ESTRO 36 Abstract Book

S451 ESTRO 36 _______________________________________________________________________________________________

Results

the bilateral breasts, the lungs, the heart, the left anterior descending artery (LAD), and the thyroid, were compared to those of non-coplanar 3D-CRT and coplanar VMAT. A p value < 0.05, derived using the paired Student’s t-test, was considered to reflect a significant difference. Results Figure 1-B shows the averaged dose-volume histograms for all 24 patients in terms of their PTVs and OARs. The Table summarizes the mean dose volume indices for the targets and OARs, and the MU for each technique. Significant difference was not observed in doses to the PTV. When DWA was employed, the average V 20Gy , V 10Gy , and V 5Gy to the ipsilateral lung; the average V 10Gy and V 5Gy to the heart; the D 2% for the planning organ at risk volume of the LAD (PRV_LAD); and the V 50% of, and the mean dose to the ipsilateral breast, were significantly lower than those of non-coplanar 3D-CRT and coplanar VMAT. The average D 2% to the contralateral breast and the V 5Gy to the contralateral lung did not differ significantly among the techniques. Furthermore, the mean prescribed MU for DWA was 486.22 MU, which was only 9.8% higher than that for non-coplanar 3D-CRT (442.67 MU) (p < 0.05).

Left breast results: Identical coverage was achieved with a D95 of 95.2% ±0.4 (DIBH) vs. D95= 95.4%±0.6 (FB)(p=0.27). No statistical difference were found in D98, median and D2 (Figure1.A). Left lung results: No statistical difference was also found (Figure1.B). Heart results: Doses were significantly lower with DIBH; Dmean at 1.5Gy±0.8 (DIBH) vs. 2.9Gy±1.6 (FB) (p<0.001) and Dmax at 36.3Gy±14.7 (DIBH) vs. 46.2Gy±6.6 (FB) (p=0.004). DVH metrics for V10, V20, V30, and V40 were all significantly better in DIBH (Figure1.C). LAD coronary results: Doses were significantly lower with DIBH with Dmean at 9.5Gy ± 7.2 (DIBH) vs. 21.8Gy±11.4 (FB) (p<0.001) and a Dmax at 29.2Gy±17 (DIBH) vs. 42.3Gy±12 (FB) (p=0.003). DVH metrics favored the DIBH plans significantly all across the range of doses (Figure.1D). Conclusion In the treatment left breast cancer, 3D-DIBH showed superior dosimetric advantages in comparison to 3D-FB. Both heart and LAD were significantly spared without compromising left breast coverage. The LAD was spared for doses ranging from the low dose spectrum to the highest dose. PO-0837 Dosimetric advantages afforded by Dynamic WaveArc therapy accelerated partial breast irradiation Y. Ono 1 , M. Yoshimura 1 , K. Hirata 1 , N. Mukumoto 1 , T. Ono 1 , M. Inoue 1 , M. Ogura 1 , T. Mizowaki 1 , M. Hiraoka 2 1 Kyoto University- Graduate School of Medicine, Department of Radiation Oncology and Image-applied Therapy, Kyoto, Japan 2 Japanese Red Cross Wakayama Medical Center, Department of Radiation Oncology, Wakayama, Japan Purpose or Objective We identify dosimetric advantages of the novel volumetric modulated arc therapy (VMAT) featuring continuously varying non-coplanar trajectories. This is the Dynamic WaveArc (DWA) therapy used for accelerated partial breast irradiation (APBI). The dose distribution of DWA therapy was compared to that of non-coplanar three- dimensional conformal radiotherapy (3D-CRT) and coplanar VMAT. Material and Methods We evaluated APBI dose distributions, delivered via DWA, in 24 left-side breast cancer patients via non-coplanar 3D- CRT from November 2011 to April 2016 at our institution. The prescribed dose was 38.5 Gy in 10 fractions. The Vero4DRT enables DWA by continuous gantry rotation and O-ring skewing with moving dynamic multi-leaf collimator (MLC). Thus, the Vero4DRT delivers non-coplanar VMAT without couch rotation, minimizing dose delivery to adjacent organs at risk (OARs). We created two sets of 11 control points (at angles 315-35° to the O-ring angle, and 110-155° and 290-355° to the gantry angle), for two non- coplanar DWA trajectories. DWA, non-coplanar 3D-CRT, and coplanar VMAT treatment plans were created by a clinical treatment planning system, Raystation, using a collapsed cone dose-calculation algorithm (Figure 1-A). The mean DWA doses to the planning target volume (PTV),