A prone approach for partial breast radiation has been tested at NYU in a clinical trial sponsored by an IDEA grant of the Department of Defense (NYU 00-23). Results of the first 47 patient accrued originally demonstrated feasibility. Eligibility to this study was limited to post-menopausal women with non-palpable, mammographically detected tumors. In addition, the protocol required patients to have first refused to undergo standard six-week radiotherapy. Five fractions of 6 Gy were delivered to the Planning Target Volume (PTV) over ten days (Monday-Wednesday-Friday, Monday-Wednesday). The dose and fractionation was based on radiobiological modeling, aimed at determining a dose to deliver in five fractions that would achieve equivalence to the tumor control estimates of 50 Gy in 25 fractions, while maintaining a risk of fibrosis at the tumor bed comparable to that of a standard regimen of 60 Gy in 30 fractions. An α/β = 4 for tumor control was used, and its validity has been recently confirmed by the results of a prospective randomized trial comparing accelerated to standard whole breast radiotherapy.