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congenital melanocytic nevus

Thursday 25 February 2010

congenital melanocytic naevus; neonatal nevus

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Congenital melanocytic nevus / neonatal naevus

Congenital melanocytic nevi (CMN) occur in 1% to 2% of newborns.

Large CMN greater than 10 cm in diameter have an increased risk of malignant transformation and malignant melanoma.

Cases of CMN and CMN with secondary proliferations can be assigned to six groups according to the predominant histological pattern (12368190):
group I, bland congenital nevi
group II, congenital nevi with foci of increased cellularity
group III, CMN with a proliferation simulating superficial spreading melanoma in situ
group IV, CMN with a proliferation simulating nodular melanoma
group V, proliferating neurocristic hamartoma
group VI, melanoma arising in congenital nevus

Small and intermediate congenital melanocytic nevi have a lifetime risk of developing melanoma estimated to range from 0% to 5%.

Secondary benign melanocytic proliferations commonly arise in congenital melanocytic nevi.

Congenital nevi are benign neoplasms that are present at birth and composed of nevomelanocytes. Approximately 1-3% of all newborns have congenital pigmented nevi, and the number of nevi increases with age, peaking by late adolescence to 20-40 nevi in an individual.

Giant congenital nevi are often defined as nevi that are greater than 20 cm in diameter in an adult, or nevi that occupy 2% or more of the body surface area.

Histologically, nevi are transformed melanocytes, which are normally highly dendritic cells interspersed among basal keratinocytes.

The genetic basis of these lesions is not known. Findings of a culture of melanocytes from such a lesion from a showed chromosome rearrangements involving 1p,12q, and 19p.

The giant nevi might be associated to several diseases:

neurocutaneous melanosis
diffuse lipomatosis
structural brain malformations
hypertrophy of skull bones
limb atrophy
skeletal asymmetry involving both soft tissue hyper-and hypoplasia
von Recklinghausen’s disease
vitiligo

The risk of malignant change in giant nevi is probably the most contentious issue in its management. The consensus is that lesions are pre-malignant, but the purported incidence of malignancy varies wildly from 0-42%.

Surgical excision remains the mainstay of treatment for large congenital melanocytic nevi, and most giant nevi are managed by staged excision and resurfacing with skin grafts or tissue expanders and flaps.

Evolution

Large congenital melanocytic nevi (CMN) are at an increased risk of developing melanoma. Several forms of secondary proliferations can arise in congenital nevi on rare occasions. Although some of these closely resemble melanoma both clinically and histologically, metastasis is rare.

During the neonatal period several types of melanocytic tumors can develop within CMN, many of which are thought to be distinct from melanoma.

These range widely in size, and can grow very fast and ulcerate.

Even the most clinically worrisome tumors arising within the neonatal period often have a benign course and tend to stabilize or regress after a period of rapid growth.

Because true melanoma can occur in the neonate, the development of any secondary proliferation in a CMN is of great concern.

These lesions can be extremely difficult to classify histologically.

Four different histological patterns of secondary proliferations in CMN during the neonatal period have been described:

1) simulants of superficial spreading melanoma (SSM), in which the epidermis and superficial dermis contain large epithelioid melanocytes, sometimes with pagetoid spread in the epidermis;
2) simulants of nodular melanoma with a nodular proliferation of large melanocytes with uniform nuclei in the dermis;
3) cases described as proliferative neurocristic hamartoma, characterized by a deep dermal or subcutaneous proliferation with a variety of forms of neural or mesenchymal differentiation;
4) true melanoma, most of which show small blast-like melanocytes with hyperchromatic nuclei, scant cytoplasm, and a high mitotic rate.

Chromosomal aberrations are common in nodular proliferations, and are absent from conventional congenital nevi.

However, in contrast to melanoma in which structural chromosomal aberrations are found in the vast majority of cases, the aberrations in these secondary proliferations are predominantly numerical changes. (12368190)

These findings point toward a qualitatively different type of genomic instability in atypical nodular proliferations in congenital nevi possibly explaining their less aggressive behavior. The different genomic characteristics may also be useful in the classification of histopathologically ambiguous cases. (12368190)

Proliferative nodules

Appearance at birth or later of a nodular or hyperpigmented area within a CMN simulates malignant melanoma and prompts biopsy.

Melanoma

The risk of malignant melanoma is increased in patients with large CMN.

Patients with congenital melanocytic nevi (CMN) have an increased risk of developing melanoma within lesional skin.

Whereas in small (@<@1.5> 20 cm) carry a 5 to 15 times increased risk of developing melanoma and, rarely, other neural crest-derived malignancies.

Variants

giant congenital melanocytic nevus
proliferative nodule in congenital melanocytic nevus

CGH

No aberrations were found in groups I to III, whereas seven of nine cases of group IV, and one of one case of group V, showed aberrations. In group IV six of seven cases with aberrations (86%) showed numerical aberrations of whole chromosomes exclusively. (12368190)

This pattern differed significantly from the findings in melanoma that arose within CMN, group VI, or independent of CMN in which only 5% showed numerical changes only. (12368190)

A case in group V showed aberrations similar to melanoma. (12368190)

The finding of frequent numerical chromosomal aberrations in atypical nodular proliferations arising in CMN identifies these as clonal neoplasms with a genomic instability consistent with a mitotic spindle checkpoint defect. (12368190)

This difference compared to the aberration pattern found in melanoma might explain their more benign clinical behavior and may be of diagnostic value in ambiguous cases. (12368190)

congenital melanocytic nevus

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