Precursors To Melanoma And The Problematic Nevomelanocytic Proliferation
Section 6 -
The Precursors of Malignant Melanoma
Neil Crowson, MD
Cynthia M. Magro, MD
Martin C. Mihm, Jr., MD
Previously precursors to melanoma were generally considered to be nevi, either acquired or
congenital. In recent decades the systematic study of patients with malignant melanoma has revealed the
presence in a significant number of patients of precursor lesions which could be more specifically
subclassified; a certain group of lesions was found that in themselves were precursors but the presence
of which were a clinical sign that the patients harbored a genetic anomaly that placed them at an
increased risk for the development of malignant melanoma and were carriers of susceptibility trait for
the disease, now known as dysplastic nevi. We intend to elucidate recent advances in our understanding
of melanoma precursors from the standpoint of the biological events and the histologic clues which help
to predict malignant transformation. We will further discuss other precursors of the disease that
include: lentigo maligna, the congenital nevus, mucosal melanocytoses of conjunctival, nasopharyngeal,
penile, vulvar and gastrointestinal tract mucosa, and the atypical Spitz tumor (Crowson et al, 2001;
Crowson et al., 2002). In the course of this discussion we must also consider those pigmented lesions
which mimick melanoma and its precursors.
Case # 6: Dysplastic Nevus.
- Crowson AN, Magro CM, Mihm MC, Jr. The melanocytic proliferations : a comprehensive textbook of
pigmented lesions. New York : John Wiley and sons, 2001.
- Crowson AN, Magro CM, Sanchez-Carpintero I, Mihm MC Jr. The precursors of malignant melanoma.
Recent Results Cancer Res 2002;160:75-84.
The Dysplastic Melanocytic Nevus and the Dysplastic Nevus Syndrome
Lynch and co-workers (Lynch et al, 1978) coined the term familial multiple
atypical mole melanoma syndrome for a symptom complex of multiple atypical nevi which placed
family members at increased risk for developing melanoma in 1978. Clark and co-workers (Clark et al,
1978) characterized the dysplastic nevus as a defining element of this syndrome. Due to controversy surrounding the both the clinical and histologic
definition of these lesion , the National Institutes of Health (NIH) consensus conference in 1992
recommended supplanting the term dysplastic nevus with the appellation "nevus with architectural disorder
and cytologic atypia" (Crowson et al., 2001). In our view the original
term dysplastic nevus is preferred because clinicians are familiar with this
term suggested by Clark in his original article. Furthermore, it is clear that subgroups of congenital
and acquired nevi (such as those in acral and genital sites) manifest architectural disorder and
cytologic atypia but have no association with subsequent malignant melanoma. Also, the NIH term does not
encompass grading which we believe is integral to assessment of any dysplastic nevus (Shea et al, 1999;
Crowson et al., 2001). The incidence of dysplastic nevi is likely in the 5–10% range in Cauacasians
(book; Nordlund et al, 1985). The concern raised by the dysplastic nevus relates to the inherent risk of
transformation to malignant melanoma and to its being a marker for the development of melanoma at other
sites. The risk of progression in an individual dysplastic nevus is unknown, but certain facts have been
gleaned with regard to incidence. Melanoma patients with two or more dysplastic nevi may also be at
increased risk for a second primary. Up to 92% of melanomas occurring in patients with dysplastic nevus
syndrome have evidence of a dysplastic nevus precursor. Dysplastic nevi are precursors for up to 18% of
all nonfamilial melanomas (Grob etal, 1988). Roughly 95% of malignant melanomas arising in dysplastic
nevi are of superficial spreading type.
