ESTRO 2020 Abstract Book

S902 ESTRO 2020

combination, averaged over three central slices and ten respiratory bins. Peak-to-Peak value (PKP): included amplitude range after OR. Reconstruction Completeness (RC) : Percentage of bin-slice combinations containing at least one image after binning.

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To determine the ISV, the mean and standard deviation (SD) of a QP over six 4DMRIs was evaluated for each volunteer, see Fig. 1B. From this, distributions from the SD over all volunteers were generated. A paired Wilcoxon’s signed rank test was used to test differences in ISV over all volunteers. Results Fig. 1B displays a typical case of a volunteer where applying Min95 OR resulted in less variation in PKP with SDs of 10.7 mm for MaxIE and 6.3 mm for Min95. Fig. 2 shows boxplots of the SDs over all volunteers per OR strategy. Average differences in PKP were significant between Phase and MaxIE, and Min95 (7.6 mm and 4.4 mm respectively) as well as between Min95 and MeanIE (4.4 mm and 3.7 mm). IBV only showed a significant average difference between Phase and Min95 (2.0 mm and 1.5 mm respectively). For RC, differences were significant between Phase and Min95 (1.0% for Phase, 5.4% for Min95). Conclusion The Min95 strategy has a lower intersession variability compared to applying no outlier rejection and the MeanIE strategy.

PO-1645 Development of protocol for EPID IVD based adaptive radiotherapy in a small RT unit. P. Borowska 1 , M. Paluszyńska 1 , U. Sobocka-Kurdyk 1 , M. Peszyńska-Piorun 1 , S. Kubicka 1 1 Wielkopolskie Centrum Onkologii, Zakład Radioterapii III, Kalisz, Poland Purpose or Objective This study was designed to develop a procedure allowing implementation of an adaptive radiotherapy (ART) protocol in a radiotherapy unit with limited staff and technical resources. Due to these limitations, the method should neither be time-consuming, nor require purchase of additional equipment or computer software. This work presents the combination of EPID transit dosimetry with existing IGRT protocol in development of an ART protocol. Material and Methods 15 patients, undergoing RT for prostate (7), lung (3) and breast (5) cancer were enrolled in the study. 722 treatment fields (450 IMRT, 272 3DCRT) delivered in 104 fractions (64, 40, IMRT, 3DCRT, respectively) were tested. Planned dose distributions were calculated with Eclipse TPS. Measurements were performed with EPID integrated with TrueBeam linac, the main therapeutic accelerator in the unit. The γ evaluation method was used to compare a reference EPID image and an IVD EPID image obtained in a daily treatment fraction. Reference image selection algorithm has been determined in course of this study. By means of Portal Dosimetry application, the γ analysis was performed for each field for two sets of the pass criteria: SD (signal difference) and DTA 2%, 2mm and 3%, 3mm, respectively, in order to select the criteria and tolerance levels to be implemented in the developed protocol. In existing IGRT protocol patient set-up is controlled, depending on tumor localization, by the following imaging procedures: • breast: kV + MV during 1st fraction followed by daily kV and MV every 3 days, • prostate, H&N: kV during 1st fraction and onward, CBCT on the 2nd fraction and every 3 days, Results For γ tests the tolerance levels were fixed for the area of γ<1 >80%, γ mean <0,5 and area of γ<1 >90%, γ mean <0,34 for 2%, 2mm and 3%, 3mm pass criteria respectively. For 2%, 2mm and 3%, 3mm criteria respectively, out of tolerance (OOT) results for the area of γ<1 were obtained for: 0,9% of IMRT and 4,04% of 3DCRT, 1,1% of IMRT and 4,41% of 3DCRT fields and for 6,25% IMRT, 17,5% 3DCRT and 4,7% IMRT, 17,5% 3DCRT fractions. • lung: kV daily.