ESTRO 2023 - Abstract Book

S1646

Digital Posters

ESTRO 2023

Conclusion Stomach position and volume are stable throughout a 30 minute duration in fasted patients imaged with abdominal compression. Some variation is observed in the intra-fraction gastric gas and gastric contents volume. Further studies are required to assess the difference in dosimetry at each timepoint with respect to the volume and COM position changes. The minimal intra-fraction stomach variation observed in this study is unlikely to affect the dose received to the tumour and the OARs.

PO-1900 Is the PTV redundant in high dose MR-guided and beam-gated SBRT of liver metastases?

I. Wahlstedt 1,2,3 , E. van der Bijl 4 , K. Boye 2 , S. Ehrbar 5 , M. van Overeem Felter 3 , T.M. Janssen 6 , S.L. Risumlund 2 , J.E. van Timmeren 4 , I.R. Vogelius 2,7 , C.P. Behrens 3,1 1 Technical University of Denmark, Department of Health Technology, Kongens Lyngby, Denmark; 2 Copenhagen University Hospital - Rigshospitalet, Department of Oncology, Copenhagen, Denmark; 3 Copenhagen University Hospital - Herlev and Gentofte, Department of Oncology, Herlev, Denmark; 4 Radboud University Nijmegen Medical Center, Department of Radiation Oncology, Nijmegen, The Netherlands; 5 University Hospital of Zurich, Department of Radiation Oncology, Zürich, Switzerland; 6 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands; 7 University of Copenhagen, Department of Health and Medical Sciences, Copenhagen, Denmark Purpose or Objective Magnetic resonance-guided radiotherapy (MRgRT) with automated beam-gating interrupts the beam whenever the target moves outside a gating window. We used an empirical (data-driven) approach for finding a planning target volume (PTV) margin ensuring adequate dose to the gross tumor volume (GTV) in stereotactic body radiotherapy (SBRT) of liver metastases in beam-gated MRgRT. Our results were compared to conventional analytical PTV margin calculations. Materials and Methods We included 19 consecutive SBRT treatments of liver metastases in inspiration breathhold in three (n=13) or five (n=6) fractions with the GTV (or a structure overlapping the GTV) as the beam-gating target. Only the first three fractions were analyzed for the five fraction treatments. We analyzed the sagittal cine images recorded during treatment (4 or 8 Hz) with an inhouse script. This script extracted labels (“beam on” or “beam off”) as well as 2D sagittal motion traces representing the center of mass of the GTV relative to the gating window. We re-planned each clinical treatment plan both without a PTV and with six anisotropic GTV-to-PTV margins. We prescribed 3x22.5 Gy to the GTV and all plans were normalized to GTV D99%=95% and planned with GTV Dmax<140% and PTV D99%>67% according to local guidelines for SBRT of colorectal liver metastases. Subsequently, we rigidly transformed GTV and healthy liver in the planned dose matrices according to the motion traces and accumulated dose per voxel during beam-on (motion-compensated dose) using a script that has been validated with film dosimetry. We compared the motion-compensated delivered GTV dose to biologically equivalent dose ( / =10 Gy), BED10, of 117 Gy (equivalent to 46.11 Gy in three fractions) which has been associated with 90% 1 year local control in SBRT of colorectal liver metastases. Inter- and intrafraction random and systematic errors were extracted for a conventional margin calculation. In this calculation we compensated for planned dose inhomogeneity and the distance between the 95% isodose prescribed to the GTV and the 67% isodose used for evaluation. Results The applied PTV margin had little impact on motion-compensated GTV dose and all combinations of motion traces and dose plans yielded a BED 10 >117 Gy to the GTV (Figure 1). This was consistent with the margin calculation which resulted in negative margins of -1.8 mm (superior-inferior) and -0.6 mm (anterior-posterior). Margins were negative due to the distance of 3.9 mm between the 95% and 67% isodose. Plans without PTVs yielded an increase in healthy liver V <15 Gy of median (1st quartile, 3rd quartile) [cc] 70 (48, 99) compared to plans with the largest PTV (Figure 2).