Patients with two or more dysplastic nevi are said to have the dysplastic nevus syndrome that occurs
sporadically or in a familial complex with an autosomal dominant pattern of inheritance, the latter most
often encoded by a gene found on chromosome 9 at 9p21 (Cannon-Albright et al., 1994). In a given
sibship, if one of the members with dysplastic nevi develops melanoma, other members with dysplastic nevi
have a 100% lifetime probability of developing melanoma (Greene et al, 1985). Some patients with
sporadic dysplastic nevus syndrome may resemble a person with familial dysplastic nevus syndrome by
virtue of numerous large, atypical nevi. These patients may be the spontaneous mutants or be members of
a family in which melanoma has not yet appeared or where history is unavailable. Most patients with
sporadic dysplastic nevus syndrome present in the 4th-5th decades with only a few
atypical moles on sun-exposed areas. UV radiation may play a role in the development of sporadic
Mutations and loss of heterozygosity of p16 and its alternate reading frame p14 ARF and p53 genes have been detected in blood lymphocytes from members of kindreds with
hereditary cutaneous malignant melanoma, most of the mutations being of the C -> T transitional type known
to be a signature for UV light-induced point mutation. One study showed areas of chromosomal loss at
regions encoding for p16 (9p21–22) and p53 (17p13) in 78% of dysplastic nevi, with no loss of
heterozygosity in benign intradermal nevi (Park et al., 1998); loss of heterozygosity at 9p21 appears to
be restricted to melanoma and to dysplastic, as opposed to banal nevi (Brindelli et al,2000). The CDKN2A gene responsible for melanoma susceptibility in most families with melanoma
linked to 9p encodes a cyclin-dependent kinase inhibitor, the dysfunction of which is also implicated in
several sporadic cancers; the second most frequent cancer in such kindreds linked to CDKN2A gene mutations is pancreatic carcinoma, which occurs in up to 17% of
patients (Vasen et al, 2000).
Patients with familial dysplastic nevi develop multiple large atypical moles distributed everywhere
on the body surface including the scalp, doubly covered areas (breasts of women and the bathing trunk
area of men and women), and lower legs. Banal acquired nevi usually spare the scalp and are rarely found
below the belt line. The characteristic features of the dysplastic nevus include: size greater than 6
mm. In diameter, irregular borders, variegated pigmentation in shades of brown dark brown and black and
are either flat or slight elevated with a pebbly surface. Rarely, the lesions are tan and erythematous,
a sign of inflammation often associated with more marked cytologic atypia in our experience. There is
striking heterogeneity of the lesions, no one lesion looking exactly the same as the other. The nevi
also form patterns, such as linear, arciform or even round arrays. Foci of hypopigmentation in the
lesion appear in areas of partial regression. Halo dysplastic nevi may also occur. It must be
emphasized that dysplastic nevi only a few millimeters in diameter occur as the lesions first appear.
Even lesions of this small size show both the other clinical and the histologic features of the larger
lesions. All these changes are in stark contrast to common acquired nevi that are less that 6 mm in
size, exhibit smooth regular borders and regular patterns of pigmentation so that the lesions all
resemble each other. Patients with multiple primary melanomas with or without
familial dysplastic nevus syndrome may show a diffuse pattern of irregular pigmentation resembling
freckling, but with an histology comprising intraepidermal lentiginous melanocytic dysplasia.
Epiluminescence microscopy of dysplastic nevi reveals a pattern of patchy interruptions in the
pigment network (the "broken network"), which is distinctive from common banal nevi and melanoma. We
have established that the use of near infrared spectroscopy to probe molecular vibration of chemical
bonds and so to assay tissue biochemistry non-destructively can be applied in-vivo to distinguish
dysplastic nevi from banal nevi and from lentigines with a high degree of accuracy (McIntosh et al,
2001). There is no doubt that dysplastic nevi differ from banal nevi from all of clinical, histological,
biochemical and molecular standpoints.
The histology of the dysplastic nevus is so reproducible that a diagnosis can usually be rendered on
scanning magnification. Interobserver variability lies in the area of grading of atypia, which should
not be done at scanning magnification as it requires assessment of cytology which can only be assessed at
x40 or higher magnification. The constellation of histological findings in the dysplastic nevus
encompasses two broad components: architecture and cytology (Clemente et al,1991).
