ESTRO 2022 - Abstract Book

S1503

Abstract book

ESTRO 2022

(R) was defined as the maximum difference between different DIBH levels. The inter-fraction reproducibility (T) was determined as the difference between the mean breath-hold level per fraction and the mean breath-hold level during the first fraction. Correlations between ABC tidal volume and surface imaging deviations were investigated. Results Each treatment fraction was completed using 4-6 breath-holds. 75 treatment fractions and 625 breath-holds during beam- on time were analyzed. Overall ABC intra-session lung volume variation was <1.5%. With regard to intra-breath-hold stability, the median linear deviations S were 1.3 mm (95%-CI: [0.0–2.2] mm), 2.4 mm (95%-CI: [0.3–6.7] mm) and 0.8 mm (95%-CI: [0.0–1.7] mm) ) in the VER, LNG and LAT directions, respectively. Results for the LNG direction indicated that the drift over time during breath-hold are not negligible, up to 9 mm even at constant inspired volumes. With regard to intra- fraction reproducibility, the averaged results reported R median values of 1.3 mm (95%-CI: [0.0–1.9] mm), 1.4 mm (95%-CI: [0.0–2.9] mm) and 0.8 mm (95%-CI: [0.0–1.3] mm) in the VER, LNG and LAT directions, respectively. Median inter-fractional reproducibility was found 0.0 mm (95%-CI: [-0.6–0.6] mm), -0.3 mm (95%-CI: [-1.4–0,2] mm) and 0.0 mm (95%-CI: [-0.2–0,2] mm) in the VER, LNG and LAT directions, respectively. No correlations were found between ABC breath-hold levels and surface monitoring deviations in all directions. Figure shows an example of the ABC spirometer (upper) and AlignRT time trajectories along VER and LNG directions (lower) for three representative breath-

holds.

Conclusion Despite the use of spirometer-controlled lung volume, breast immobilization may not be stable and reproducible as expected. The integration of spirometry with an optical surface-guided guidance system may increase surface position reproducibility during repeated DIBHs.

PO-1704 Dosimetric effect of uncorrected rotational errors during prostate SBRT Cyberknife treatments.

D. Sevillano 1 , J.D. García-Fuentes 2 , A. Hervás 3 , C. Vallejo 3 , F. López-Campos 3 , R. Colmenares 4 , R. Morís 4 , B. Capuz 4 , M. Cámara 4 , P. Retorta 4 , P. Galiano 4 , S. Williamson 4 , M.J. Béjar 4 , D. Prieto 4 , F. García-Vicente 4 1 H. U. Ramón y Cajal. IRYCIS, Medical Physics, Madrid, Spain; 2 H. U. Ramón y Cajal, IRYCIS, Medical Physics, Madrid, Spain; 3 H. U. Ramón y Cajal, IRYCIS., Radiation Oncology, Madrid, Spain; 4 H. U. Ramón y Cajal, IRYCIS., Medical Physics, Madrid, Spain Purpose or Objective The Cyberknife system does only allow to perform rotational corrections if the prostate position is within certain tolerances (±5 degrees in pitch and ±3 degrees in yaw and ±2 degrees in roll). When it is not possible to setup the patient correctly, the user faces the decision of whether it is better to treat without rotational corrections or try to reposition or even replan that treatment. To ease this decision, this work studies the dosimetric effect of not applying rotational corrections in prostate SBRT treatments with Cyberknife. Materials and Methods 17 prostate SBRT treatments with Cyberknife were studied. Treatment plans consist on delivering 36.25 Gy to the prostate+1/3 of seminal vesicles (SV) with an isotropic CTV-to-PTV margin of 5mm. For each plan, the dose delivered to