Hormone replacement therapy (HRT) is a treatment primarily used to alleviate moderate to severe symptoms associated with menopause, such as hot flashes, night sweats, and vaginal dryness. By restoring hormone levels, HRT significantly improves quality of life for many women experiencing these changes. While its benefits for symptom relief and bone health are well-established, HRT is also linked to certain risks that require careful consideration. A well-documented concern involves the potential for an increased risk of breast cancer, which necessitates a clear understanding of how different hormone regimens affect the body.
How HRT Composition Affects Risk
The composition of the hormone regimen is the primary factor influencing the degree of breast cancer risk. HRT is broadly divided into two categories: Estrogen-only Therapy (ET) and Combined Hormone Therapy (Estrogen plus Progestin, EPT). The choice between these forms is determined by whether a woman still has her uterus.
ET is typically prescribed for women who have undergone a hysterectomy. This is because providing estrogen without a counterbalancing progestin hormone significantly increases the risk of developing endometrial cancer, or cancer of the uterine lining. Studies show that ET use is associated with little or no increased risk of breast cancer, especially when used for a short duration.
EPT is the standard treatment for women who still have a uterus, as the progestin component protects the uterine lining from estrogen-driven proliferation. This combination is consistently associated with a higher increase in breast cancer risk compared to ET. Research suggests that the progestin component, when combined with estrogen, promotes breast cell growth. The specific type of progestin used may also influence the magnitude of this risk increase.
Understanding Absolute and Relative Risk Increases
It is necessary to differentiate between relative risk and absolute risk to accurately evaluate the data. Relative risk describes how much the risk increases compared to a baseline group of women who have never used HRT. For instance, a relative risk of 1.6 means a 60% higher chance of developing breast cancer compared to the non-user group.
This relative figure can sometimes sound alarming to the public because it does not convey the actual likelihood of the event occurring. Absolute risk refers to the actual number of extra cancer cases per a specific number of women over a defined period. This metric provides a more tangible measure of personal risk.
For women in their 50s taking combined HRT for five years, the baseline risk is approximately 63 cases per 1,000 women up to age 69. Using continuous combined HRT for five years results in about 20 extra cases per 1,000 women by age 69, totaling 83 cases. This represents a small increase in the overall risk for any individual woman. For estrogen-only therapy used for five years, the increase is significantly lower, estimated at about five extra cases per 1,000 women.
Duration of use is a significant element in interpreting risk data, as the increase typically emerges after several years of use. There is little or no increase in breast cancer risk during the first year of HRT use. The risk begins to climb noticeably after five years of combined therapy. Once HRT is stopped, the elevated risk begins to decline, returning close to the baseline level within about five years.
Personal Factors That Modify Risk
Individual factors can modify a woman’s personal baseline risk, even with a general HRT risk profile. Cumulative exposure is a primary modifier, meaning the risk increases with the total number of years a woman uses systemic HRT. The increased risk is higher for women who start HRT at an older age because their baseline risk of breast cancer is naturally higher.
The route of administration is another factor, though its effect on breast cancer risk is less clear than its effect on other health concerns. Oral HRT is metabolized differently than transdermal methods, such as patches, gels, or sprays. While transdermal delivery is associated with a lower risk of blood clots compared to oral preparations, studies have not shown a consistent difference in breast cancer risk between oral and non-oral combined HRT.
Individual health history also plays a role in the overall risk assessment. Women with a higher pre-existing risk, such as a family history of breast cancer or a personal history of benign breast disease, must weigh the HRT data against their elevated baseline. Lifestyle factors, including a higher body mass index (BMI) and increased alcohol consumption, also independently contribute to breast cancer risk.
Recommended Screening and Monitoring
Enhanced monitoring and screening are important for women using systemic HRT. Regular breast cancer screening, typically involving routine mammograms, remains the primary tool for early detection. Women on HRT should adhere to current national screening guidelines, which often recommend annual mammograms depending on age and individual risk factors.
Combined HRT can increase breast density, which may make it more challenging for mammograms to detect cancerous lesions. This effect sometimes leads to a higher rate of call-backs for additional imaging after an initial mammogram. Women should also receive regular clinical breast exams performed by a healthcare provider and report any changes immediately.
The decision to continue HRT requires an annual risk-benefit reassessment with a healthcare provider. This review ensures that the lowest effective dose is used to manage symptoms and that the duration of use is limited to the shortest time necessary. For women whose symptoms are confined to the vaginal area, low-dose vaginal estrogen preparations are recommended, as they do not appear to increase the systemic risk of breast cancer.