The current standard of care in our clinic when delivering a conventional course of whole breast radiation is to deliver between 60 and 66 Gy to the tumor bed when negative margins are achieved, with the initial \~45-50 Gy delivered to the whole breast and the 10-16 Gy TB boost delivered in 5-8 fractions.The current standard of care in our clinic when delivering a hypofractionated course of whole breast radiation is to deliver between 50 and 52 Gy to the tumor bed when negative margins are achieved, with the initial \~40-42.5 Gy delivered to the whole breast in 15 or 16 fractions respectively followed 10 Gy TB boost delivered in 4 or 5 fractions (total 20). The clinical target volume for the TB boost in this study is quite similar to conventional TB boost. The only difference is that the planning target volume margin is smaller in this study (5 mm instead of 10 mm) due to the reduced set up uncertainties with the breast cup immobilization and localization devices. The current institutional standard is covering the TB + 15 mm dosimetric margin. Since the reproducibility is improved by 5 mm with the aid of the breast immobilization cup, the TB + 10 mm dosimetric margin will be used on this study. Using the radiobiological equivalent dose (BED) formula, {BED = n D(1 + D/(α/β))}, the 8 Gy single fraction dose is equivalent to 16 Gy delivered in 8 fractions. Wherein the BED formula, n is the number of fractions, D is the dose per fraction and α/β is estimated to be between 3 and 4. On this study, the investigators will deliver either 40 Gy in fifteen (15) fractions (hypofractionated) or 50Gy in twenty-five (25) fractions (conventional) to the whole breast following a single 8 Gy boost using the Gamma Pod™. The summed dose to the boost region will be radiobiologically equivalent to a total dose of 52 (hypofractionated) or 66 Gy (conventional).

The safety and feasibility of delivering the boost dose to the tumor bed using a single fraction external beam is supported by past clinical trials. Besides the use of electrons and external beam from a linear accelerator, intracavitary balloons and intraoperative x-rays and electron beams have also been used to deliver a Tumor Bed (TB) boost. In the T A R G I T trial, a single dose of 20 Gy is delivered to the TB using the Intrabeam™ device while the dose drops to between 5 and 7 Gy 1 cm into the normal breast. With long term follow up (median follow-up of 60.5 months), the 5- year local recurrence rate was less than 2 %. In the largest intraoperative electron experience, investigators delivered 9 to 10 Gy in a single treatment prior to whole breast radiation using 9 (million electron volts) electrons. Long-term local control rates and cosmesis appear similar to or better than more conventional techniques. In this proposed study, the investigators plan to deliver 8 Gy to the TB rather than 20 Gy with low energy x-rays or 9-10 Gy with electrons before whole breast irradiation, based on 1) there is no benefit of dose escalation over the biological equivalent dose of 16 Gy at 2 Gy per fraction which is equal to 8 Gy in 1 fraction and 2) known increased side effects with higher doses to the tumor bed. The investigators believe that the proposed dose should be similarly tolerated as observed in these trials, since the volume of normal breast irradiated is similar using a lower dose. In addition, this treatment will similarly shorten treatment by 1 to 1½ weeks by replacing the 8 treatments to the TB.