ESTRO 2021 Abstract Book

S1502

ESTRO 2021

Results Each treatment fraction was completed using 6-8 breath-holds. 525 breath-holds during beam-on time were analyzed. The mean amplitude of the ABC tidal volume was 2.0±0.05 L. The mean amplitudes of displacements were 8.0 mm (CI95%: 3.3–11.7 mm), 13.4 mm (CI95%: 10.1–21.1 mm) and 4.4 mm (CI95%: 0.3– 8.8 mm) in the AP, SI and LL directions, respectively. The SD of the breath-hold level during DIBH was used as measure of the intra-breathhold stability: median values were 0.3 mm (95%CI: 0.1–1.1 mm), 0.8 mm (95%CI: 0.2–2.5 mm) and 0.2 mm (95%CI: 0.1–0.9 mm) in the AP, SI and LL directions, respectively . I nter-fractional reproducibility in the breathing amplitudes was found 5.9, 4.9 and 4.4 mm in the AP, SI and LL directions, respectively. Displacements up to 8-10 mm between different sessions were observed in the AP and SI direction, despite the almost constant inspired volume supply by the ABC spirometer. No correlations were found between ABC breath-hold levels and surface monitoring deviations for the AP (R2=0.10), SI (R2=0.20) and LL axes (R2=0.14). Figure 1 shows an example of the ABC spirometer (upper) and AlignRT time trajectories along the three spatial directions (lower) for three representative breath-holds.

Conclusion Optical surface monitoring reported that the use of spirometer-guided radiotherapy alone does not guarantee a stable and reproducibility breast positioning during DIBH. Optical surface-guided systems are strongly recommended for improving the breast surface position reproducibility during DIBH. PO-1775 Effect of intra-fraction patient stability on treatment margins in brain SRS with Cyberknife D. Sevillano 1 , J.D. García-Fuentes 1 , M. Cámara 1 , E. Fernández-Lizarbe 2 , M. Martín-Sánchez 2 , C. De la Pinta 2 , R. Colmenares 1 , B. Capuz 1 , R. Morís 1 , F. Orozco 1 , P. Retorta 1 , P. Galiano 1 , M.J. Béjar 1 , D. Prieto 1 , F. García- Vicente 1 1 H. U. Ramón y Cajal, Medical Physics, Madrid, Spain; 2 H. U. Ramón y Cajal, Radiation Oncology, Madrid, Spain Purpose or Objective To study intra-fraction residual errors occurring in brain SRS treatments with the Cyberknife system depending on the characteristics of patient. Materials and Methods Data from intra-fraction displacements of 70 brain SRS patients (150 treatment fractions) treated in our department with the Cyberknife system were analyzed in this work. Geometrical uncertainties for four different situations were obtained: no intra-treatment tracking, real treatment and treatment if only half and one third of the tracking images were employed. Margins needed to account for intra-fraction uncertainties were assessed by means of the Van Herk´s formula assuming a 70% isodose level and a beam penumbra of σ p =2mm. Margins due to rotations were calculated by assuming a mean distance between target and imaging center of 5 cm and calculating the effect of rotations on displacements of the target. As tumors can be isotropically distributed around the imaging center, symmetric uncertainties were applied. All analysis were performed for the whole patient population and for a division of it in three groups according to the mean standard deviation of the corrections observed during treatment. Results Treatment margins for the whole patient population are 0.5mm isotropically if no tracking is applied and 0.1 mm if residual errors of the real treatments are considered. Margins in AP and SI directions were increased to 0.2mm when skipping the 50% and 66% of the images, showing little dependency on imaging frequency. Results when dividing the population according to their in-treatment motion (based on the standard deviation of displacements) are shown in Table 1. It can be observed that margins are much larger for the group with the most unstable patients (group 3) than for the other 2 groups. Margins for group 3 in case no tracking is applied are close to 1mm and almost the double than in the other 2 groups, showing the importance of identifying