Classification of malignant melanomas: a view of the current controversy.

The Clark classification of malignant melanomas into four types is well known and widely used. This classification system has been recently challenged by Ackerman and subsequently defended by Elder, Jucovy, Tuthill, and Clark. Four areas of disagreement among these opposing authors are discussed. They include the reliability of the criteria used for distinguishing the four types of malignant melanomas, the possible site dependence of the four types of malignant melanomas, the differing definitions given to radial-growth phase, horizontal-growth phase, and vertical-growth phase; and whether a malignant melanoma can exist in a "pure" vertical-growth phase (nodular melanoma). An analogy to the classification of warts is made. The view of this paper is that all classification systems are arbitrary and thus are types of models. Like other models, they should be evaluated only in terms of their usefulness in explaining pathogenesis or predicting clinical outcome.