ESTRO 2023 - Abstract Book

S1650

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ESTRO 2023

Conclusion DIBH treatments with the ETD system combining SGRT and stereoscopic IGRT were feasible. The successful verification of the ETD X-ray positioning by standard IGRT techniques paved the way for clinical implementation. The cause of residual deviations – systematic differences between the imaging modalities or the limited reproducibility of the DIBH manoeuvre – remains subject for further investigations.

PO-1903 Intra-fractional target shift during stereotactic treatment of peripheral lung lesions

T.B. Nyeng 1 , S.N. Thomsen 1,2 , D.S. Møller 1,2 , R. Hansen 1 , A.A. Khalil 3 , L. Hoffmann 1,2

1 Aarhus University Hospital, Department of Oncology, Medical Physics, Aarhus, Denmark; 2 Aarhus University, Department of Clinical Medicine, Faculty of Health Sciences, Aarhus, Denmark; 3 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Lesions in the thorax may be prone to both respiratory motion and geometrical shifts during radiotherapy delivery. Treatment of small peripheral lung lesions with stereotactic radiotherapy (SRT) is delivered in very few fractions, making geometrical miss of the target in any fraction more severe. We investigated the intra-fraction target displacement and the impact of resting time after patient positioning. Materials and Methods Fifty-one lung lesions were treated with SRT of 45-67.5Gy in three fractions, with a planning target volume (PTV) margin of 4mm. Patients were positioned in the treatment room in an individualized immobilization device (End of positioning, Time: T0). Subsequently, a cone-beam CT (CBCT) image was acquired (Start of acquisition, Time: T_CBCT0), and the couch position was corrected using an online soft tissue match on the target to the planning CT (pCT). Hereafter, 7-8 static treatment fields were delivered and a post-treatment CBCT was acquired (Start of acquisition, Time: T_CBCT1). Retrospectively, CBCT0 and CBCT1 were matched to pCT on the target and subsequently, the vertebral column, and the resulting 3D vector shift of the target and the shift of the vertebra between CBCT0 and CBCT1 were calculated. Correlation between target and vertebra shifts was tested by calculating the Spearman’s rank correlation coefficient. T0 was identified in the treatment machine logs (TrueBeam 2.7) for each fraction of each patient (153 fractions in total) as the time for the final movement of the treatment couch to the planned setup-position. T_CBCT0 and T_CBCT1 were found in the treatment management system (Varian Aria 16.1). The resting time, defined as the time interval from T0 to T_CBCT0 was calculated. Results The median [range] target and vertebra shifts were 2.2mm [0.2;16.1] and 0.9mm [0;5.7], respectively. In 54 of the 153 fractions (in 30 of the 51 lesions), target shifts of more than 3mm were seen (see Figure 1), three of these above 8mm.