Survival estimates of a prognostic classification depended more on year of treatment than on imputation of missing values

BACKGROUND AND OBJECTIVE: The International Germ Cell Consensus (IGCC) classification defines good, intermediate, and poor prognosis groups among patients with nonseminomatous germ cell cancer. In the database used to develop the IGCC classification (n = 5,202), >40% of patients were excluded because of missing values (n = 2,154). We looked for effects of this exclusion on survival estimates in the three IGCC prognosis groups. STUDY DESIGN AND SETTING: We imputed missing values using a multiple imputation procedure. The IGCC classification was applied to patients with complete data (n = 3,048) and with imputed data (n = 2,154), and 5-year survival was calculated for each prognosis group. RESULTS: Patients with missing values had a lower 5-year survival than those without missing values: 76% vs. 82%. Five-year survival in the complete and imputed data samples was 92% and 87% for the good prognosis groups and 80% and 70% for the intermediate prognosis groups, whereas 5-year survival for the poor prognosis groups in both samples was similar (50% and 47%, respectively). This difference in survival was largely explained by a higher proportion of missing values among patients treated before 1985, who had a worse survival than patients treated after 1985. CONCLUSION: Multiple imputation of the missing values led to lower survival estimates across the IGCC prognosis groups, compared with estimates based on the complete data. Although imputation of missing values gives statistically better survival estimates, adjustments for year of treatment are necessary to make the estimates applicable to currently diagnosed patients with testicular cancer.