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David J. Margolis, MD; Allan C. Halpern, MD; Timothy Rebbeck, PhD; Lynn Schuchter, MD; Raymond L. Barnhill, MD; Judith Fine, BA; Marianne Berwick, PhD, MPH

Arch Dermatol. 1998;134:1597-1601.

Background  A "clinically accessible," 4-variable (patient age, patient sex, tumor location, and tumor thickness) prognostic model has been published previously. This model evaluated variables that were commonly available to the clinician. Because models are heuristic, validity of a prognostic model should be evaluated in a population different from the original population.

Objective  To evaluate the external validity of this 4-variable melanoma prognostic model.

Design  To estimate the external validity of this model, we used a population-based cohort of individuals with melanoma. We also evaluated a 1-variable model (tumor thickness). Estimates of the external validity of these logistic regression models were made using the c statistic and the Brier score.

Settings and Patients  A total of 1261 patients with melanoma evaluated in a multispecialty, university-based practice and 650 patients with melanoma from throughout Connecticut.

Main Outcome Measure  Death from melanoma within 5 years of diagnosis.

Results  The c statistics for the 4-variable model were 0.86 (95% confidence interval [CI], 0.83-0.89) for the university-based practice data set and 0.81 (95% CI, 0.75-0.86) for the Connecticut data set. For thickness alone, the c statistics were 0.83 (95% CI, 0.80-0.86) and 0.79 (95% CI, 0.74-0.85), respectively. Brier scores for the 4-variable model were 0.09 (95% CI, 0.08-0.10) and 0.08 (95% CI, 0.06-0.09) and for the 1-variable model were 0.09 (95% CI, 0.08-0.10) and 0.08 (95% CI, 0.07-0.10), respectively. No significant differences exist between the data sets for the 4- and 1-variable models.

Conclusions  The 4- and 1-variable models are generalizable. The simpler 1-variable model—tumor thickness—can be used with a relatively small loss in accuracy.

From the Departments of Dermatology (Dr Margolis) and Biostatistics and Epidemiology (Drs Margolis and Rebbeck), University of Pennsylvania School of Medicine, and University of Pennsylvania Cancer Center (Dr Schuchter), Philadelphia; the Department of Dermatology (Dr Halpern) and the Epidemiology Service (Dr Berwick), Memorial Sloan-Kettering Cancer Center, New York, NY; the Department of Dermatology, Johns Hopkins University School of Medicine, Baltimore, Md (Dr Barnhill); and Rapid Case Ascertainment Shared Resources, Yale University School of Medicine, Yale Cancer Center, New Haven, Conn (Ms Fine).

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