The risk of cutaneous melanoma in melanocytic nevi.

The data on melanoma associaed with melanocytic nevus are controversial. A longitudinal prospective study of 107 cases of cutaneous melanoma revealed that 9 (8.4%) cases were presumed to be linked to a precursor lesion, but only in 1 (0.9%) out of these cases the histopathological examination showed an associated melanocytic nevus. The vague information of a preexisting lesion of cutaneous melanoma is not sufficient to consider it a tumour precursor and it requires histopathological evidence to confirm the diagnosis.

The risk of malignant transformation in the congenital melanocytic nevi (CMN) generally ranges, according to the literature, from 5 to 40% and from 4.5 to 10%.[1-3] The risk of developing melanoma in small CMN is more common in adults (around 60 years).[4]

The diagnosis of melanocytic nevus (congenital or acquired) is primarily clinical, and its presence at or immediately following birth is conclusive for the diagnosis of CMN. The acquired melanocytic nevi (AMN) arise from two years of age in the majority of cases, reaching a peak in the second and third decades and disappearing between the seventh and ninth decades. Its development seems to be related to some factors such as skin type, ethnicity, genetic predisposition and exposure to ultraviolet light. AMN tend to be more numerous in chronically sun-exposed areas and among children with light skin, blond hair and blue eyes. This pattern is not described for CMN.[5] The histopathological examination of 1101 cutaneous melanomas has revealed that 23.3% have been associated with melanocytic nevi, 56.5% being histopathologically classified as AMN, 5.8% as CMN, and 37.7% as dysplasic.[6] In other large series it has been demonstrated by histopathological examination that small CMN have been the precursor to melanoma in 8.1% of cases.[7] Forty-four (21.6%) of the total of 204 cases of melanoma have resulted from the transformation of small CMN, defined as based exclusively on anamnesis data.[8]

A longitudinal cross-sectional study of 107 cases of cutaneous melanoma in HUCFF/UFRJ between 1993 and 2008 revealed the four clinical and eight histopathological types ( Tables 1 and 2).[9,10]

Table 1

Distribution of cases according to clinical type

Clinical Type	No of cases
Total	107
Extensive superficial	63
Acral lentiginous	23
Nodular	12
Lentigo maligna	9

Table 2

Distribution of cases according to histopathological type

Histopathological Type	No of cases
Total	107
Extensive superficial	31
In situ	24
Nodular	15
Lentigo maligna	8
Acral lentiginous	5
Desmoplasic	2
Amelanotic	1
Associated with melanocytic nevus	1 (0.9%)
Not classified	20

In nine (8.4%) of the total of 107 cases, a precursor lesion was reported compatible with melanocytic nevi - in eight cases since childhood; in one case there was no information on the length of time it took to evolve. The histopathological types of these cases were defined as superficial spreading (4), nodular (2), desmoplastic (1), unclassified (1), histologically associated to nevus (01).

The dimensions of the melanoma discarded CMN of medium, large, and giant sizes, remaining the definition of small CMN (<1.5cm) and AMN.[5] The presented data allow the conclusion that the association between melanocytic nevus and melanoma should be confirmed by histopathology. The possible total occupancy of the melanocytic nevi by neoplastic cells imposes itself as a variable in the study of the magnitude of the risk of transforming melanocytic nevi into melanoma.