Survivors in the study had to have a tumour that was estrogen receptor positive. In general, it can be said that for breast cancer tumours, they either respond to estrogen or they don't. It is seen as a hormonal disease. Tamoxifen is an estrogen blocker. There is actually an estrogen receptor status test.
Some tumours are very receptor positive and others less so.
There is a wide range. Until now, tamoxifen, a relatively cheap drug has been very successful in manipulating cells on a hormonal basis as well as being well tolerated by most women with breast cancer. It can prevent the cancer's systemic spread and most women will accept its side effects and risks. It is mostly prescribed to women with breast cancer who are post-menopausal. The success rate has been well documented in this group.