10791 Background: The clinical application of IMRT for adjuvant treatment of breast cancer has been the subject of increasing study in recent years. IMRT plans have improved target coverage and reduced dose inhomogeneities observed within the breast in standard plans. IMRT was able to reduce doses delivered to the heart, lungs, and right breast at clinically significant doses, but this has been at the cost of larger volumes of low dose radiation to these structures and thus, increasing the risk for second malignancy. Our goal was to develop an IMRT beam arrangement that did not result in additional low dose spill to risk organs while maintaining equal or better target coverage. METHODS: Five patients with early stage left-sided breast cancer, who underwent breast conservation surgery, and adjuvant radiation using standard wedged tangential fields, were chosen for this comparative study. An IMRT plan consisting of 6 tangential beams (3 medial and 3 lateral) was generated by using the gantry, collimator and table angles of the standard plan used for the conventional radiation (CRT), and moving the table +10 and -10 degrees on each side. The prescription dose for both CRT and IMRT plans was 45 Gy, 1.8 Gy/fraction, prescribed to the isocenter which was placed near the center of the breast. RESULTS: IMRT plans provided significantly better coverage of the left breast than CRT plans, (p=0.03). Although dose heterogeneity was greater with the IMRT plans, the difference was not significant (p = 0.68). The mean volumes of the heart, lung, and right breast were lower in patients planned with IMRT at all dose levels from 5% to 100% dose (5% increments). This difference was significant for volumes receiving 2.25 Gy for the heart (p = 0.003), volumes receiving 2.25 Gy, 4.5 Gy, 6.75 Gy, 33.75 Gy, 36 Gy, 38.25 Gy, and 42.75 Gy for the lung (p = 0.014, 0.04, 0.044, 0.05, 0.049, 0.045, 0.05, respectively). Surprisingly, breast IMRT resulted in significantly lower right breast volumes irradiated at all dose levels compared to CRT. CONCLUSIONS: A six-tangential field IMRT technique achieved significantly better left breast coverage while maintaining lower doses to risk organs at all dose levels and therefore, reducing the potential for induction of a second malignancy. No significant financial relationships to disclose.