Dosimetry of Alternative Techniques for Accelerated Partial Breast Irradiation Hanh Pham, B.S, CMD, Thanh Nguyen, BS, Christina Henson, MD, Salahuddin Ahmad, PhD, DABR, Tania De La Fuente Herman, PhD, DABR. Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma Purpose In radiation therapy, all methods available can provide excellent target coverage, but it is of great interest to choose a technique that reduces the risks of excessive toxicity to the normal structures and poor post-treatment cosmesis. This study investigates the dosimetry of accelerated partial breast irradiation (APBI) using intensity modulated radiation therapy (IMRT), non-coplanar (nc) IMRT, and volumetric modulated radiation therapy (VMAT) treatment planning techniques. Results Maximum dose to the PTV was 115.23% on average higher in the nc-IMRT plans (range 112.3 to 119.5%). Dose conformity and uniformity to the PTV was also outperformed by the nc-IMRT plans. On average, the nc-IMRT plans showed larger percentage of ipsilateral breast volume receiving 15 Gy, and doses to 5 and 10 mm depth skin of 107% and 113% of the prescribed dose respectively. On average, VMAT plans adequately covered the PTV, and reduced the V15Gy by about 40% the volumes that resulted with IMRT and nc-IMRT; nevertheless, they had much higher heart volume receiving 3 Gy and ipsilateral lung receiving 10 Gy. In overall performance, the IMRT plans were the most acceptable because of adequate PTV coverage, less skin dose and lower ipsilateral breast V15Gy than nc-IMRT. Materials and Methods Twelve original 6 MV IMRT plans of 5 to 7 fields from breast cancer patients treated with an accelerated partial breast irradiation (APBI) to a course of 30 Gy given in 5 fractions were used for comparison. New nc-IMRT plans with 7 fields, and VMAT plans with 2 partial arcs were created for each patient. All plans were calculated with Eclipse AAA Varian Medical Systems v11.0.31. The organs at risk (OAR) were ipsilateral (ipsi) and contralateral (contra) breasts, skin (5 mm and 10 mm depth), heart, and ipsi and contra lungs. Comparisons were based on minimum, mean, and maximum doses, and specific dose-volume metrics. Conclusion While all techniques produce reasonable plans and provide very conformal coverage, all have limitations regarding doses to the normal organs. For this patient population, IMRT technique seems to offer the best balance between heart and lung doses and surrogate measures for cosmesis. E-Mail: Hanh-Pham@ouhsc.edu