Melanomas arising from naevi and de novo melanomas--does origin matter?

BACKGROUND: It is widely accepted that some melanomas arise from pre-existing naevi, while others appear de novo. The proportions involved and the effect of melanoma origin on prognosis is unclear.
OBJECTIVES: To determine whether melanomas reported by the patient to have developed from a pre-existing naevus are associated with a better or worse prognosis compared with those arising de novo when adjusted for confounding variables.
METHODS: All patients attending a dedicated melanoma screening clinic between March 1997 and March 2002 were included. The distinction between melanoma arising without any pre-existing lesion (de novo) and those derived from a pre-existing lesion (naevus melanoma) was based on patient history. We categorized patients into three groups: those who gave a history of their lesion arising within a pre-existing naevus, those in whom the melanoma developed de novo and those in whom no conclusive history could be obtained. We compared prognostic indicators between the naevus and de novo melanoma groups.
RESULTS: Of 8593 patients screened, 377 had a positive diagnosis of melanoma (in situ or invasive). Of these 42% had naevus melanomas, 34% new melanomas and 24% were uncertain. Patients presenting with a melanoma arising from a pre-existing naevus had a greater Breslow thickness despite presenting sooner than the de novo group, although no significant difference in thickness was found when other prognostic factors were controlled for.
CONCLUSIONS: This prospective study shows that naevi that undergo malignant change may result in melanomas that are thicker and thus potentially have a worse prognosis than de novo melanomas. Although our results were not statistically significant when other risk factors were also taken into account, it is possible that a larger study would identify a significant association.