Characterization of ionization chambers in magnetic fields for MR guided proton beam therapy
PH-0046
Abstract
Characterization of ionization chambers in magnetic fields for MR guided proton beam therapy
Authors: Fuchs|, Hermann(1)*[hermann.fuchs@meduniwien.ac.at];Padilla Cabal|, Fatima(1);Fetty|, Lukas(1);Georg|, Dietmar(1);Palmans|, Hugo(2);
(1)Medical University of Vienna, Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria;(2)MedAustron Iontherapy center & National Physical Laboratory, Medical Physics, Wr. Neustadt & London, Austria;
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Purpose or Objective
MR-linac systems have become commercially available, are currently clinically explored and provide promising possibilities for adaptive radiation therapy. In proton beam therapy, research towards the integration of an MR and a beam line has already started. In clinical practice, air-filled ionization chambers (IC) are the most important dosimetry systems. The necessity of magnetic-field related correction factors for ICs has been reported for MR-linacs. For proton beam therapy, the performance of ICs in magnetic fields has not yet been studied. This work characterizes the performance of ICs in proton beam therapy in the presence of external magnetic fields.
Material and Methods
A resistive dipole magnet with a pole gap of 12.5cm provided a magnetic field oriented orthogonal to the beam path. Magnetic field strengths were verified to be within 4 mT of the target value using a portable hall probe. The magnet was placed at the isocenter of a horizontal research beam line.
A thimble-type Farmer chamber (TM300013, 0.6mm³, PTW, Freiburg, Germany) and a plane-parallel Roos chamber (TM34001, 0.35cm³, PTW, Freiburg, Germany) were investigated. Read out was performed using a UNIDOS webline (T10021, PTW, Freiburg, Germany). The chambers’ reference points were positioned in PMMA slabs at water equivalent depths of 21.3 and 21.1mm for the Farmer and Roos chamber, respectively. Both chambers were oriented orthogonal to the beam direction; the Farmer chamber orthogonal, the Roos chamber in-line with respect to the magnetic field (see Fig. 1). Measurements used a scanned 10x8cm² field of 252.7MeV protons at three magnetic field strengths of 0, 0.5 and 1T, respectively. Cross-calibrations in terms of absorbed dose to water were performed without magnetic field following the IAEA TRS 398 protocol adapted to scanned proton beams. Data evaluation was done after correcting for temperature and pressure. Statistical evaluation was based on the unpaired Student’s T-test.
Figure 1: Sketch of the experimental set-up.
Results
For the Roos chamber virtually no difference were found between measurements at 0 and 1T (differences on average 0.01%, STDev 0.2). Chamber behavior at 0.5T showed on average differences (w.r.t. 0T) of -0.72% (STDev 0.29), which was statistically significant (p>0.001).
For the Farmer chamber, a similar behavior was observed. Measurements at 1T were not found to differ from measurements at 0T (differences on average 0.01%, STDev 0.26). Again, for 0.5T statistically significant differences (p>0.001) were found, with average deviations of -0.19% (STDev 0.25).
Conclusion
Differences in chamber readings were found to be magnetic field dependent. For 1T no noticeable influence of the magnetic field on the chamber reading was detected. However, the encountered differences were more pronounced for the intermediate magnetic field strength. In addition, the effect seems to be dependent on the chamber type. Especially for the plane-parallel Roos chamber the observed differences need to be corrected for.