuality, nearly all small flat or very slightly elevated pigmented melanocytic nevi are Clark's nevi, most of them being junctional and when
being characterized also histopathologically by having but a few small nests of melanocytes dead center in a papillary dermis not widened by them. For purposes practical, never is a Clark's nevus intradermal wholly. A stereotypical Clark's nevus is recognized readily for what it is at scanning magnification of a microscope conventional, it having the silhouette of a benign neoplasm (symmetrical and well circumscribed), raised only slightly, if at all, above the skin surface, and typified by being compound with small nests of melanocytes (possessing a tiny monomorphic nucleus) positioned entirely at the dermoepidermal junction and with a few small nests of similar-appearing melanocytes situated in the papillary dermis in the very center of the lesion. Suffice it to say, there are no melanoyctes with large, pleomorphic nuclei; in short, there is no "random cytologic atypia" insisted on by Clark and fellow workers as being the
sine qua non
for diagnosis of a "dysplastic nevus."
In contrast, his congenital melanocytes nevi, for purposes practical, are mostly compound or intradermal. Among the most common of the congenital nevi are ones "superficial" and "superficial and 'deep.'" Neither of those are flat or elevated only very slightly above the surface of the skin; both are raised appreciably, the former ("superficial") because large nests of melanocytes are present at the dermoepidermal junction and in large number in a widened papillary dermis, whereas the latter ("superficial and 'deep'") because no small number of melanocytes are present, too, in the upper third, at least, of the reticular dermis, they often being splayed between bundles of collagen there and being distributed in fashion angiocentric and adnexocentric.
In brief, there are big differences clinically and histopathologically between a Clark's "dysplastic" nevus and a "superficial" or "superficial and 'deep'" congenital nevus (
). Clark's nevus is more common by far and is much smaller, nearly always less than 10 mm and usually less than 5 mm in diameter greatest. The differences morphologic profound between a so-called dysplastic nevus and the two types of congenital nevi so often confused with it were elaborated on by Harada and Ackerman as recently as 2006 and in these words:
"A clue helpful remarkably in distinguishing a superficial congenital nevus from a Clark's nevus utilizes scanning magnification of a conventional microscope. When, at first glance, a formidable number of melanocytes is seen in the uppermost part of the dermis and across what seems to be most of the entire front of the lesion, it is superficial congenital and not Clark's."
They went on to say this:
"The riveting disparity between the size of aggregations of melanocytes in the epidermis and in the upper part of the dermis is helpful in differentiating a superficial nevus from a Clark's nevus. In Clark's nevus, nests of melanocytes at the dermoepidermal junction tend to be small, but, as a rule, they are larger than the nests in the dermis. As is apparent in this particular nevus [illustrated here], which is a superficial congenital one, aggregations in the dermis are larger than those in the epidermis, which is the case at times for a superficial congenital nevus but not for a Clark's nevus."
Clark"s nevus, compound, stereotypical. The lesion is very slightly elevated and characterized by an increased number of melanocytes disposed as solitary units and in small nests at the dermoepidermal junction and in the papillary dermis in the center of the lesion. Nuclei of melanocytes are small, oval, and monomorphic, and cytoplasm is pale and scant.
Congenital nevus, superficial, compound. Melanocytes disposed as solitary units and in nests, some of which are large, are positioned at the dermoepidermal junction and in nests mostly in a papillary dermis widened markedly by the sheer number of them. Nuclei of melanocytes are small and monomorphic, and cytoplasm is paltry.
Congenital nevus, superficial and "deep".
"We classify congenital nevi histopathologically as superficial, superficial and deep, and deep, the lesion shown here being stereotypical of the superficial type, encroaching as it does on the uppermost part of the reticular dermis. In contrast, a superficial and deep congenital nevus affects the upper third, at least, of the reticular dermis, where melanocytes in loci tend to be splayed between bundles of collagen and to be arrayed also in angiocentric and adnexocetntric fashion. A deep type of congenital nevus is characterized by a dense, diffuse infiltrate of melanocytes throughout the entire dermis, within fibrous septa in the subcutaneous fat (and not uncommonly in the wall of large muscle-containing vessels there, especially veins) and at times, in fascia and in skeletal muscle."
For decades, the mantra of Clark and devotees of him has been that the "dysplastic nevus" is the commonest precursor of melanoma, it being the nevus present most often in conjunction with a melanoma. Two studies recent seem to give the lie to that assertion. These are the conclusions of the authors of the articles in which the results of those studies are published:
"In this large series, we found a rate of 37.8% for superficial congenital nevus remnants in all nevus-associated melanomas." "We conclude that BMN components in naevus-associated melanomas constitute a heterogeneous group morphologically, consisting mainly of dysplastic and superficial congenital nevi. This finding indicates a more important role for superficial congenital nevus as a precursor lesion of nevus-associated melanomas than presently recognizes."
"We undertook to assess both the incidence in Caucasians of a nevus in conjunction with melanoma and the character of that nevus. We did that by studying sections cut from 359 biopsy specimens of primary cutaneous melanoma (from 359 separate patients) submitted in formalin to the Ackerman Academy of Dermatopathology during an 18-month period between July 2002 and December 2003."
"We found that 50(14%) of 359 melanomas were associate with a nevus; 309(86%) melanomas seemed to have developed
Of the 50 nevi, 47(94%) exhibited attributes histopathologic that marked them as being superficial or superficial and "deep" congenital nevi and 3(6%) displayed findings histopathologic of Clark's 'dysplastic' nevus. . . . In sum, we found that approximately 85% of melanomas in Caucasians begin
only 15% arising in association with a melanocytic nevus. From that information it can be extrapolated that less than 10% of melanomas, worldwide, are associated with a nevus; the overwhelming majority of melanomas in Africans and Asians begin
Of nevi affiliated with melanoma, the overwhelming majority are congenital, not Clark's 'dysplastic' type.
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