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Objectives: For cytostatic cancer trials, Growth Modulation Index (GMI) defined by an intrapatient progression-free survival (PFS) ratio, has been proposed to evaluate the efficacy of new target agent. The purpose of this study was to suggest new methods for the estimation of GMI with censored data in the first PFS (PFS1) interval, and subsequent second PFS (PFS2) interval. Methods: The proposed methods include latent variable approach based on Rank Preserving Structural Failure Time (RPSFT) model and Accelerated Failure Time (AFT) model. Simulations were conducted to compare the performance of proposed GMI estimates and estimates based on the Kaplan-Meier method in terms of bias and mean squared error (MSE) by varying dependency of two PFS and censoring rates. Results: Simulation results show that new GMI estimates using latent variable approach and AFT model exhibited smaller bias and MSE than the previous estimates based on the Kaplan-Meier survival function. As censoring rates increased in PFS1, bias and MSE increased in the previous GMI estimates. When the AFT model was applied in the case of high censoring rates, bias was relatively higher than those of latent variable approach. Conclusions: When using GMI as primary endpoint in cancer clinical trials, cautious statistical application and interpretation is needed, particularly for the presence of censored data in the first PFS interval.

Estimation of Growth Modulation Index Using Latent Variable and Accelerated Failure Time Model