ESTRO 2020 Abstract book

S916 ESTRO 2020

rpm) were simulated using a programmable motion phantom (QUASAR Modus QA). The phantom with imaging inserts of density 1.41 gcm -3 , including 2 spheres S1 and S2 of 1 and 2 cm diameter, respectively, and a cube (D3) of edge 3 cm was scanned by 4D-CT (Philips big Bore) and 4D- CBCT (acquired with Elekta XVI) under the 16 breathing modes registered. The image datasets were transmitted to the MIM workstation, and the inserts were automatically contoured on each phase by HU thresholding, and then verified by the reference radiation oncologist. The volumes of each insert were recorded and the results obtained in the different phases for both modalities were analysed and compared with Friedman test, and p < 0.05 was considered statistically significant. Results The comparison between the volumes of the inserts and their corresponding 4D-CT and 4D-CBCT contoured volumes was performed when all the breathing patterns were considered. The results showed a mean percent increase in the volume of 60.9%±29%, 33.4%±16.1%, and 30.2%±14.1% for the 4D-CT, 56%±10.1%, 7.6%±6.6%, and 17.2%±6.5% for the 4D-CBCT and for the S1, S2, and C3 inserts, respectively. Figure 1 shows the volumes of the three contoured inserts under different imaging modalities and breathing patterns vs breathing phases. The results obtained for the 4D-CT S1, S2, C3 contoured volumes (Figure 1a, b, c, respectively) show a plateau for each breathing patterns between phases 40 to 70%. In this range of phases, the mean percent increase of the volume, was reduced to 35.6%±8.4%, 16.2%±4.9%, and 23.2%±4% for S1, S2, C3, respectively. The graphs in Figure 1d, e, f, representing the 4D-CBCT S1, S2, C3 volumes, respectively, show a lower increase of the contoured volumes, and a lower volume variability along the whole interval of phases. Moreover, the mean percent increase in the S1, S2, C3 volumes calculated in the phase interval 40-70% is 50.0%±12.0%, 8.3%±6.9%, and 16.6%±5.2%, respectively, and remain close to the values obtained using the whole interval of phases.

Conclusion Compensation of intrafractional prostate rotations with gating and tracking was investigated experimentally for the first time. All three techniques largely improved the dose, but residual errors remained due to the lack of rotation correction. PO-1592 Effect of respiratory motion on lung target volume during 4D-CT and 4D-CBCT imaging S. Liu 1 , X. Liao 1 , J. Li 1 , L.C. Orlandini 1 , J. LANG 1 1 Sichuan Cancer Hospital & Institute- Sichuan Cancer Center- School of Medicine- University of Electronic Science and Technology of China, Department of Radiation Oncology, Chengdu, China Purpose or Objective To study the effect of respiratory motion on lung target volume during 4D-CT and 4D-cone beam CT (4D-CBCT) imaging to improve the accuracy of volume contouring and registration in gated techniques. Material and Methods Breathing patterns with different amplitudes (peak-peak 5, 10, 20, and 30 mm) and frequencies (10, 12, 15, and 20

The mean absolute volumes of the 4D-CT and 4D-CBCT contoured volumes, for each phase between 40 and 70%