Difficulties encountered in the application of Clark classification and the Breslow thickness measurement in cutaneous malignant melanoma.

The purpose of this study was to analyse the reproducibility of Clark's classification and Breslow's measure of tumor thickness to determine if there is general agreement among pathologists regarding these parameters and whether these pathologists reach consistent conclusions concerning each slide. Three successive workshops including a panel of pathologists were held at Amsterdam, in May 1977; at Villejuif in June 1977, followed by circulation of slides and at Villejuif again, in June 1978, followed by slide projection. This was necessary because of the great divergencies of opinion noted at the first meeting at Amsterdam. These sessions gave a better understanding of the divergencies and finally indicated that general agreement could only be reached by slide presentation followed by discussion. Regarding histogenesis, agreement between pathologists was improved after discussion from 6/11 cases to 11/13 cases. Regarding the level, no initial agreement was reached (Amsterdam). At the second meeting an agreement was obtained in 4/13 cases and finally in 11/13 cases (Villejuif). Clark's classification is more difficult to use than Breslow's because it depends on a discontinuous variable, the choice of the infiltration level. Breslow's classification, on the other hand, is based on a continuous variable. Because of frequent discrepancies, the classification of patients into various therapeutic groups according to Clark's level and Breslow's thickness necessitates that all cases including in therapeutic trials be reviewed by a panel of pathologists.