Impact of Preexisting Mental Illnesses on Receipt of Guideline-Consistent Breast Cancer Treatment and Health Care Utilization Rohan Mahabaleshwarkar, PhD, 1 Rahul Khanna, MBA, PhD, 2 Benjamin Banahan, PhD, 3 Donna West-Strum, PhD, 2 Yi Yang, MD, PhD, 2 and Jeffrey S. Hallam, PhD, CHES 4 Abstract This study determined the impact of preexisting mental illnesses on guideline-consistent breast cancer treatment and breast cancer-related health care utilization. This was a retrospective, longitudinal, cohort study conducted using data from the 2006–2008 Medicaid Analytic Extract files. The target population for the study consisted of female Medicaid enrollees who were aged 18–64 years and were newly diagnosed with breast cancer in 2007. Guideline-consistent breast cancer treatment was defined according to established guidelines. Breast cancer-related health care use was reported in the form of inpatient, outpatient, and emergency room visits. Statistical analyses consisted of multivariable hierarchical regression models. A total of 2142 newly diagnosed cases of breast cancer were identified. Approximately 38% of these had a preexisting mental illness. Individuals with any preexisting mental illness were less likely to receive guideline-consistent breast cancer treatment compared to those without any preexisting mental illness (adjusted odds ratio: 0.793, 95% confidence interval [CI]: 0.646–0.973). A negative association was observed between preexisting mental illness and breast cancer-related outpatient (adjusted incident rate ratio (AIRR): 0.917, 95% CI: 0.892–0.942) and emergency room utilization (AIRR: 0.842, 95% CI: 0.709–0.999). The association between preexisting mental illnesses and breast cancer-related inpatient utilization was statistically insignificant (AIRR: 0.993, 95% CI: 0.851–1.159). The findings of this study indicate that breast cancer patients with preexisting mental illnesses experience disparities in terms of receipt of guideline-consistent breast cancer treatment and health care utilization. The results of this study highlight the need for more focused care for patients with preexisting mental illness. (Population Health Management 2015;18:449–458) Introduction B reast cancer is the most common cancer among women, after skin cancer. Breast cancer is also the second leading cause of cancer-related mortality in women, exceeded only by lung cancer. 1 In the year 2013, roughly 300,000 inci- dent cases of breast cancer were expected to occur and 39,620 women were estimated to die from breast cancer in the United States. 2 It has been estimated that nearly 12% of the women in the United States will develop breast cancer in their lifetime. 2 Despite being associated with considerable morbidity and mortality, breast cancer is one of the most treatable cancers if detected early. The primary treatment of breast cancer consists of surgical removal of the tumor. Some of the aggressive types of breast cancer surgeries include radical mastectomy, modified radical mastectomy, and total mastectomy. These procedures involve removal of the whole breast that has cancer. A rela- tively less aggressive surgical breast cancer treatment regimen is breast-conserving surgery, a procedure performed to remove the cancer but not the breast itself. Adjuvant treatments such as radiotherapy, hormonal therapy, chemotherapy, and tissue- targeted therapies have been found to reduce the likelihood of breast cancer recurrence and are an integral part of the breast cancer treatment regimen. 3 Major medical organizations, including the American Society of Clinical Oncology, the National Comprehensive 1 Dickson Advanced Analytics, Carolinas Healthcare System, Charlotte, North Carolina. 2 Department of Pharmacy Administration, School of Pharmacy, The University of Mississippi, University, Mississippi. 3 Center for Pharmaceutical Marketing and Management, School of Pharmacy, The University of Mississippi, University, Mississippi. 4 Department of Social and Behavioral Sciences, College of Public Health, Kent State University, Kent, Ohio. POPULATION HEALTH MANAGEMENT Volume 18, Number 6, 2015 ª Mary Ann Liebert, Inc. DOI: 10.1089/pop.2014.0146 449