ESTRO 2021 Abstract Book

S1676

ESTRO 2021

A correlation between the reduction of the CUS volume (ΔV) and its mobility (∆GC-Fx) was also found (R= 0.71). The mean CTV coverage of 95% for all the considered fractions was obtained with a CTV-PTV margin of at least 15mm.

Conclusion CUS mobility is not regular throughout the treatment and 15 mm CTV-PTV margins are required for appropriate dose coverage. The use of adaptive strategies may allow to tailor individualised margins, offering potential solutions to maximise CTV coverage while minimising OAR dose thanks to better therapy volumes visualization provided by on-board MRI. PO-1970 4D-CBCT (Symmetry Elekta X-ray) for ITV generation in thoracic stereotactic radiotherapy S. Marcucci 1 , L. Caravatta 1 , C. Rosa 1 , M. Nuzzo 1 , M. Di Tommaso 1 , M. Taraborrelli 1 , C. Di Carlo 1 , F.C. Di Guglielmo 1 , L. Gasparini 1 , N. Adorante 1 , D. Genovesi 1,2 1 SS. Annunziata Hospital, Department of Radiation Oncology, Chieti, Italy; 2 G. D’Annunzio University, Department of Neuroscience, Imaging and Clinical Sciences, Chieti, Italy Purpose or Objective In order to evaluate organ motion, the use of 4D-Computed Tomography simulation is suggested in stereotactic radiotherapy for thoracic lesions. This study aimed to assess the capability of a 4D-Cone Beam Computed Tomography (CBCT) (Symmetry Elekta X-ray volume imaging system) to generate Internal Target Volume (ITV) and evaluate set-up errors. Materials and Methods Twenty-two lesions in 21 patients were evaluated: 20 pulmonary nodes and 2 lymphadenopathies. Patients underwent a simulation-CT, with 2 mm slice thickness. For immobilization the Body Pro-lok TM ONE (CIVCO) was applied in all patients and abdominal compression was performed in 16 patients (11 lesions: ONEBRIGE, 5: thoracic thermoplastic mask, 1: Respiratory Belt). Five patients were not fit to receive an abdominal compression. Thoracic lesions were delineated on CT images as Gross Tumor Volume (GTV). A preliminary plan with isocenter at the center of the GTV was made to allow the 4D-CBCT acquisition on Elekta Agility Linear Accelerator. Before treatment planning, the patients underwent 4D-CBCT with Simmetry software. Ten-phase images were reconstructed from the 4D-CBCT. The GTV was contoured on each phase and an ITV was generated. ITV was expanded of 3-4 mm to generate Planning Target Volume (PTV). VMAT treatment planning were created. According to site and size lesions, patients were treated with total doses of 35-50 Gy (5-10 Gy/fraction). Prior to each treatment fraction, a Symmetry 4D-CBCT was acquired with dual-image registration (clipbox followed by a mask registration) for bone (set-up error) and soft-tissue (organ motion) evaluation. Translations were measured in medio-lateral (x), supero-inferior (y) and antero-posterior (z) directions, as well as 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. Mean (M) and standard deviation (SD) of the 4D-CBCT displacements were calculated (figure 1).