Histopathology

Oral melanoma may have in-situ (radial) and invasive growth phases, but the histological classification is not analogous to cutaneous lesions. Mucosal lesions are similar to acral lentiginous melanoma of the skin {2652}, with junctional activity and upward migration but Pagetoid invasion is unusual. Atypical melanocytic lesions may progress to malignant melanoma but there is little evidence for progression of oral benign melanocytic naevi to invasive malignancy {1085, 2652}. Oral mucosal melanoma is, therefore, classified as in-situ oral mucosal melanoma, invasive oral mucosal melanoma, and mixed in-situ and invasive lesions. Borderline lesions may be termed atypical melanocytic proliferations {122,1085,2652}. Most lesions at presentation are invasive or have mixed invasive and in-situ com

Fig. 4.57 A in-situ growth phase showing atypical and enlarged melanocytes at the epithelial-connective tissue interface. Melanocytes may show upward migration into the epithelium. B Invasive lesions showing considerable junctional activity, with atypical melanocytes invading into the underlying connective tissues.

Fig. 4.57 A in-situ growth phase showing atypical and enlarged melanocytes at the epithelial-connective tissue interface. Melanocytes may show upward migration into the epithelium. B Invasive lesions showing considerable junctional activity, with atypical melanocytes invading into the underlying connective tissues.

ponents. Less than 20% are solely in-situ lesions. Typically, an oral melanoma is composed of sheets or islands of epithe-lioid melanocytes, which may be arranged in an organoid, or alveolar pattern. The cells have pale cytoplasm and large open nuclei with prominent nucleoli and occasionally they may be plasmacy-toid. Sheets and fascicles of spindle cells may also be seen, but are usually a minor part of the lesion. Occasional lesions may be predominantly or wholly spindled. Over 90% of lesions contain melanin pigment that can easily be demonstrated with stains such as Masson-Fontana or Schmorl's. When present, the in-situ component shows atypical naevoid cells arranged singly or in nests at the epithelial-connective tissue interface. Upward migration of the cells is common, but Pagetoid islands, similar to those of superficial spreading cutaneous melanomas, are not frequent. Invasion may be difficult to determine but the presence of obviously malignant cells in the lamina propria indicates invasion and islands of cells larger than those seen within the epithelium suggest an invasive growth phase. Mitoses are surprisingly sparse but are seen more frequently in invasive lesions. The overlying epithelium is usually atrophic and just over half of lesions are ulcerated.

Immunoprofile

Over 95% of lesions are S100 positive and negative for cytokeratins {144}. Although sensitive, S100 is not specific. More specific markers include HMB45, Melan-A or anti-tyrosinase, which stain about 75% of lesions {2079}.

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