Geographic access to cancer care is an important dimension of quality of cancer care. Previous studies have shown that the more uncertain medical evidence is, the more geographic variation is observed in the medical care utilization that is attributable to local care health care system capacity and local area patient/physician preferences. Chemotherapy for metastatic breast cancer (MBC) is such a case. Although clinical trials have proven the efficacy of chemotherapy in treating MBC, whether to treat elderly MBC patients with chemotherapy is uncertain because of the underrepresentation of elderly patients in the clinical trials. As age advances, uncertainties increase due to competing causes of death, limited life expectancy, and higher risk of toxicities. As a result, geographic access may matter more in chemotherapy choice for older patients than for younger patients. Literature has shown that older patients are less likely to be treated with chemotherapy. In this study, we examined the effect of access to cancer care on agerelated difference in chemotherapy use for elderly MBC patients. Access to cancer care is measured by four variables, including travel time to the nearest oncologist practice, local area per capita number of oncologists among stage IV cancer patients, local area per capita number of hospices among stage IV cancer patients, and local area chemotherapy percentage among stage IV cancer patients. The retrospective cohort study used the 1992-2002 SEER-Medicare database. Chemotherapy use was defined as at least one chemotherapy-related claim within 6 months post diagnosis. To examine the age variant effect of access on chemotherapy choice, the analysis adopted both interaction term approach and subgroup analysis. In interaction term analysis, product term between age and access dummy variables were specified in the multivariate logistic regression model controlling for other covariates; in subgroup analysis, age subgroups were specified consistently with interaction term 2 approach. For each age subgroup, we used multivariate logistic regression to estimate the effect of access to cancer care on immediate chemotherapy use controlling for covariates. Among 4533 elderly patients with MBC, 30.16% used chemotherapy. Chemotherapy rate decreased with age. Interaction term approach did not show significant interaction between age and access in each specification. Both interaction term and subgroup analysis showed that the local area treatment rate was positively associated with immediate chemotherapy use across patient age. In addition, subgroup analysis showed among patients who were 85+ years old, the local area oncologist supply was negatively associated with chemotherapy use. This effect was not observed among younger age groups. Our results suggest that estimating all patients in one equation with dummies and interactions can hide results. By estimating each group separately, subgroup analysis showed that provider access is paramount for age subgroup 85 years or older. Our access measures suggest th...
