ESTRO 2021 Abstract Book

S1487

ESTRO 2021

PO-1761 Exactrac X-ray imaging during stereotactic radiotherapy of bone metastases. Comparison with kV-CBCT A. Hadj Henni 1 , S. THUREAU 1 , E. COLARD 1 , G. BULOT 1 , M. ROGE 1 , C. HANZEN 1 , D. GENSANNE 1 1 Centre Henri Becquerel, Radiotherapy, Rouen, France Purpose or Objective The objective of this study was to investigate the possibility of using the 2D-kV Exactrac X-ray for 6 degree-of- freedom (6D) image guidance in stereotactic body radiation therapy (SBRT) of bone metastases and to propose a patient management procedure that takes the results into account. Materials and Methods The analyses were obtained from measurements on a pelvic phantom and on 19 patients treated for bones metastasis SBRT. The phantom study consisted of applying known offsets in the three translations (1, 2, 5, and 10mm) and evaluating the Exactrac X-ray level of accuracy by comparing with the CBCT. The impact of the digitally reconstructed radiographs (DRRs) slice thickness (0.625, 1.25, 2.5, and 5mm) was also investigated. Two groups of patients, 10 spinal and 9 non-spinal SBRT cases, were analyzed to evaluate the feasibility of Exactrac X-ray imaging for the 6D setup of different bone localisations. A comparison was made between the Exactrac X-ray verification and CBCT acquisition prior to the treatment fraction. During the treatment 2 others Exactrac X-ray/CBCT images pairs were also acquired in the same order. A total of 224 6D shifts given by the 2 imaging systems were compared. Results In the phantom study the root mean square (RMS) of the translational and rotational discrepancies between Exactrac X-ray and CBCT were < 0.6mm and < 0.4° respectively. The impact of the DRRs slice thickness was minimal in translation with a difference between the different configurations < 0.2mm. The differences were greater on Yaw (Ry) rotation with up to 0.9° between 1.25mm and 5mm configurations. For both groups of patients the RMS of the setup discrepancies observed between the imaging systems were greater than the phantom experiment while remaining < 1mm and < 0.7°. In terms of frequency, for the spinal group 99% of the discrepancies between Exactrac X-ray and CBCT are ≤ 2mm and ≤ 2°. 85% and 93% of the differences are ≤ 1mm and ≤ 1° respectively. For the non-spinal group, 98% of the discrepancies are ≤ 2mm and ≤ 2° and 83% and 79% are ≤ 1mm and ≤ 1° respectively. In the non-spinal group, for three patients (2 scapulas, 1 humerus) the Exactrac X-ray could not provide consistent shift values due to a lack of anatomical information and were therefore excluded from the comparison study. Conclusion The results obtained validate the use of Exactrac X-ray for 6D intrafraction image guidance during a bone metastasis stereotactic treatment in our clinical routine. These changes in our practices have enabled us to reduce the treatment time per fraction by 30 to 40%. However a threshold must be established, depending of the PTV margin, beyond which the setup corrections proposed by the Exactrac X-ray requires verification with a CBCT providing a 3D patient´s imaging with wider anatomical information. The registration of bone lesions