Hormone replacement therapy and elevated breast cancer risk: An artifact of growth acceleration?

BackgroundAvailable data on accelerated proliferation and increased breast cancer risk due to hormone replacement therapy (HT) are inconsistent. Data on long-term effects of HT are limited. The interaction between several key factors was examined using a model-based approach. MethodsCohorts of 50 year old women, BCs were randomly generated for 30 years based on the age-specific incidence. A control group received a HT that increased the growth of occult BCs. In a 3rd cohort BCs were additionally induced by HT. This model illustrates the interrelationship of important parameters and allows the simulation and comparison of previously published clinical studies. ResultsUsing plausible parameters for BC growth factor (GF) and HT-related effects it was demonstrated that HT caused accelerated growth of occult BCs with an apparent increase in incidence and shortened time to diagnosis. The Womens Health Initiative (WHI) study was reconstructed assuming a GF of 1.43 induced by HT. The decision of millions of women to discontinue or forego HT based on the published risks of the WHI-study in 2002 could explain the marked jump of 6.7% in incidence within a few months. If additional BCs were induced by HT, then these BCs may become apparent after 10 or more years together with those appearing according to the normal incidence. At this time conclusive data on type, timing, and molecular characteristics of HT induced BCs are not yet available. ConclusionThe acceleration in growth of occult BC has been underestimated. Initially HTs can cause an apparent increase in BC incidence thereby explaining the WHI-dependent decrease in 2003. A HT associated BC risk should only be detectable with a delay of ten and more years.