From the article:
The research by Chan and her colleagues drew on data from 834 postmenopausal women enrolled in the Nurses' Health Study – which has tracked the health of more than 100,000 female nurses since 1976 – who had been diagnosed with colon cancer between 1976 and 2000. The researchers found that, compared to women who had never used estrogen, those using the hormone at the time of diagnosis had a 36 percent lower chance of dying of colon cancer and a 26 percent lower chance of dying of any cause. The benefit was most pronounced in women who used estrogen for less than five years. Using it for longer, however, did not seem to provide a similar advantage.
The authors do not know why using estrogen for more than five years did not improve women’s chances of survival. They theorize that colon tumors that developed in women who had used estrogen for many years may have been less susceptible to estrogen’s growth-slowing effects.
The precise way by which estrogen foils the growth of colon cancer cells is unclear, as well. The study authors offer three possibilities: the hormone directly suppresses the cells' growth; it decreases the production of bile acids which are known to spur cancer; or it blocks certain genetic changes within colon cells that lead to cancer.