Major Criteria for Diagnosis of a Dysplastic Nevus
- Asymmetric basilar proliferation of nevomelanocytes along the
dermoepidermal junction extending laterally beyond the confines of a preexisting dermal component if
- Cells have 1 or both of 2 characteristic cytological and
- Lentiginous dysplasia: randomly disposed single cells are located along
and between elongate rete with nests of varying sizes; nuclei are hyperchromatic, angulated and are
similar in size to or larger than adjacent keratinocytes.
- Epithelioid dysplasia: epithelioid melanocytes are disposed in variably
sized junctional nests as well as in a single-cell array along the dermoepidermal junction of an often
normal or hyperplastic epidermis. The cells have round to oval nuclei with delicate chromatin, nucleoli,
thick membranes, and diameters greater than those of adjacent keratinocytes. Rounded cytoplasmic
contours encompass cytosols ranging from amelanotic to coarsely melanized with giant melanosomes.
Minor Criteria for Diagnosis of a Dysplastic Nevus
Both major and two of four minor criteria must be met for the diagnosis to be made. Some lesions
exhibit cytologic atypia without the architectural changes and vice versa. We suggest that the
diagnosis be rendered as a junctional or compound nevus with atypia or a junctional or compound nevus
with the architectural changes of the dysplastic nevus, respectively. Changes suggestive of dysplasia
may be seen in other lesions (Toussaint and Kamino, 1999). 70% of congenital nevis show elongate rete
with coarse periretal fibrosis, but the other changes of atypia, increased vascularity and patterned
inflammation are not present. Similarly acral and genital nevi may at first glance resemble dysplasia,
but should only be diagnosed as dysplastic when requisite criteria are met. Lentigo maligna often has
focal reactive hyperplasia of the epidermal component and may suggest dyplastic nevus.
- Papillary dermal collagen shows concentric eosinophilic fibrosis in
which a dense zone of hypocellular collagen envelops rete ridges and/or lamellar fibroplasia in which
delicate layers of collagen are interspersed with presumptive neural crest-derived facultative
fibroblasts laying collagen along the tips of hyperplastic retia in parallel arrays.
- Lymphocytic infiltrates in the papillary dermis.
- Telangiectasia and/or vascular proliferation.
- Fusion of retia by confluent growth between adjacent melanocytic
Studies have shown that criteria for grading dysplasia can be learned and reproducibly applied by
pathologists with a consistency that is maintained in both the three tier grading system which we employ,
namely, mild, moderate and severe, or a two tier system, namely low and high grade dysplasia (Shea et al,
1999; Clemente et al., 1991; Murphy and Mihm, 1999). Some observers have found reasonable concordance in
grading of architectural, but not cytologic features, implying that the interpretation of cytology is
more challenging (Hastrup et al, 1994; Tannous et al, 2000). We grade atypia on the basis of both
architecture and cytology, which are assessed separately, although architectural and cytologic grades of
atypia tend to correlate. There is correlation between the degree of architectural and nuclear atypia
(Shea et al, 1999). Readers are referred elsewhere for grading criteria (Crowson et al., 2001).
At the first patient visit, we recommend that a clinically atypical nevus be excised to confirm the
impression of dysplastic nevus. Any changing or suspicious lesion should be removed. It is our practice
that excisional biopsy with a few millimeters margin is appropriate. If a partial biopsy has been
performed, we advise removal of any clinically apparent residuum with a few millimeter margin. If slight
atypia is present at a histologic margin without a clinically-evident residuum, we do not advise
reexcision. If moderate atypia is present at a margin, we advise conservative reexcision with a few
millimeter margin. If severe dysplasia is present at a margin, or if a margin is only clear by a
millimeter or two, we advise reexcision to obtain a 5 mm margin of normal skin. Follow-up of any patient
is dependent on the number of lesions and the degree of clinical or histological atypia. Photographic
documentation is desirable.
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