@Marakesh My oncologist uses Tamoxifen as the fall back position as well.
The idea is to stop oestrogen. They both do it, but with different chemical processes.
Your body fat produces estrogen. Your ovaries still produce a little bit after menopause. Both are enough to feed estrogen positive breast cancers. Removing the ovaries stops that source of production.
The estrogen that's made in body fat uses an enzyme called aromatase. Aromatase inhibitors stop this enzyme from working. Letrozole & Anastrolzole are aromatase inhibitors.
Tamoxifen doesn't stop estrogen. It competes with it to bind to estrogen receptors in a breast cancer.
AIs are better at preventing a recurrence. Tamoxifen acts like estrogen everywhere else other than your boob. Theory is that it's less disruptive to your normal body processes if you're still ovulating.
Once you've stopped ovulating then why not reduce risk even further by preventing estrogen production all together.
Ovaries out, no estrogen from there at all. AI, no estrogen from anywhere else either. If cancer is present, it's starved, goes into stasis and eventually dies.
Clear as mud?!