Pigmented Naevi

Oral melanocytic naevi are much rarer than cutaneous lesions with a prevalence of 0.1% of the general population [75]. The most common sites are the hard palate, buccal mucosa and labial mucosa. Women are affected twice as frequently as men and most cases are seen in the third and fourth decades. They are usually single and

ICD-O:8720/3

Oral mucosal melanomas are rare and account for about 0.5% of oral malignancies [35, 75]. However, they appear to be more common in Japan, India and Africa. Most arise in adults with a peak incidence between 40 and 60 years. Large series show a male predominance. About 80% of cases involve the palate and maxillary alveolus and gingivae. About a third of cases are preceded by long-standing areas of oral hyperpigmentation, but they rarely arise from pre-existing benign melanocytic naevi. The majority of cases are painless in the early stages and form irregular, black or brownish flat, raised or nodular areas that are frequently multicentric. Rarely, they are amelanotic and may be reddish in colour. Nodular areas are usually a feature of more advanced tumours and may be ulcerated and associated with pain and bleeding. Invasion of the underlying bone is common and teeth involved may loosen or exfoliate. In most cases there is involvement of the cervical lymph nodes at presentation and half of patients have distant metastases.

Purely nodular melanomas are relatively rare and most tumours have a radial growth element similar to that seen in cutaneous acral lentiginous melanoma together with evidence of upward migration. Oral melanomas have been divided into:

1. In situ oral mucosal melanomas;

2. Invasive oral mucosa melanomas;

3. Mixed in situ and invasive oral mucosal melanomas.

About 15% of oral mucosal melanomas are in situ and 30% are invasive [9]. Fifty-five percent of melanomas have a combined pattern. Borderline lesions have been termed atypical melanocytic proliferations [75].

Microscopically, in situ melanomas show an increase in atypical melanocytes. Although these atypical mela-nocytes have angular and hyperchromatic nuclei, mitoses tend to be sparse. The melanocytes may form aggregates or be irregularly distributed in a junctional location. The characteristic nested or thequal pattern commonly seen in cutaneous melanomas is less frequently observed in mucosal lesions. Sometimes the melano-cytes are dispersed throughout the epithelium and this may be combined with a junctional pattern (Fig. 3.14). Sequential biopsies have shown increases in the density of the junctional atypical melanocytes over time. Atypical melanocytes can extend down the excretory ducts of the underlying minor salivary glands. However, there is usually no inflammatory response to in situ lesions.

The melanocytes present in invasive melanomas show a variety of cell types including epithelioid, spindle and plasmacytoid. They typically have large, vesicular nuclei with prominent nucleoli. Mitoses may be present, but usually not in large numbers. They are usually aggregated into sheets or alveolar groups and less commonly neurotropic or desmoplastic configurations are seen. About 10% of cases are amelanotic. Over 95% of lesions are anti S-100 antigen-positive [10] and more specific markers include HMB45, Melan-A and antityrosi-nase [144].

Atypical melanocytic proliferation or hyperplasia is the term used for lesions with equivocal histopathologi-cal features, but the criteria for inclusion in this category are rather ill-defined [176]. These include oral mucosal lesions with melanocytes containing angular or hyper-

Fig. 3.14. In situ melanoma showing atypical melanocytes widely dispersed throughout the epithelium

chromatic nuclei with very infrequent mitoses. Melano-cytic atypia can vary from mild to severe [75].

Oral melanomas are much more aggressive than their cutaneous counterparts. The prognosis of oral melanomas is poor with a 5-year survival rate of less than 20% [9], and even Stage I tumours have a 5-year survival rate of less than 50%. The conventional depth of invasion indicators such as Breslow thickness and Clark's levels tend to be of little value in mucosal melanomas, as many present at an advanced stage and most are deeper than 4 mm [145]. Histological features associated with a poor prognosis include evidence of vascular invasion, cellular pleomorphism, necrosis and amelanotic tumours [14, 75, 123, 145].

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