ESTRO 2021 Abstract Book

S1280

ESTRO 2021

Figure 2: In each plot the accumulated doses with all five algorithms are evaluated. The clinical metric assessed for the healthy liver (liver structure with the GTV subtracted) is the volume receiving below 15 Gy. The labels are the same for all plots (see bottom right). Conclusion Comparison of accumulated DVHs suggests differences between the algorithms in the preliminary analysis. It may be necessary to use multiple accumulation algorithms to ensure failsafe operation when the treated volume is to be minimized in MRgoART. PO-1555 The effect of respiratory motion on the planned dose in VMAT treatment of right sided breast cancer M. Mankinen 1 , T. Virén 2 , J. Seppälä 2 , H. Hakkarainen 3 , T. Koivumäki 1 1 Central Finland Hospital Nova/University of Jyväskylä, Department of Medical Physics/Department of Physics, Jyväskylä, Finland; 2 Kuopio University Hospital, Department of Radiotherapy, Kuopio, Finland; 3 Central Finland Hospital Nova, Department of Oncology and Radiotherapy, Jyväskylä, Finland Purpose or Objective The interplay effects of respiratory motion and MLC movement in right sided whole breast irradiation (WBI) using VMAT and field-in-field (FinF) treatment techniques were studied. Materials and Methods Structures for RT planning were automatically segmented using MIM Maestro software. CTV structures were contoured by an oncologist and a 5mm margin used to generate the PTV. Three types of WBI treatment plans were generated for 6 patients: Monaco VMAT, Varian RA and FinF. All the patients had a free-breathing 4D CT series with 10 phases. The plans were generated on the end-inspiration phase image. With RA and VMAT, an 8mm PTV extension with an 11mm optimization bolus was used in dose optimization. Each plan was segmented into 10 subplans based on the patient respiratory data acquired during the 4D CT acquisition. Each subplan contained the irradiation for the corresponding respiratory phase. This procedure was repeated 5 and 15 times for each plan to model two hypofractionated dose protocols, 5/26 Gy and 15/40.05 Gy. The starting phase was randomly selected for each fraction. The phase specific dose distributions were deformed and superimposed to the planning image by using MIM Maestro software. The PTV was cropped by 5mm margin from the skin surface to form PTV-5 structure, which was used for dose normalization for the original plans. The resulting dose distributions were compared to the original ones to analyze the effects of respiratory motion. The mean skin doses were evaluated in two consecutive 3mm layers beneath the skin surface (PTV0-3 and PTV3-6). Wilcoxon signed rank test was used to evaluate the statistical significance (p < 0.05). Results The average PTV coverage decreased for both RA and VMAT techniques (Table 1). The maximum dose to 1cc volume (D1cc) was unchanged for RA and VMAT, while a decrease was found for FinF technique. A slight decrease in mean PTV skin dose was observed in the PTV0-3. Changes in DVH parameters were small for organs at risk. The highest dose differences were observed above and below the target and in the sternum area (Fig. 1). Table 1: The mean planned and simulated DVH parameter values and standard deviations across 6 patients in this study. The parameters are presented for 5 and 15 fraction treatments per technique. For convenience of comparison between fractionations, the dose parameters (mean, D1cc) are presented as percentages of prescribed dose and not in Gy.