ESTRO 2023 - Abstract Book

S567

Sunday 14 May 2023

ESTRO 2023

Conclusion The PETITION PET detector shows the capability to detect range shifts due to anatomical changes both during and following delivery of treatment fields. Low statistic in-beam images provide qualitative evidence of overdosage. Shifts in the reconstructed PET images are correlated to shifts in the dose due to the anatomical changes. Further work is being done to investigate the use of spot reduced planning and higher beam intensities to improve in-beam detection of anatomical changes. MO-0673 Experimental characterization of two novel MR-conditional ionization chambers in magnetic fields M. Schneider 1 , S. Frick 2 , M. Nachbar 1 , R. Kapsch 2 , D. Thorwarth 1 1 University Hospital and Medical Faculty, Eberhard Karls University Tuebingen, Department of Radiation Oncology, Section for Biomedical Physics, Tuebingen, Germany; 2 Physikalisch-Technische Bundesanstalt (PTB), 6.21 Dosimetry for radiotherapy, Braunschweig, Germany Purpose or Objective The availability of real-time MRI allows for online motion management in MRgRT. So far, ionization chambers generated artifacts in MRI, potentially irritating gating algorithms and obstructing the use of MRI for dosimetry and QA of MRgRT. In this work we investigated the dosimetric characteristics of two novel ionization chambers, Semiflex3D-MR (PTW31024) and PinPoint3D-MR (PTW31025), specifically developed for artifact-free usage in MR-Linacs. and beam qualities, measurements were done in a water phantom using a standard linac (Elekta Precise) with an electromagnet insert. The ionization chambers were placed in 10 cm water equivalent depth, with their chamber axis perpendicular to the magnetic field. One response curve per chamber type was acquired once for 6 MV, with . The correlation of chamber response and beam quality was determined for 6, 10 and 15 MV and = 0.35 and 1.5 T. Those measurements were repeated three times. Materials and Methods To investigate the chamber readings for various magnetic flux densities

Magnetic field correction factors of each type for 6 MV and 0.35/1.5 T.

were assessed by doing five independent measurements for five chambers

The influence of rotating the chamber axis was evaluated by deriving two chambers of each type at the 1.5 T MR-Linac (Elekta Unity).

, which was measured five times for

Results Both chamber types showed a continuous response curve with decreasing response for increasing absolute magnetic flux densities. The decrease of response is higher for both chambers, if the Lorentz force points to the tip of the chamber (fig.1). Both types of ionization chambers showed a linear dependency on the beam quality (R2 > 0.99 for = 0.35/1.5 T). The five Semiflex3DMR chambers showed consistent results with mean = 1.0470±0.0014 (STD) and = 1.0000±0.0005 for 1.5 and 0.35 T respectively. The PinPointMR chambers showed increased deviations (fig.2), was observed as 1.0427±0.0051 (1.5T) and 1.0043±0.0022 (0.35T). At the 1.5 T MR-Linac, crot was determined as 0.9412±0.0019 (PTW31024) and 0.9600±0.0023 (PTW31025). Using , crot, the linear interpolation between beam qualities and from literature [1], was determined to be 1.051±0.004/0.989±0.005 for PTW31024 and 1.050±0.007/1.008±0.007 for PTW31025 in perpendicular, respectively anti-parallel orientation.

Figure 1: Effect of variating magnetic flux densities on the chamber readings.