Postmenopausal Breast Cancer and Physical Function Change: A Difference-in-Differences Analysis

BACKGROUND/OBJECTIVES: While breast cancer and associated therapies can influence physical function (PF), findings for breast cancer's effects on PF among postmenopausal women are not consistent. This research evaluates the effect of breast cancer on change in PF. DESIGN: Longitudinal cohort study analyzed using difference-in-differences (DID) method to compare the changes in PF over time between women with and without breast cancer, by invasive status, stage, and age. SETTING AND PARTICIPANTS: Women's Health Initiative participants, aged 50 to 79 years at baseline, with four completed functional status assessments were eligible for inclusion (mean age = 62.1 ± 6.9 years). Women with breast cancer diagnosed between the 3rd and 11th enrollment year (n = 1636) were compared to women without breast cancer (n = 35 660). MEASUREMENTS: PF was measured using the Medical Outcomes Study Short Form 36 at baseline and years 3, 11, and 12. RESULTS: Compared with women without breast cancer, women with breast cancer experienced greater PF decline (P <.0001), after adjustment for age, race, education, income, study/trial arm, and body mass index. Overall, functional declines were greatest among women with invasive breast cancer (DID = −4.87; 95% confidence interval = −7.12 to −2.62). In stratified analyses, the breast cancer effect on PF was greater among women in older age groups. CONCLUSION: Women diagnosed with breast cancer experienced a significant decline in PF beyond what is observed in typical aging in this cohort. To optimally address survivorship issues, disease sequelae need to be distinguished from normal aging in studies incorporating a cancer-free comparison group. J Am Geriatr Soc 68:1029–1036, 2020.