Sentence: In 2005 , after a significant disease-free survival benefit was reported for letrozole as compared with tamoxifen , a protocol amendment facilitated the crossover to letrozole of patients who were still receiving tamoxifen alone ; Cox models and Kaplan-Meier estimates with inverse probability of censoring weighting ( IPCW ) are used to account for selective crossover to letrozole of patients ( n=619 ) in the tamoxifen arm .
Drug combination annotation:
[{'class': 'NEG', 'spans': ['letrozole', 'tamoxifen'], 'is_context_needed': True}]