What does “estrogen-induced” cancer mean?
The clearest example of an estrogen-induced cancer is cancer of the endometrium (the lining of the uterus). Continuous exposure of the endometrium to estrogen, in the absence of a second hormone called progesterone, greatly increases the risk of developing cancer. Before this was recognized, many of these cancers developed in women taking an early form of birth control pills (called sequential OCPs, which are no longer used) and in women taking high doses of estrogen for menopause symptoms. For unclear reasons, women who develop endometrial cancer while taking estrogen seem to do a little better than those whose cancer occurred spontaneously. The direct role of estrogen in the development of breast cancer is a little less clear. Women who take estrogen replacement therapy may be at a slightly increased risk (this is still being argued), but this is nowhere near the extra risk seen for endometrial cancer.
