ESTRO 2024 - Abstract Book

S5544

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

Despite the limited number of observations, these preliminary results highlight the opportunity of introducing a 4D CBCT acquisition into the lung SBRT workflow. Anatomy-based 4D CBCT could provide a promising and more accurate motion assessment than conventional surrogate-based 4DCT. An underestimation of the ITV volume was shown in the 4DCT-based contouring. The main limitations of this study are the poor CBCT image quality, although not so limiting in the delineation of well-defined nodules, and the small amount of data. Further studies and more data will be needed to confirm these findings.

Keywords: Respiratory motion management, 4D CBCT, lung SBRT

1529

Digital Poster

Irradiation of the entire femur using IGRT with the AP/PA technique, while also increasing the SSD.

Lean L. Krikke, Tessa Harderwijk, Annelies J. Zoetelief, Kim Verhoef, Marlies H.E. de Vries, Simon J. Woodings

University Medical Center Utrecht, Radiation Oncology, Utrecht, Netherlands

Purpose/Objective:

In our department, palliative external beam irradiation of the entire femur is performed using an Anterior Posterior/Posterior-Anterior (AP/PA) technique with an increased Source-Skin Distance (SSD). The increased SSD ensures that the entire femur fits within the radiation beam. This technique previously prevented the use of Cone Beam Computer Tomography (CBCT) due to collision. The position was verified by a projection of the radiation beam on the patient's skin. Since January 2022, we have been using daily online CBCT for position verification. The radiation is delivered using two opposing beams in AP and PA directions (gantry angles 0° and 180°), each with a different isocenter that varies in height. This robust planning technique ensures the patient can receive treatment one day after the planning CT scan is acquired. The current treatment plan includes a third isocenter; a setup isocenter. The setup isocenter has no increased SSD, allowing the CBCT to be performed without collision. After the online correction, the irradiation follows, for which the treatment table needs to be shifted to the AP or PA isocenter. These shifts are automatically executed by Theraview/TSCA (Cablon Medical, Leusden, The Netherlands) using the Parent-Child feature.

The purpose of this retrospective study is to evaluate whether the irradiation with the AP/PA technique and increased SSD is more accurate with IGRT opposed to without IGRT.

Material/Methods:

All patients had their entire femur irradiated, PTV lengths ranging from 40 to 50 cm. The margin from the clinical target volume (CTV) to the planning target volume (PTV) was 8 mm, which was the same as before the implementation of IGRT. All matches were performed in the X-ray Volumetric Imaging system (5.0.7, Elekta AB, Stockholm, Sweden) using a clipbox and Bone Translations and Rotations registration method (chamfer algorithm), followed by an online