It is well recognized that primary cutaneous melanomas can occasionally regress. Interestingly, regression seems to occur almost exclusively in the radial growth part of melanoma. Several studies have shown that virtually all melanomas that show regression are thin.1 One might argue that this is because the regression resulted in a thinning of a previously thick melanoma. Several observations argue against such an interpretation. Once melanomas form a nodule, regression of the nodular portion is highly unusual. The histologic telltale sign of regression—the presence of significant accumulations of melanophages—almost always occurs in areas of radial growth. Regression of a melanoma in the vertical growth phase would lead to a significant pigment deposition, resulting in a large accumulation of melanophages. Although, this phenomenon—tumoral melanosis—does exist, it is exceptionally rare. The current explanation of regression is an immune-mediated process. The association of regression with a distinct phase of progression suggests that cancer-autonomous mechanisms are contributing to the phenomenon of regression.
Bastian BC. Hypothesis: A Role for Telomere Crisis in Spontaneous Regression of Melanoma. Arch Dermatol. 2003;139(5):667–668. doi:10.1001/archderm.139.5.667