ESTRO 2023 - Abstract Book

S617

Monday 15 May 2023

ESTRO 2023

Conclusion Different upper extremity positioning in LF and HF MRI scanners allowed for reproducible setup scenarios in both scanners. Comparisons of LF and HF MRI scans with the same lying position and different arm orientations suggest that LF MRI can be further explored as an alternative to HF MRI. Comparisons of repositioned arm LF scans encourage applicability in patient setup verification. Therefore, the unique arm abducted 90° positioning in an open LF MRI scanner, which enables a free path for charged particle beams, would allow for in-beam MRI-guided PT of ESTS. Finally, HF scans comparisons advise patient positioning on the HF MRI scanner based on the most comfortable position. PD-0751 Experimental verification of accumulated dose of multifraction plan adaptations on the MR-linac M. van den Dobbelsteen 1 , S. Hackett 1 , B. van Asselen 1 , S. Oolbekkink 1 , J. Wolthaus 1 , W. de Vries 1 , B. Raaymakers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective During a MR-linac treatment, every fraction is planned and delivered on the daily patient anatomy. Currently, plan quality assurance of a MR-linac treatment focuses on verifying individual fractions instead of verifying series of adapted fractions. The goal of this research is to determine the accuracy of experimental accumulated doses of multifraction plan adaptations on the Unity MR-linac. To verify the total dose of a series of adapted plans, a phantom was modified during each fraction and multiple adapted plans were generated and irradiated. Materials and Methods Three modifications to the phantom were considered: translations, rotations and body modifications. For each modification a five fraction MR-linac treatment was planned and delivered. Measurements were performed on the QUASAR™ MRI ⁴ ᴰ Motion Phantom (Modus Medical Devices Inc., Canada), comprising an external body and two internal cylinders. One cylinder in the middle of the phantom contains a film insert with Gafchromic EBT3 film (Ashland Inc., USA) and the other (empty) cylinder on the side of the phantom represents the lung. A schematic view of the phantom is illustrated in Figure 1. The measured dose over the accumulated treatment was compared to the calculated dose. For each fraction the geometry of the phantom was modified, as shown in Table 1.