The histogenesis of malignant melanoma in relation to pre-existing pigmented lesions

One hundred and twenty-nine cases of malignant melanoma seen at Bangour General Hospital over an eleven year period were reviewed clinically and histologically. In 51 patients (39.5%) there was a history of pre-existing pigmented lesion at the site of the melanoma. In 14 cases (10.8%) histology confirmed traces of a benign melanocytic naevus. The relationships between naevus cells and malignant melanocytes are described. The patterns most frequently observed were those of intradermal naevus cells in the deeper part of an invasive melanoma and of a compound naevus undergoing malignant change in its junctional component. From these histological observations a histogenetic sequence of events is postulated. In the 37 patients with a positive history of a preceding pigmented lesion but no histological traces of a naevus, the percentage incidence of Lentigo Maligna (LM) and Superficial Spreading Melanoma (SSM), as compared with the 78 patients with a negative clinical history, was significantly higher than the incidence of nodular melanoma (NM). Although LM and SSM are thought to have separate histogenesis and aetiology, they share an important clinical and histological factor, i.e. in both, the radial growth phase is preceded by a, more or less, prolonged stage of intraepidermal horizontal growth. It is postulated that this pre-existing lesion of atypical melanocytic proliferation rather than a hypothetical benign naevus is in most cases the precursor of an invasive melanoma.