Abstract Book

S1068

ESTRO 37

abdominal compression. CT images were exported to the TPS Oncentra External Beam v.4.5 (Elekta, Crawley) to contour pulmonary lesions and mediastinal nodes as Gross Tumor Volume (GTV). A preliminary isocenter at the center of the GTV was defined to allow the 4D-CBCT acquisition on Elekta Agility Linear Accelerator. The patients underwent 4D-CBCT with Symmetry software that sorted in 10 breath phases. Subsequently, GTVs were delineated on all phases of 4D-CBCT to define Internal Target Volume (ITV). ITVs were expanded of 5 mm to define PTV. Definitive VMAT treatment planning were created; 4 patients received 50 Gy (10 Gy/fraction) and 2 patients received 35 Gy (7 Gy/fraction). During treatment course a total of 30 4D-CBCT were acquired. Translations were measured in medio-lateral (x), supero- inferior (y) and antero-posterior (z) directions, as well as in rotation around axes. Translational displacements ≥ 2 mm were corrected on-line. Cut-off rotation should be ≤ 3°; if rotation was > 3°, the patients were repositioned. Results ITV was obtained from union of GTVs delineated on all breath phases of 4D-CBCT (Table 1). The mean setup error was 0.18 cm (range 0.18-0.33) in lateral direction, 0.24 cm (range 0.12-0.46) in longitudinal direction, and 0.23 cm (0.08-0.53) in vertical direction. Mean values distribution of translation displacements for each patient were reported in Figure 1.

Conclusion Simplified contours for locally advanced HNC treatment planning could be created in minutes and resulted in clinically acceptable automated HNC treatment plans. This shows that it is possible to actively spare clinically relevant salivary and swallowing OARs without the need for numerous high-accuracy, time consuming delineations. Such strategies may help to reduce the resources needed to provide advanced HNC radiotherapy and aid clinical implementation, especially when combined with automated planning. EP-1964 Use of 4DCBCT Symmetry Elekta to evaluate organ motion and set-up error in lung SBRT. A pilot study. C. Di Carlo 1 , M. Nuzzo 1 , A. Allajbej 1 , R. Jimenez De Lorenzo 2 , M. Taraborrelli 1 , L. Caravatta 1 , M. Trignani 1 , M.D. Falco 2 , D. Genovesi 1 1 "SS. Annunziata" Hospital- "G. D'annunzio" University, Radiotherapy, Chieti, Italy 2 "SS. Annunziata" Hospital- "G. D'annunzio" University, Medical Physics, Chieti, Italy Purpose or Objective To evaluate organ motion and set up errors in patients treated with stereotactic radiotherapy for lung cancer or mediastinal lymphadenopathy, using 4D-Cone Beam CT (CBCT) (Symmetry Elekta X-Ray volume imaging system) both in treatment planning and delivery. Material and Methods From September 2016 to April 2017, 6 patients were selected to receive stereotactic radiotherapy to thoracic lesions. Patients characteristics were: mean age 75 years (range 68-85); M/F: 6/0; pulmonary nodules: 4 (66.7%), mediastinal lymphadenopathy: 2 (33.3%). Patients underwent a 3D simulation computed tomography (CT) without contrast, with 3 mm slice thickness and acquired from humeral heads to include the 12 th rib. A 4 fixation points thoracic thermoplastic mask was used with manual

Conclusion From our preliminary experience, acquisition of 4D-CBCT with Symmetry Elekta X-Ray software, seems to be a useful method for evaluating organ motion in thoracic stereotactic radiotherapy. It is certainly necessary to increase the number of patients in order to confirm these data and in the optics of PTV margin personalization.