P229] New radiotherapy approach for simultaneous breast-nodes cancer treatment: Comparison between 3d and IMRT techniques

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Purpose Radiotherapy treatments of simultaneous breast-nodes cancer are usually performed using 3-dimensional static fields (3D) or intensity modulated radiotherapy (IMRT). IMRT improves 3D regarding conformity (CI) and maximum doses to organs at risk (OR). However, IMRT implies longer treatment times due to an increment of Monitor Units (MU). In this study a new hybrid radiotherapy technique (HYB) is presented and its differences compared to 3D and IMRT are analyzed. Methods Three dosimetric plans were performed for 15 patients with breast-nodes cancer in Eclipse (Varian Medical System). 1. 3D:2 half tangencial beams for the breast matching the isocenter plus 2 or more half-beams for the nodes using 6 and 18 MV photon energy. 2. IMRT:7 coplanar, single-isocentric and different angled 6 MV fields covering the breast-nodes volume simultaneously. 3. HYB:3 IMRT 6 MV fields (2 tangencials and 1 confronting the PTV) delivering 70% of the prescribed dose to the breast-nodes plus 2 coplanar 6 MV half-arcs with the same isocenter delivering the remaining 30%. The distributions of Dmax, Dmean, D98-2%, CI and Homogeneity Index (HI) for the PTV; D2%, Dmean, V5Gy for the contralateral breast; V2%, Dmean, V10Gy for the contralateral lung; D2%, Dmean, V5-10-20-30Gy for the ipsilateral lung and D3%, Dmean, V5-25Gy for the heart, were compared for the three techniques using Kruskal–Wallis and Wilcoxon tests. The correlation between MU nad PTV volume was also analyzed. Results All the PTV dose distributions are statistically equivalent for 3D, IMRT and HYB. CI is inferior and HI superior for 3D, while equivalent for IMRT and HYB. All the aforementioned OR variable distributions for HYB and IMRT are statistically equivalent, but different in respect to 3D for some of them, in accordance with literature. The MU distributions are statistically different: IMRT has more MU, followed by HYB and finally 3D. The MU increase more rapidly with the PTV volume with IMRT than with HYB and 3D. Conclusions Regarding dosimetry, IMRT and HYB techniques are equivalent. However HYB implies fewer MU than IMRT, regardless of the PTV volume. Moreover, the higher the PTV volume, the more benefit the HYB seems to have in respect to the MU, since this implies less treatment time, less inconvenience for the patient, and better treatment reproducibility.