ESTRO 2022 - Abstract Book

S16

Abstract book

ESTRO 2022

Online beam-gating had little dosimetric effect on coverage and dose to organs at risk (Figure 1). All but two patients would have received D 95% >100% with a 3 mm prostate-to-PTV margin instead of the clinical 5 mm margin (Figure 2). Patient 12 had the poorest prostate coverage in the initial plan and is indicated with a triangle in both figures.

Conclusion For the patients in our study online-beam gating had little dosimetric effect. For 18 out of 20 patients, the clinically used prostate-to-PTV margin of 5 mm could have been reduced to 3 mm while still retaining 95% coverage even without the online gating.

OC-0042 Hybrid 2D/4D MRI-guided MLC tracking on the Unity MR-linac

K. Keijnemans 1 , P. Borman 1 , P. Uijtewaal 1 , P. Woodhead 1,2 , B. Raaymakers 1 , M. Fast 1

1 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands; 2 Elekta AB, Elekta AB, Stockholm, Sweden Purpose or Objective MRI is useful to estimate respiratory motion in radiotherapy. Conventionally, motion estimation is done using either fast 2D imaging of a focal lesion or (retrospective) respiratory-correlated 4D acquisitions of the wider anatomy. Ideally, fast 2D imaging of the target for MLC tracking, and respiratory-correlated 4D imaging for dose accumulation could be performed