cancer molecular subtype. 14 Breast Cancer Facts & Figures 2019-2020 invasive breast cancer, women diagnosed with DCIS have a small increased risk for developing a new breast cancer. A recent study estimated that 3%-5% of women diagnosed with DCIS were diagnosed with in situ or invasive breast cancer in the opposite breast within 10 years of their initial diagnosis.70 In contrast, LCIS is not generally considered a breast cancer precursor, but is associated with increased risk of developing breast cancer. A recent population-based study of US women diagnosed with LCIS between 1983 and 2014 reported that the 10- and 20-year risk of being diagnosed with DCIS or an invasive breast cancer was 11% and 20%, respectively.71 Notably, the study also reported that the 20-year breast cancer survival in those diagnosed with DCIS or invasive breast cancer exceeded 95%. Pleomorphic LCIS, a more aggressive subtype, is linked to a higher risk of invasive cancer than classic LCIS and is often treated as though it is a cancer precursor.72 Benign breast disease Benign breast conditions are categorized into 3 general groups reflecting the associated degree of cancer risk: nonproliferative lesions, proliferative lesions without atypia (abnormal cells or patterns of cells), and proliferative lesions with atypia. • Nonproliferative lesions are not associated with overgrowth of breast tissue and include fibrosis and simple cysts (also known as fibrocystic changes) and mild hyperplasia. Nonproliferative conditions are associated with little to no increased breast cancer risk.73 • Proliferative lesions without atypia are associated with a small increase in the risk of breast cancer (1.5 to 2 times the risk of those who do not have one of these lesions) and include usual ductal hyperplasia (without atypia) and fibroadenoma.73 • Proliferative lesions with atypia are associated with about 4 times higher than average breast cancer risk. These include atypical ductal hyperplasia and atypical lobular hyperplasia.73, 74 Recent studies indicate that the 15-year risk of developing in situ or invasive breast cancer exceeds 30% in women diagnosed with atypical hyperplasia.74, 75 Women should keep detailed records of any benign breast biopsy results because they are valuable for risk assessment, screening, and counseling for chemoprevention and other risk-reduction strategies. What is the difference between absolute, lifetime, and relative risks? Absolute risk is the likelihood of being diagnosed with cancer over a certain period of time. For example, the absolute risk of breast cancer increases with age: 12 out of 10,000 women ages 40-44 versus 23 out of 10,000 women ages 50-54 will be diagnosed with breast cancer in the next year.8 Lifetime risk is the absolute risk of being diagnosed with cancer anytime between birth and death. Lifetime risk of breast cancer reflects the average probability of a female being diagnosed with breast cancer in the US. A woman living in the US has a 13% chance of being diagnosed with invasive breast cancer in her lifetime (Table 2). Another way to say this is that 1 out of every 8 women will be diagnosed with breast cancer in her lifetime. Relative risk compares the absolute risk of disease among people with a particular risk factor to the risk among people without that risk factor. If the relative risk is above 1.0, then risk is higher among those with the risk factor than among those without the factor. Relative risks below 1.0 reflect an inverse association between the exposure and the disease, or a protective effect. For example, one study found women ages 50-59 who were current users of combined estrogen and progestin menopausal hormones had a relative risk of developing breast cancer of 1.21, meaning they had a 21% increased risk compared to women who have not used hormone therapy.69 While relative risks are useful for comparisons, they do not provide information about the absolute risk of the exposed group. In this example, 27 breast cancers per year would be expected to be diagnosed among 10,000 women ages 50-59 who had never used menopausal hormones (their absolute risk) compared to 33 breast cancers among 10,000 women of the same ages who had used estrogen and progestin. Thus, the 21% increased relative risk is the equivalent of 6 additional breast cancers per 10,000 women per year. Breast Cancer Facts & Figures 2019-2020 15 Breast density Breast tissue density is an indicator of the amount of glandular and connective tissue relative to fatty tissue measured during a mammogram and is not determined by how “firm” the breast feels. Following a mammogram, doctors categorize a woman’s breast tissue according to standardized system developed by the American College of Radiology called the Breast Imagine Reporting and Data System (BI-RADS) as A) fatty; B) scattered areas of fibroglandular tissue; C) heterogeneously dense; and D) extremely dense. Women with breasts classified as C (heterogeneously dense) or D (extremely dense) are referred to as having “dense breasts.” The risk of breast cancer increases with increasing breast density. Women with dense breasts (BI-RADS C or D) have a 1.5- to 2-fold increased risk of breast cancer compared to those with average density (BI-RADS B).76 High breast density can also mask the appearance of breast tumors on a mammogram.77 Dense breasts are common. About 36% of US women