ESTRO 2020 Abstract book

S842 ESTRO 2020

Gamma (1% / 1 mm) 100 million photons

Gamma (1% / 1 mm) 4 million photons

Protocol Name

CBCT 5s DR Body Care CBCT 12s DCT Large Volume L CBCT 12s DCT Large Volume P CBCT 6s DCT Body CBCT 20s DCT Head 70kV CTCT 20s DCT Head 109kV CBCT 10s DCT Head

84.5%

84.7%

68.2%

72.7%

94.3%

96.4%

95.0%

96.1%

Figure 1: Comparison of the PT plans made by the PT center (A) and Erasmus-iCycle (B). PO-1476 Monte Carlo based 3D treatment planning using intraoperative CBCT scanning for image guided IORT L. Probst 1 , L.D. Jiménez-Franco 1 , S. Clausen 1 , V. Steil 1 , F.A. Giordano 1 , F. Schneider 1 1 Universitätsmedizin Mannheim, Klinik für Strahlentherapie und Radioonkologie, Mannheim, Germany Purpose or Objective Intraoperative radiotherapy (IORT) allows delivering high doses of irradiation to the tumor bed with no to little involvement of healthy tissues. With the aim to more precisely calculate intraoperative dose distributions, we investigated if conventional intraoperative cone-beam CT (CBCT) imaging is suitable for IORT treatment planning. Material and Methods Seven different HU to electron density (ED) curves were acquired for corresponding CBCT (Artis ZeeGo TM , Siemens, Germany) protocols, using the RMI 467 ED phantom (Gammex, USA). The reference CT image data set was achieved from a conventional treatment planning CT (Brilliance Big Bore, Philips, Netherlands) (PCT). The acquired CT images provide 1 mm slice thickness and lead to a 400 mAs tube current-time product with 120 kV anode- cathode potential. Furthermore, PCT images with 150 mAs and 250 mAs were generated to confirm consistency of the analyzing method. All treatment plans were calculated on the CIRS model 057A triple modality 3D abdominal phantom (CIRS, USA) using Radiance (version 4.0.3, GMV, Spain), which employs the Intrabeam (Carl Zeiss Meditec GmbH, Germany) head model to simulate the dose with a Monte Carlo algorithm. To estimate the variance of the MC simulations, the dose distributions on the 400 mAs PCT were repeatedly calculated with several particle numbers on a 0.5 mm grid, using a 4 cm spherical applicator. All dose distributions simulated on CBCTs were then compared to the reference dose distribution calculated on PCTs. Plan evaluation was performed with Matlab, applying a 3D local gamma with 1%/ 1 mm passing criteria, after converting the grid to 0.25 mm via interpolation of the dose distribution. The region of interest was a shell of 2 cm thickness around the applicator. Results Five CBCT protocols reached more than 94% local 3D Gamma passing rates for 100 million photons (Table 1). The passing rate increased up to 3% for certain protocols using 4 million photons. The calculation times ranged between 1 and 200 minutes when simulating 500.000 up to 100 million photons (Figure 1). Consequently, the averaged standard deviations (obtained from ten individual plans on the same PCT) decrease in parallel from roughly 16% to less than 1%.

94.9%

97.9%

98.2%

99.1%

98.9%

99.3%

PCT 150 mAs 99.2% PCT 250 mAs 99.2%

99.5% 99.5%

Table

1

Figure 1: Variances and calculation time in dependency of number of photons used for MC simulations. Conclusion Conventional intraoperative CBCT scanning is suitable for Monte Carlo based online 3D kV treatment planning, paving way for image-guided intraoperative radiotherapy. However, trade-offs between simulation times and variance of doses need to be considered. PO-1477 Comparison of 3D-CRT, IMRT and VMAT plans (including low body dose) for whole breast irradiation B. Czeremszynska 1 , A. Rygielska 2 , A. Walewska 2 , M. Gabor 2 , D. Pruska-Pich 2 , K. Osowiecka 3 , L. Kepka 1 1 Military Institute od Medicine, Radiotherapy Department, Warsaw, Poland ; 2 Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland ; 3 University of Warmia and Mazury, Department of Public Health, Olsztyn, Poland Purpose or Objective It is believed that dynamic radiotherapy techniques may improve the therapeutic ratio of radiotherapy for breast cancer. We compared the PTV coverage and doses in organs at risk (OAR) for 3D-CRT, IMRT and VMAT plans in whole breast irradiation (WBI). Material and Methods 22 (15 left breasts, 7 right breasts) consecutive patients addressed for WBI without lymph node irradiation were included. Patients were treated with free breathing; total