Malignant melanoma can arise de novo or from a pre-existing melanocytic nevus.
Its appearance can vary, ranging from an irregularly contoured, relatively flat lesion with variegated pigmentation, to a sharply demarcated blue-black nodule. Prognosis is determined by Clark's level of invasion and by Breslow's thickness measurement , determined by measuring in millimeters the distance between the top of the epidermal granular cell layer and the deepest extent of the melanoma into the dermis or subcutis. The exhibited slide shows a nodular melanoma of Clark's level V, which indicates involvement of the subcutaneous fat. Among primary malignant melanomas, a tumor with these characteristics tends to have the poorest prognosis. A nodular malignant melanoma of the forehead.