ESTRO 2021 Abstract Book

S1475

ESTRO 2021

Po-1750 Plan Robustness Analysis Of Single-Isocenter Multiple Brain Metastases Srs J. Calvo-Ortega 1 , J. Casals-Farran 1 1 hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain

Purpose or Objective To validate the CTV-PTV margin used in our department for single-isocenter multiple metastases SRS, by using the plan robustness based on DVHs. Materials and Methods 15 treated SRS plans were included. PTV for each lesion (CTV) was designed using an isotropic 2 mm margin. Number of PTVs per plan ranged from 2 to 10. Distances from the PTV center to the isocenter ranged from 0.3 to 8.7 cm. Median prescription dose (PD) was 19 Gy. The SRS plans were designed with the Eclipse TPS (AAA v 10.0.28) using non-coplanar IMRT technique, and delivered using 6 MV photons from a Varian Clinac 2100 CD linac with a Millennium 120 MLC. SRS was delivered using an on-line approach avoiding systematic patient setup errors (Med Dosim. 2013 Autumn;38(3):291-7). Based in our internal estimation, systematic errors (∑) due to gantry and couch isocenter wobble are 0.4, 0.3 and 0.3 mm in X (Left-Right), Y (Anterior-Posterior) and Z (Superior-Inferior) directions, resp. Random errors (σ) due to intrafraction head motion were estimated in 0.2, 0.4 and 0.9 mm in X, Y and Z, reps., and 0.1, 0.4 and 0.1 degrees in tilt, yaw and roll axis, resp. Robustness of each SRS plan was analyzed using the CERR software (https://github.com/cerr/CERR). The dose of the SRS plan was shifted according to the ∑ and σ errors. The shifts applied to the original dose distribution were sampled from a normal distribution centered around 0 and having a standard deviation equal to the input ∑ and σ values. Sampling was done for 100 trials and an average DVH (±2 SD) was computed. The minimum dose to each CTV (Dmin) was extracted using 2 approaches: taken from the DVH-2 SD lower bound (Dmin_low) and from the average DVH (Dmin_av). Dmin were compared to the PD: [(Dmin/PD) -1] x 100. Results Over the 15 SRS plans, dose differences (Dmin vs PD) were 10.6% (SD: 2.9%) and 4.1% (SD: 4.0%) for Dmin_av and Dmin_low, respectively. Minimum dose differences of 2.9% and -3.3% were observed for the Dmin_av and Dmin_low, respectively. Conclusion The isotropic 2 mm CTV-PTV margin used in our single-isocenter multiple brain metastases SRS procedure guaranties the dose coverage of the CTVs with the planned PD. PO-1751 Inter- and intra-fractional uncertainties in deep inspiration breath-hold (DIBH) breast treatment P. Retorta 1 , D. Sevillano 1 , M. Cámara 1 , A.B. Capuz 1 , R. Colmenares 1 , J.D. García 1 , R. Morís 1 , F. Orozco 1 , M.J. Béjar 1 , D. Prieto 1 , P. Galiano 1 , F. García 1 1 Hospital Universitario Ramón y Cajal, Medical Physics, Madrid, Spain Purpose or Objective The incorporation of Catalyst System impels us to question the accuracy of our current breast treatments system. With that in mind, we studied and analyzed inter- and intra-fractional uncertainties of both DIBH and We studied 62 DIBH and 36 non-DIBH patients. For DIBH patients, Catalyst allows us to perform a CT simulation while in deep inspiration and defines a gating window (3-4 mm) in that position for SGRT treatment guidance. Spatial displacements during irradiation intervals were obtained from the cMotion study in C-RAD software (Figure 1). We registered the mean displacements of the patient population, µ , the systematic uncertainty, Σ , and the inter- and intra-fraction random uncertainties, σ inte r and σ intra . In addition, a split in different groups was carried out: older vs younger than 55 years old and 3D vs VMAT technique. There were 27 patients younger than 55, 35 older than 55, 51 patients undergoing a 3D treatment, and 11 VMAT. non-DIBH treatments. Materials and Methods

Results We plotted the results for the differences between DIBH and non-DIBH cases in Table 1. The margins presented herein were calculated through the Van Herk’s formula. It is remarkable that, for DIBH, either