Why does HRT cause breast cancer?
The link between hormone replacement therapy (HRT) and breast cancer risk is complicated. Several key factors are involved:
- Type of HRT: Combination estrogen-progestin therapy is associated with a higher breast cancer risk than estrogen-only therapy. The progestin component seems to drive increased risk.
- Duration of HRT use: Longer duration (5 years) confers greater breast cancer risk than short-term use.
- Existing breast cancer risk factors: Family history of breast cancer, reproductive history, genetic mutations, and breast density modulate risk from HRT use. Women with other risk factors may be more predisposed to HRT-associated breast cancers.
Proposed mechanisms for the increased risk include:
- Stimulation of breast cell proliferation by supplemental estrogen and progestin
- Development of new blood vessels in the breast to "feed" cancer cells
- Interference with cancer detection due to increased breast density
The increased risk appears to return to baseline within 5 years after stopping combination HRT. For this reason, the lowest effective HRT dose taken for the shortest time is recommended. Careful screening and risk-benefit assessment is vital, especially for long-term HRT use.
I aimed to provide a balanced overview based on current evidence. Medical decisions should ultimately be made in consultation with one's doctor. Please let me know if you would like me to modify my answer further.