ESTRO 2020 Abstract Book

S884 ESTRO 2020

time series representing prostate movement, and k -means clustering up to 30 clusters. In this retrospective study, the prostate was allowed to move more than 3 mm. Dose distribution was reconstructed for each cluster and mapped back to the original CT image. CT images were created for each cluster by translation of a rigid prostate (CTV). Femoral heads were chosen to remain rigid and fixed. Soft tissues, and to some extent also pelvic bones, were deformed and transferred in relation to the prostate movement. Results Since the treatment system did not record beam on times in relation to the movement data, worst case scenarios were chosen to study what could have happened without real-time monitoring during one fraction. The method allowed prostate movement induced D95 reductions to be analysed per fraction series. Assuming a stationary prostate for the other four fractions, D95 reductions for the five patients were 7.6, 19.4, 0.3, 0.1 and 4.4 %. This represents a minimum error for the treatment. A maximum error would occur if the prostate movement was similar in all the five fractions. In that case the D95 reductions would be 38, 97, 1, 1 and 22 %. Fig.1. Recorded prostate movement data. The crosshair represents the point of origin for movement data simulating a match based on CBCT image. The red and green stripes correspond to beam on times for the two arcs of the VMAT plan.

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

Fig.2. Original and reconstructed dose distributions of a cranial CT slice. The lack of dose in the PTV is the result of a cranial movement.