https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2574428/ 'Caffeine Consumption and Risk of Breast Cancer in a Large Prospective Cohort of Women' : "Comment" section
It tells about caffeine's effect of other hormones, but it seems it only talks about its effects on cancerous tissue. It is inconclusive whether caffeine on its own.
"consumption of herbal tea was significantly associated with a reduced risk of breast cancer.51"
"The mechanisms by which caffeine may affect breast carcinogenesis are complex and remain unclear. Caffeine has been reported to suppress [tumor] cell cycle and proliferation and induce apoptosis.52 Caffeine also has been positively associated with blood levels of estrone53 and sex hormone-binding globulin,53–55 but negatively associated with plasma free estradiol.55 Caffeine and coffee can both stimulate and suppress the development of mammary tumors, depending on the phases of tumorigenesis (initiation/promotion) when caffeine and coffee are administered in rodents.2, 56 Caffeine is a known antagonist of the adenosine receptor.2, 57 Adenosine, an endogenous bioactive substance, exerts its diverse biologic effects through the activation of specific cell surface adenosine receptor.57 In breast cancer cell lines, high concentrations of adenosine inhibited cell growth and induced cell cycle arrest at G2-M phase, but had no effect on ERα levels,58 suggesting that, through antagonism of adenosine receptor, caffeine might be able to stimulate breast cell proliferation independent of ERα pathway."
Caffeine raises human estrone, which has carcinogenic properties.
It then suggests that caffeine lowers Estrogen and Progesterone related cancers, but might increase risk of non-hormonal cancers. Then after that it says a study found no evidence of that. There are also inconclusive results on caffeine's effect on existing benign tumors, on whether they can become cancerous.
