ESTRO 2020 Abstract book

S936 ESTRO 2020

for treatments. The intrafraction motion evaluation has been performed by investigating the residual isocenter shifts during beam on time. Mean, minimum, maximum and SD displacements in isocenter position over all the treatment fractions of each patient were evaluated for the two sites separately. Results Intra-fraction mean isocenter shift over all the patients was 2.7±0.5 mm for tangential and 2.5±0.4 mm for locoregional treatments. The isocenter average shift for each patient ranged between 1.8mm-3.4mm for tangential and 1.9mm-3.1mm for locoregional treatments. The box plot of interfraction mean shift during beam on time of treatment delivery is shown in fig.1. Results indicate that locoregional treatments are characterized by slightly lower isocenter shifts than tangential although this difference is not statistically significant (t test p-value 0.2). This is probably due to the tighter tolerance set for the gating window amplitude and the isocenters shifts for locoregional treatments. However, locoregional treatments have usually a longer duration than tangential ones (4 beams for locoregional instead of 2 for the tangential treatments) and therefore the patient movements during the treatment can be larger. locoregional

Conclusion Prostate movement for 5 times 7 Gy fractionated PCa radiotherapy without intra-fractional monitoring could lower D95 of PTV as much as 19.4 % based on actual prostate movement in only a single fraction. PO-1621 Surface Guided Radiotherapy for locoregional DIBH treatment: Intrafraction motion evaluation A. Gnerucci 1 , S. Russo 2 , M. Esposito 2 , A. Ghirelli 2 , S. Pini 2 , F. Rossi 3 , L. Paoletti 3 , R. Barca 3 , S. Fondelli 3 , P. Alpi 3 , B. Grilli 3 , P. Bastiani 3 1 University of Florence, Experimental and Clinical Biomedical Sciences "Mario Serio", Florence, Italy ; 2 Azienda USL Toscana Centro- Florence, Medical Physics Unit-, Florence, Italy ; 3 Azienda USL Toscana Centro- Florence, Radiotherapy Unit, Florence, Italy Purpose or Objective Aim of this work was to assess the reproducibility of a DIBH tecnique based on optical surface tracking technologies for patients undergoing adjuvant RT for left sided breast cancer with and without lymph nodes inclusion. Material and Methods 58 patients (38 tangential and 20 locoregional) were enrolled in this study. 3D conformal tecnique for 50Gy median dose in 25 fractions was adopted. C-RAD Sentinel™ laser scanning system and a Siemens CT scanner were used for the prospective DIBH CT study. The respiratory signal was reconstructed by tracking a region of interest defined on the surface above xiphoid process. Base line and gating window levels of the respiratory signal were established during CT simulation procedure. Gated treatments delivery was supported by the C-RAD Catalyst™ system connected with an Elekta Synergy. The treatment beam was turned on only when the patient signal is within the previously established gating window. Visual coaching through video goggles were provided to help the patient following the optimal breathing pattern. A daily surface reference image (REF) was captured the first time when the patient was breathing into the gating window. The live surface obtained during the rest of the treatment fraction was matched with REF by using a novel nonrigid algorithm and the isocenter shifts between the two surfaces were tracked in real time. Tolerances for beam triggering based on the isocenter shifts were set equal to 5mm for tangential and 4mm for locoregional treatments while the gating window amplitude was 5mm for tangential and 3mm

Conclusion DIBH procedure guided by optical systems for left breast irradiation is a reproducible and stable tecnique with a limited intra-fraction DIBH variability both for tangential and locoregional treatments.

Poster: Physics track: Adaptive radiotherapy and inter- fraction motion management

PO-1622 Impact of positioning errors in the dosimetry of VMAT left-sided post mastectomy irradiation X. Liao 1 , F. Wu 1 , J. Wu 1 , Q. PENG 1 , X. Yao 1 , S. Kang 1 , Y. Zhao 1 , L.C. Orlandini 1 1 Sichuan Cancer Hospital and Institute, Radiation Oncology, Chengdu, China Purpose or Objective Post-mastectomy radiation therapy (PMRT) is technically difficult given the complexity of the target volume and its proximity to critical structures. Intensity modulated radiation therapy can achieve highly conformal dose distributions, nevertheless, inaccuracy in the positioning