Utilizing Liquid Biopsy for Treatment Management in Bone-Dominant Metastatic Breast Cancer: A Case Report

Breast cancer is the most commonly diagnosed cancer and the second leading cause of cancer death among women in the United States. In clinical practice, the standard method to confirm metastatic disease and tailor treatment is to biopsy a metastatic site. Bone is the most common metastatic site, occurring in up to 70% of patients with advanced breast cancer. Standard-of-care management includes a needle biopsy with histopathological analysis to confirm tumor status and to evaluate for mutations. However, bone biopsies can be technically challenging and are oftentimes painful for patients. Given the challenges in acquisition and analysis of bone samples in metastatic breast cancer (mBC), a liquid biopsy is a less invasive alternative that can reveal clinically relevant alterations. Here, we report two cases of bone-dominant hormone-positive (HR+) mBC, in which circulating tumor DNA (ctDNA) was extracted from blood samples using two different next-generation sequencing (NGS) platforms to identify molecular targets for FDA approved treatment. In both patients, PIK3CA mutations were detected and subsequently started on alpelisib along with aromatase inhibitor or fulvestrant treatment. These cases demonstrate a feasible real-world clinical application to liquid biopsies.