Balloon breast brachytherapy is a catheter-based technique to deliver high local concentration of radiation following breast-sparing surgery. Although this technique is logically appealing--providing more directed radiation to sites at high risk of local failure--there remains little empirical support that this intervention is non-inferior to external beam radiotherapy, a well-established standard. Additionally, observational studies suggest that balloon brachytherapy is associated with high rates of local complications, and higher rates of subsequent mastectomy, a marker of local failure. Here, I explore regulatory and clinical considerations that lead to the widespread adoption of breast brachytherapy. I argue that the therapy spread before its efficacy was confirmed. Breast brachytherapy illustrates ongoing complexities in the approval of novel devices.