LAUSR

BACKGROUND Although social exposures have complex and dynamic relationships and interactions, the existing literature on the impact of rural-urban residence on stage at breast cancer diagnosis does not examine heterogeneity of effect. We examined the joint effect of social support, social relationship strain, and rural-urban residence on stage at breast cancer diagnosis. METHODS Using data from the Women's Health Initiative (WHI) (n = 161,808), we describe the distribution of social, behavioral, and clinical factors by rural-urban residence among postmenopausal women with incident breast cancer (n = 7,120). We used rural-urban commuting area (RUCA) codes to categorize baseline residential addresses as urban, large rural city/town, or small rural town, and the surveillance, epidemiology, and end results staging system to categorize breast cancer stage at diagnosis (dichotomized as early or late). We then used univariable and multivariable logistic regression to estimate odds ratios (ORs) and associated 95% confidence intervals (95% CI) for the relationship between rural-urban residence and stage at breast cancer diagnosis. We included separate interaction terms between rural-urban residence and social strain and social support to test for statistical interaction. RESULTS Of the social, behavioral, and clinical factors we examined, only younger age at WHI enrollment screening was significantly associated with late stage at breast cancer diagnosis (p = 0.003). Contrary to our hypothesis, rural-urban residence was not significantly associated with stage at breast cancer diagnosis among postmenopausal women ([adjusted OR, 95% CI] for urban compared with small town: 1.08 [0.76-1.53]; large town compared with small town: 1.16 [0.74-1.84]; and urban compared with large town: 0.93 [0.68-1.26]).The associations did not vary by social support or social strain (p for interaction between RUCA and social strain and social support, respectively: 0.99 and 0.17). CONCLUSIONS Future studies should examine other potential effect modifiers to identify novel factors predictive or protective for late stage at breast cancer diagnosis associated with rural-urban residence.