Melanoma arising on a naevus is different from melanoma arising de novo

Linked Article:   Dessinioti et al . Br J Dermatol 2021; 185 :101–109. Skin melanoma usually arises on healthy‐looking skin and this is called de novo melanoma (70% of melanoma). When melanoma arises on a pre‐existing naevus (skin mole) it is called naevus‐associated melanoma (NAM, 30% of melanoma). The differences between de novo and NAM are not clear because de novo melanoma are thicker tumours and there may have been a naevus component that was later replaced by tumour growth. To investigate if de novo melanoma is distinctly different from NAM, we conducted a multicentre study (in Europe, Australia and the USA) focusing on thin melanoma (Breslow thickness ≤1 mm). We found that in a total of 9474 localized melanoma (i.e. ones that have not spread to other parts of the body), de novo melanoma compared with NAM were associated with thicker tumours and there were body‐site clinical differences and histological differences (changes seen under the microscope). When looking at a subset of 5307 thin melanoma, we observed similar body‐site differences: de novo melanoma compared with NAM was more likely to affect older individuals (≥70 years) when located on the head/neck, trunk or the upper extremity; to affect females when located on the lower extremities; to be of the nodular melanoma subtype when located on the trunk; and this type of melanoma was less likely to have histological regression present (a focal decrease in the number of melanoma cells, with changes similar to those observed in a scar). We did not find a difference in survival between patients with de novo melanoma compared with NAM, but our study had an average follow‐up time of 3 years. In conclusion, these differences suggest that de novo melanomas are different from NAM and point to different mechanisms of development that depend on the body site.