Purpose Brachytherapy is a method for delivering partial-breast irradiation after breast-conserving surgery (BCS). It is currently used in the community setting, although its efficacy has yet to be validated in prospective comparative trials. Frequency and factors influencing use have not been previously identified. Methods In a nationwide database of 6,882 Medicare beneficiaries (age ≥ 65 years) with private supplemental insurance (MarketScan Medicare Supplemental), claims codes identified patients treated with brachytherapy versus external-beam radiation after BCS for incident breast cancer (diagnosed from 2001 to 2006). Logistic regression modeled predictors of brachytherapy use. Results Frequency of brachytherapy use as an alternative to external-beam radiation after BCS increased over time (< 1% in 2001, 2% in 2002, 3% in 2003, 5% in 2004, 8% in 2005, 10% in 2006; P < .001). Increased use correlated temporally with US Food and Drug Administration approval and Medicare reimbursement of brachytherapy technology. Brachytherapy use was more likely in women with lymph node–negative disease (odds ratio [OR], 2.19; 95% CI, 1.17 to 4.11) or axillary surgery (OR, 1.74; 95% CI, 1.23 to 2.44). Brachytherapy use was also more likely in women with non–health maintenance organization insurance (OR, 1.81; 95% CI, 1.24 to 2.64) and in areas with higher median income (OR, 1.58; 95% CI, 1.05 to 2.38), lower density of radiation oncologists (OR, 1.78; 95% CI, 1.11 to 2.86), or higher density of surgeons (OR, 1.57; 95% CI, 1.07 to 2.31). Conclusion Despite ongoing questions regarding efficacy, breast brachytherapy was rapidly incorporated into the care of older, insured patients. In our era of frequently emerging novel technologies yet growing demands to optimize costs and outcomes, results provide insight into how clinical, policy, and socioeconomic factors influence new technology diffusion into conventional care.