However, further investigation is needed to understand potential differences between 2 and 5 years of therapy. Regarding tamoxifen, direct comparison between 2 years of 40 mg once daily as given in the STO-5 trial with today's at least 5 years of 20 mg once daily is difficult. The optimal dose and duration of endocrine therapy in premenopausal patients have not been established and may differ from that for postmenopausal patients.
There are limitations to this study. In the ER-positive subset of the STO-5 trial, sample size is a limitation and caution should be taken in the interpretation. Because of sample size, further subanalysis, for instance, on age was not statistically justifiable. Comparable with today's treatment approach, the STO-5 trial design allocated chemotherapy to lymph node–positive patients. Because of trial design, the additional effect from chemotherapy cannot be explored in the STO-5 trial.
In conclusion, results from the STO-5 trial with a 20-year follow-up suggest a long-term benefit from 2 years of adjuvant endocrine therapy in ER-positive premenopausal patients. Furthermore, long-lasting benefit from tamoxifen in genomic low-risk patients with steady long-term risk of distant recurrence is observed, whereas genomic high-risk patients with early risk benefit from goserelin.