Most primary melanomas go through phases of growth—a radial growth phase and a vertical growth phase—while staying confined to the outer skin layer (epidermis) and inner skin layer (dermis). During the radial growth phase, melanoma grows out into skin on the side but only slightly down into the skin layers. Cancer cells stay in the epidermis, the area between the epidermis and dermis, and the upper dermis. Primary melanomas rarely spread (metastasize) to other sites in the body in this phase. Thin, radial-growth–phase primary melanomas are easily cured by surgical removal (excision).

During the vertical growth phase, the cancer grows down into skin, and a small raised bump (nodule) may develop on the surface of the melanoma. Melanoma that has entered the vertical growth phase is more likely to spread to other parts of the body and is more difficult to cure than is radial-growth–phase melanoma.¹