Hormone Replacement Therapy (HRT) offers relief for many navigating the symptoms of menopause, yet its association with breast cancer remains a common concern. This article aims to clarify the available data on HRT and breast cancer risk, providing a structured overview of contributing factors.
Establishing the Baseline Breast Cancer Risk
Before considering the impact of HRT, it is important to understand the typical risk of breast cancer in women who do not use hormone therapy. For women in their 50s, the chance of developing breast cancer over the next 10 years is approximately 2.4%, or about 1 in 42 women. This represents the absolute risk, which is the actual number of cases expected within a specific population. For instance, among 1,000 women in their 50s who do not take HRT, around 24 would develop breast cancer over a decade.
In contrast, relative risk describes how much more or less likely an event is to occur compared to a baseline. If a factor doubles the risk, the relative risk is 2.0. Understanding this difference helps interpret how additional factors, such as HRT, might shift these baseline probabilities. The lifetime risk of breast cancer for a woman born in the United States today is estimated to be about 1 in 8, or 12.9%. This baseline provides a reference point for evaluating the additional risks associated with various factors.
Risk Associated with Different HRT Types
The type of HRT used significantly influences the associated breast cancer risk. Combined HRT, which includes both estrogen and progestogen, is prescribed for women who still have their uterus. Studies indicate that using combined HRT for five years is associated with an increase in breast cancer cases. Another estimate suggests that five years of combined-cyclical HRT can increase the absolute risk of breast cancer by age 80 by 1.2% for an average woman, from a baseline of 9.8% to 11%.
For women who have undergone a hysterectomy and use estrogen-only HRT, the risk profile is different. Estrogen-only formulations have a minimal effect on breast cancer risk, even with extended use. For a 55-year-old woman, the additional breast cancer risk with five years of estrogen-only HRT is approximately 0.2%. Some large studies, such as the Women’s Health Initiative, even observed a lower breast cancer risk in women using estrogen-only HRT compared to those receiving a placebo.
The Influence of HRT Duration and Discontinuation
The duration of hormone replacement therapy directly impacts the associated breast cancer risk, particularly with combined HRT. The risk is cumulative, meaning it generally increases the longer a woman uses combined estrogen and progestogen therapy. One model suggests that 10 years of combined HRT taken between ages 50 and 60 could increase the risk of developing breast cancer by an estimated 2.6% for an average woman.
Upon stopping HRT, the elevated risk of breast cancer begins to decline. For women who used combined HRT, the risk is lower after discontinuation than during current use. The increased risk associated with combined HRT typically reduces over time after therapy is stopped, eventually returning to the baseline level of women who never used HRT. While some excess risk may persist for more than 10 years after stopping long-term treatment, for those who took combined HRT for less than five years, there was no increased breast cancer risk five years after discontinuation.
Personal Factors Modifying HRT Risk
The overall risk associated with HRT is not uniform and can be influenced by several individual characteristics. A woman’s age when starting HRT plays a role; women in their 50s generally experience a lower increase in breast cancer risk compared to those who start HRT later, such as in their 60s or 70s.
Body Mass Index (BMI) is another significant factor, as obesity is an independent risk factor for breast cancer, particularly after menopause. Higher BMI can compound the risk from HRT. Breast density is also a factor, as women with denser breast tissue have a higher baseline risk of breast cancer. HRT, especially combined estrogen and progestin therapy, can further increase breast density, which is an important consideration for screening and risk assessment.
Emerging evidence suggests that specific HRT formulations may also carry varying risk levels. For example, some research indicates that the type of progestogen used in combined HRT can influence risk, with certain types like norethisterone linked to higher increases in risk and dydrogesterone to lower increases. While oral pills are common, transdermal patches or gels may have different effects, though this area remains a subject of ongoing scientific investigation.