negative breast cancer; the risk is elevated in the postpartum period but decreases over time (242,244,245). Identifying interventions that may alleviate the tumor-promoting potential of recent childbirth, and determining the best therapeutic options to treat postpartum breast cancer, are areas of extensive investigation. There is evidence that breastfeeding can be protective against the development of estrogen receptor-negative breast cancer in mothers that is associated with giving birth (246-248), with increased duration of breastfeeding associated with further decrease in risk (160,244,247,249-251). Emerging data suggest that breastfeeding may also be associated with a lower risk of ovarian cancer development (252,253). The protective effects of breastfeeding are seen in both Black and White women (160,247,249,250). Notably, the increased risk of triple-negative breast cancer diagnosis associated with giving birth can be reduced by breastfeeding, with longer durations of breastfeeding further decreasing the risk (160,244,246,249,250,254). Beyond protecting the mother, emerging data suggest that breastfeeding can also reduce the risk of children developing leukemia. The protective effect increased proportionally with duration of breastfeeding (255,256). While breastfeeding is not always an option, the awareness of the benefits of breastfeeding in reducing cancer risk is low among U.S. women (258). Culturally tailored public education and implementation of health policies in support of lactation are needed, specifically for medically underserved populations, such as young Black women, who have a lower prevalence of breastfeeding and a higher incidence of aggressive breast cancers compared to all other U.S. racial and ethnic groups (259,260). Hormone Replacement Therapy Hormone replacement therapy (HRT) refers to treatments that aim to relieve the common symptoms of menopause and the long-term biological changes, such as bone loss, that take place after menopause. These changes occur due to the decline in the levels of the hormones estrogen and progesterone. Hormone replacement therapy usually involves treatment with estrogen and progestin or estrogen alone in women who have undergone a hysterectomy. This is because when estrogen is given alone, but not in combination with progestin, it is associated with an increased risk of endometrial cancer, a type of cancer that forms in the tissue lining of the uterus. Data show that women who use the estrogen and progestin combination have an increased risk of developing breast cancer (261,262). The risk is greater with longer duration of use and is nearly two-fold higher among women who have used estrogen and progestin in combination for 10 years or longer compared to those who never used HRT (263-265). Women who are no longer using HRT have a lower risk than current users but remain at an elevated risk for more than a decade after they have stopped taking the drugs (264). Individuals who seek HRT should discuss with their health care providers the advantages and possible risks, before deciding what is right for them. One area of ongoing investigation in exogenous hormone use is the differential cancer risks among individuals undergoing gender-affirming hormonal therapy (266). While current data are very limited, there is emerging evidence indicating an increased risk of breast cancer but a lower risk of prostate cancer among trans women who received gender-affirming hormonal therapy compared to age-matched cisgender men. Trans men who received gender affirming hormonal therapy had a lower risk of breast cancer compared to age-matched cisgender women (267,268). New evidence indicates that this lower risk of breast cancer in trans men may be due to protective effects of receiving androgen therapy during their transition (269). Long-term, population-based studies are needed to comprehensively assess the risk of cancers in these understudied and medically underserved populations. Women can reduce their risk of developing breast cancer by 4.3 percent for every 12 months of breastfeeding. This is in addition to the seven percent decrease observed for each birth (257). Reducing the Risk of Cancer Development AACR Cancer Progress Report 2023 53
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