An updated meta-analysis of trials of five years of treatment of early breast cancer with tamoxifen has reported that tamoxifen reduces breast cancer-specific death by about one-third throughout the first 15 years for women with estrogen receptor positive (ER+) disease. As tamoxifen trials mature, the relevance of hormone receptors and other factors to long-term prognosis can more reliably be assessed.

The meta-analysis included individual patient data from 20 trials with 21,457 participants. Patients who had been diagnosed with early breast cancer and received approximately five years of tamoxifen (with about 80% compliance) were compared to similar patients who received no adjuvant tamoxifen.

Among women with ER+ tumors (10,645 participants), five years of tamoxifen was found to reduce recurrence rates by almost half during years 0 to 4 and by over 30% during years 5 to 9. The reduction in recurrence held without increasing or decreasing during years 10 to 14. The favorable results in ER+ disease were found to be substantially independent of progesterone receptor (PR) status (or level). In other words, tamoxifen was beneficial for both ER+/PR+ and ER+/PR- disease. The reduction in recurrence was substantial even in breast cancer that was only weakly ER positive. On the other hand, tamoxifen was found to have little or no effect on breast cancer recurrence or mortality in women with ER- disease (ER-/PR- or ER-/PR+).

Breast cancer-specific death was also reduced by approximately one-third throughout the first 15 years. Non-breast cancer-specific death was not greatly affected by tamoxifen treatment since deaths from blood clots and uterine cancer (both found only in women older than 55 years) were infrequent (about 1% of study participants taking tamoxifen). As a result, all-cause mortality was substantially reduced by tamoxifen. The authors conclude that five years of adjuvant tamoxifen safely reduces 15-year risks of breast cancer recurrence and death. ER status was the only factor studied that importantly influenced the tamoxifen-related reductions.

Please see our article on what to eat during tamoxifen treatment for more information on how to optimize treatment with tamoxifen and reduce side effects.