ESTRO 36 Abstract Book

S884 ESTRO 36 2017 _______________________________________________________________________________________________

inter-session translation and rotation was shown in Fig.1. No significant differences were observed in translation in LR and SI direction, while AP translation was significantly smaller. This could be partially explained by the fixed AP level of the MR-sim couch. The yaw rotation was significantly larger than roll and pitch rotation, which may result from the relatively inadequate immobilization performance in this direction, but yet to be further investigated.

registration, translational errors were applied to correct the CT position, obtaining a tCT, i.e. CT with only translational errors correction. Then, rotational errors were corrected too, obtaining roto-translated CT (rtCT). Reference treatment plan was copied to translated tTP and to roto-transalted rtTP and dose distributions were re- calculated, obtaining two treatment plans for each fraction. DVHs dosimetric parameters were compared. Results In this study 179 CBCT were performed on 47 patients (14 with primary tumours and 33 with metastatic lesions) and 358 treatment plans were calculated (179 tTP and 179 rtTP). Geometric and dosimetric analysis are reported in Table 1. There was no correlation between translational and rotational shifts. Rotational shifts resulted greater than 1° in 40% of cases. Average variations in PTV and CTV V95% were negligible, but we observed variations of PTV V95% >2% in 30%, 8% and 12% of cases (CBCTs) respectively for brain, thorax and abdomen patients. OARs proximity to the target caused variations >2% and rotations around each axis could determine important changes depending on the symmetry of the organ. Conclusion Rotational errors have to be corrected regardless of translations magnitude. Although rotations don’t impact on CTV V95%, 6DoF corrections allow better PTV’s coverage. Rotational errors could cause considerable dosimetric changes in organs at risk and must be carefully corrected in SBRT to avoid normal tissue complications. An ongoing analysis on setup systems and margin reductions has been planned. EP-1652 A new position verification protocol for breast cancer with integrated boost K.L. Gottlieb 1 , E.L. Lorenzen 1 , J.D. Jensen 2 , M.H. Nielsen 2 , M. Ewertz 2 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective The use of integrated boost compared to sequential boost allows shortening of the overall treatment time while maintaining the same biologically equivalent dose to the boost region. However when the target is large as in breast cancer treatments there can be challenges in IGRT registrations between the boost volume and total target volume. The present study proposes and evaluates a protocol for daily IGRT using CBCT of breast cancer patients with integrated boost. A threshold is set for the allowed difference between the whole target match and the boost specific match. If the two matches differ by less than the threshold the boost volume and the total volume is treated in the same setup. If the two matches differ by more than the threshold, the total target volume is matched and treated and then secondly an additional CBCT is performed and matched on the boost volume In order to evaluate the match protocol all patients receiving RT during the period 1/1-2016 and 1/7 2016 after breast conserving surgery and with lymph node involvement were retrospectively included in the study. which is then treated. Material and Methods

Conclusion Small head and neck setup uncertainty and thus high positional repeatability could be achieved on a 1.5T MR- sim, suggesting the potentials of MRI for precise HN-RT. EP-1651 Dosimetric impact of rotations correction in Stereotactic RT. How much a 6DoF couch is useful? S. Chiesa 1 , S. Menna 2 , A.R. Alitto 1 , L. Azario 2 , G.C. Mattiucci 1 , M. Ferro 3 , M. Massaccesi 1 , A. Re 1 , A. Piermattei 2 , V. Valentini 1 , M. Balducci 1 1 Università Cattolica del Sacro Cuore -Fondazione Policlinico Universitario A. Gemelli, Radiation Oncology Department- Gemelli-ART, Rome, Italy 2 Università Cattolica del Sacro Cuore - Fondazione Policlinico Universitario A. Gemelli, Medical Physics Department, Rome, Italy 3 Università Cattolica del Sacro Cuore -Fondazione di Ricerca e Cura “Giovanni Paolo II”, Radiation Oncology Department, Campobasso, Italy Purpose or Objective Setup accuracy and organ motion control are essential in stereotactic radiation therapy (SRT) due to the use of sharp dose gradients and tight margins around the target volume. PRO-ART Project was designed to evaluate set up errors and dosimetric impact of rotational patient positioning correction using a 6-Degree of Freedom (6DoF) robotic couch. Material and Methods Patients with lung, brain and abdominal lesions were enrolled and immobilized with Uni-frame or trUpoint Arch for brain, Breast board or Body Pro-Lok for chest and abdomen (CIVCO support system) lesions. Eclipse™ Treatment Planning Systems (Varian Medical System®,Palo Alto,CA) was used for dose calculations of VMAT plans. A daily KV-Cone Beam Computed Tomography (CBCT) was performed before each treatment fraction and translational and rotational shifts were identified, recorded and applied on the Protura TM Robotic couch to correct the position. Using MIM 5.5.2 software, the simulation CT was rigidly registered with CBCT, considered as primary CT, for each fraction. After