Socioeconomic inequalities in stage-specific breast cancer incidence: a nationwide registry study of 1.1 million young women in Norway, 2000–2015

Women with high socioeconomic status (SES) have the highest incidence rates of breast cancer. We wanted to determine if high SES women only have higher rates of localized disease, or whether they also have higher rates of non-localized disease. To study this, we used data on a young population with universal health care, but not offered screening.
Using individually linked registry data, we compared stage-specific breast cancer incidence, by education level and income quintile, in a Norwegian cohort of 1,106,863 women aged 30–48 years during 2000–2015 (N = 7531 breast cancer cases). We calculated stage-specific age-standardized rates and incidence rate ratios and rate differences using Poisson models adjusted for age, period and immigration history.
Incidence of localized and regional disease increased significantly with increasing education and income level. Incidence of distant stage disease did not vary significantly by education level but was significantly reduced in the four highest compared to the lowest income quintile. The age-standardized rates for tertiary versus compulsory educated women were: localized 28.2 vs 19.8, regional 50.8 vs 40.4 and distant 2.3 vs 2.6 per 100,000 person-years. The adjusted incidence rate ratios (tertiary versus compulsory) were: localized 1.40 (95% CI 1.25–1.56), regional 1.25 (1.15–1.35), distant 0.90 (0.64–1.26). The age-standardized rates for women in the highest versus lowest income quintile were: localized 28.9 vs 17.7, regional 52.8 vs 41.5 and distant 2.3 vs 3.2 per 100,000 person-years. The adjusted incidence rate ratios (highest versus lowest quintile) were: localized 1.63 (1.42–1.87), regional 1.27 (1.09–1.32), distant 0.64 (0.43–0.94).
Increased breast cancer rates among young high SES women is not just increased detection of small localized tumors, but also increased incidence of tumors with regional spread. The higher incidence of young high SES women is therefore real and not only because of excessive screening.

社会经济地位（Socioeconomic Status, SES）较高的女性乳腺癌发病率最高。本研究旨在明确：高社会经济地位女性的发病率升高是否仅局限于局限性乳腺癌，抑或同时伴随非局限性乳腺癌发病率的上升。为开展此项研究，我们纳入了一类拥有全民医保但未被提供乳腺癌筛查的年轻人群数据。
本研究采用个体关联登记数据（individually linked registry data），针对2000-2015年间年龄介于30~48岁的1106863名挪威女性队列（共纳入7531例乳腺癌病例），按受教育水平与收入五分位（income quintile）分组，比较了不同临床分期的乳腺癌发病率。我们通过校正年龄、时期与移民史的泊松模型（Poisson models），计算了各分期的年龄标化率（age-standardized rates）、发病率比（incidence rate ratios）及率差（rate differences）。
局限性与区域性乳腺癌的发病率随受教育水平与收入水平提升呈显著上升趋势。远处转移分期乳腺癌的发病率未随受教育水平出现显著差异，但收入最高的四个五分位组发病率均显著低于最低收入五分位组。高等教育组与义务教育组女性的年龄标化率分别为：局限性癌28.2 vs 19.8、区域性癌50.8 vs 40.4、远处转移癌2.3 vs 2.6 每10万人年。校正后的发病率比（高等教育组vs义务教育组）为：局限性癌1.40（95%置信区间（Confidence Interval, CI）1.25~1.56）、区域性癌1.25（1.15~1.35）、远处转移癌0.90（0.64~1.26）。最高收入五分位组与最低收入五分位组女性的年龄标化率分别为：局限性癌28.9 vs 17.7、区域性癌52.8 vs 41.5、远处转移癌2.3 vs 3.2 每10万人年。校正后的发病率比（最高五分位组vs最低五分位组）为：局限性癌1.63（1.42~1.87）、区域性癌1.27（1.09~1.32）、远处转移癌0.64（0.43~0.94）。
年轻高社会经济地位女性乳腺癌发病率升高，并非仅源于小型局限性肿瘤的检出率提升，同时还伴随区域性扩散肿瘤的发病率升高。因此，年轻高社会经济地位女性的高发病率是真实存在的，并非仅由过度筛查所致。