Introduction
Black women in the United States are more likely to develop breast cancer at a younger age and to be diagnosed with more aggressive subtypes and more advanced stage disease, both contributing to higher rates of breast cancer mortality among Black women. (1) Light at night (LAN) has been proposed as a breast cancer risk factor because it inhibits nighttime production of melatonin, a hormone that may modulate biological pathways involved in breast cancer carcinogenesis. (2,3) Several epidemiologic studies have linked higher outdoor LAN estimated from satellite imagery to elevated incidence of breast cancer, including in cohorts predominantly comprised of White women with relatively high socioeconomic status (SES). (4,5,6) However, it remains unclear whether LAN is associated with breast cancer risk among Black women and women of lower SES.
Methods
We examined the relationship between LAN and incident breast cancer in the Southern Community Cohort Study (SCCS). (7,8) The vast majority of participants (86%) were recruited from community health centers in the southeastern United States that primarily served uninsured and underinsured populations, and ~ 2/3 were Black. Our analytic cohort included 30,518 Black and 12,982 White women who were cancer free and reported residential addresses at baseline. LAN exposures were estimated by linking geocoded baseline addresses (2002-2009) with satellite images in 2004 obtained by the U.S. Defense Meteorological Satellite Program's Operational Linescan System, and we used the high-dynamic range data to avoid saturation in high-LAN areas. (9) Incident breast cancer cases were identified via linkage to state cancer registries and vital status was ascertained from the Social Security Administration--both through 31 December 2017. Data on estrogen receptor (ER) status and cancer stage were obtained from cancer registries and supplemented by pathology reports and medical records. Race was self-reported at baseline. Institutional review boards at Vanderbilt University (Nashville, TN) and Meharry Medical College (Nashville, TN) approved the study and participants provided informed consent at the time of enrollment. We used Cox proportional hazards models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) comparing higher quintiles of LAN (Q2-Q5) with the lowest quintile, as well as for each 10-unit increase in LAN. Models were adjusted for multiple covariates as listed in table footnotes.
Results
Among all women in the cohort, we found a statistically significant increased risk of breast cancer...