Estimated risk of progression of lentigo maligna to lentigo maligna melanoma

Little is known about the risk of progression of lentigo maligna to lentigo maligna melanoma. We determine the annual risk of progression of lentigo maligna to lentigo maligna melanoma by analysing a prospective population-based survey of recently diagnosed anterior (visible in a mirror) head and neck lentigo malignas and lentigo maligna melanomas. Six hundred eighty-two consecutive patients aged 18-80 years with non-recurrent lentigo maligna or lentigo maligna melanoma, diagnosed between 1 July 2015 and 20 April 2016, were identified from pathology notifications to the New South Wales Cancer Registry (Australia) and sent survey questionnaires soon after diagnosis (median 4.6 months interquartile range: 3.8-5.7). Details of the time the lesion was present and when changes to it were noticed before diagnostic biopsy were ascertained by surveying the patients, of whom 53.5% agreed to participate. There was little difference between the proportions of lentigo maligna melanoma and lentigo maligna in the consenting and non-consenting patients (P = 0.56). Two hundred twenty-eight lentigo maligna (median age 67 years, range: 38-80) and 33 lentigo maligna melanoma (70 years, 43-80) were surveyed. There was no difference between the time lentigo maligna melanoma was present on the skin (median 18 months, range: 0-690) and the time lentigo maligna was (18 months, 0-665) (P = 0.972). The estimated risk of progression of lentigo maligna to lentigo maligna melanoma was 3.5% per year (95% confidence interval: 2.5-5.0). This equates to an average time for lentigo maligna to progress to lentigo maligna melanoma of 28.3 years (95% confidence interval: 20.0-40.5) in this population. Although our data suggests that the annual progression rate of lentigo maligna is more than 25 times greater than previously suggested, the rate is still low.