Size overlap between benign melanocytic choroidal nevi and choroidal malignant melanomas.

PURPOSE To estimate size overlap between large choroidal nevi and small choroidal melanomas by using plotted frequency distributions of tumor size. METHODS Frequency distributions of largest linear basal diameter (LBD) and thickness (TH) of choroidal nevi and melanomas were plotted from published data and cases in the senior author's practice. Relative frequencies of choroidal nevi and melanomas were estimated from published data. Relative frequency distributions of the tumors were plotted to illustrate the extent of overlap between them. RESULTS Comparison of plotted frequency distribution curves for thickness indicated that there were approximately 125 nevi for every melanoma in the TH range 1.5 to 2 mm, approximately 25 nevi for every melanoma in the TH range 2 to 2.5 mm, and approximately 5 nevi for every melanoma in the TH range 2.5 to 3 mm. Similarly, comparison of the plotted frequency distribution curves for LBD of these tumor types indicated that there were approximately 70 nevi for every choroidal melanoma in the LBD range 5 to 6 mm, approximately 10 nevi for every melanoma in the LBD range 6 to 7 mm, and approximately 3 nevi for every melanoma in the LBD range 7 to 8 mm. CONCLUSIONS Because of the markedly greater cumulative lifetime incidence of choroidal nevi, the results of this analysis suggest considerable size overlap between larger nevi and smaller melanomas. Attempts to classify small melanocytic choroidal tumors clinically as benign nevi versus malignant melanomas on the basis of tumor size appear likely to result in multiple misclassifications.