Stereotactic body radiotherapy for oligometastatic breast cancer: a new standard of care, or a medical reversal in waiting?

Metastases-directed therapy via surgery or stereotactic body radiotherapy (SBRT) has become the de facto standard of care in the United States and abroad despite a lack of high quality prospective, randomized trials. Oligometastatic tumors may behave in an inherently more indolent manner secondary to underlying biologic characteristics, including discrepant microRNA expression patterns. This biologic discrepancy suggests that historic improvements in survival observed in retrospective series may stem from the inherent biology of oligometastases and selection biases as opposed to advances in novel localized treatments. In this review, we discuss the theoretical basis for metastases-directed therapies, retrospective data supporting these approaches, recent advances in oligometastasis biology, and ongoing prospective randomized trials designed to compare SBRT and standard of care systemic therapies. We focus on metastases-directed therapy, primarily SBRT, for oligometastatic breast cancer with references to other tumor types when these other tumor types inform oligometastatic breast cancer treatment.