ESTRO 2021 Abstract Book

S690

ESTRO 2021

Average values, standard deviations and histograms were computed for each spatial coordinate (longitudinal, lateral and vertical). Systematic and random errors were calculated for each DIBH systems, utilizing its associated intrafraction motion data. To evaluate DIBH systems accuracy a two-tailed Student T-test was performed with a significance level of 0.05. To evaluate precision a Fisher-Snedecor F test was utilized. Results Figure 1 depicts each spatial component probability distribution comparing intrafraction motion results obtained when respiratory management was performed using ABC or Catalyst, respectively. Figure 2 shows average results, systematic errors and random errors for each spatial coordinate and system employed. Average values are quite similar between ABC and Catalyst. Besides, p-values obtained are not significant: 0.649 (longitudinal), 0.862 (lateral) and 0.735 (vertical). However, distribution widths are statistically different which implies that precision is not the same. P-values obtained through Fisher F-test were quite close to 0 (<<0.001). The differences obtained in variances indicates that ABC is more precise than Catalyst. The cause might be the different behavior of the respiratory cycle when DIBH is performed with both systems. When utilizing ABC the air flux is stopped and a completely flat respiratory pattern is obtained. However, Catalyst forces the patient to hold its breath and maintain the cycle inside a gating window of 4mm width. Inside that window the respiratory cycle moves and so does the patient.

Figure 1: Offset distribution comparison between ABC (red) and Catalyst (blue).

Figure 2: Sistematic and random errors and average results for ABC and Catalyst intrafraction estimations.

Conclusion Although both systems are perfectly suitable to perform deep-inspiration breathhold in lung SBRT, ABC is more precise as the variances obtained are significantly smaller compared to Catalyst. These differences might be associated with the distinct behavior of the respiratory cycle in the gated area. However, we consider both systems to be accuracy-equivalent. PD-0857 Real-time intrafraction prostate motion during dose-escalated linac-based SBRT D. Panizza 1,2 , S. Trivellato 1 , G. Montanari 1 , V. Pisoni 3 , V. Faccenda 4 , P. Caricato 4 , M.C. Daniotti 5 , R. Lucchini 3,2 , E. De Ponti 1,2 , S. Arcangeli 3,2 1 ASST Monza, Medical Physics Department, Monza, Italy; 2 University of Milan Bicocca, School of Medicine and Surgery, Monza, Italy; 3 ASST Monza, Radiation Oncology Department, Monza, Italy; 4 University of Milan, Department of Physics, Milan, Italy; 5 University of Milan Bicocca, Department of Physics, Milan, Italy Purpose or Objective Extreme hypofractionation requires tight CTV to PTV margins, high dose gradients, and strict adherence to planning criteria in terms of patient positioning and organ motion mitigation. An electromagnetic (EM) transmitter-based tracking device for prostate and urethra localization and monitoring during prostate cancer SBRT was implemented. The aim of this study was to evaluate the intra-fraction prostate motion in the very first clinical use worldwide. Materials and Methods Ten patients with organ-confined prostate cancer underwent dose-escalated SBRT in 4 or 5 fractions (BED 1.5 = 279 Gy and 253 Gy, respectively), using Volumetric Modulated Arc Therapy (VMAT) techniques with flattening filter free (FFF) beams on VersaHD linac. The EM tracking device consisted in an integrated Foley catheter with a transmitter in a dedicated lumen. Signals sent by the transmitter were detected by antennas in a specific receiver placed on the linac couch. The system was calibrated to the treatment room isocenter and, based on the known relationship between the transmitter center and the isocenter in the planning CT, the system allowed treatment localization in addition to motion tracking. Starting from the daily CBCT and during