Characterization of a Commercial Ionization Chamber Array With Scanned Proton Beams for Applications in MRI-Guided Proton Therapy.

Background: The integration of MRI-guidance and proton therapy is a current research topic. Proton therapy with the patient being placed inside an in-beam MR scanner would require the presence of its static magnetic ( $B_0$ ) field to be taken into account in dose calculation and treatment planning. Therefore, dosimetric tools are needed to characterize dose distributions in presence of the $B_0$ field of the MR scanner. Furthermore, patient-specific quality assurance (QA) and treatment plan verification measurements should also be performed within the magnetic field.

Purpose: In this work, the PTW Octavius 1500 ${}^{MR}$ ionization chamber array was characterized experimentally and tested for its suitability as a dosimetric tool for beam characterization and QA in MRI-guided proton therapy.

Methods: The dose rate response, response homogeneity and effective measurement depth of the detector were determined in experiments with scanned proton beams delivered by a horizontal beamline at OncoRay, Dresden. A patient-specific QA test including gamma analysis was performed for a realistic proton patient treatment plan at two different distances from the beam nozzle. In addition, experiments were performed in a $0.32T$ in-beam MR scanner. These included measurements of square reference scanning patterns at different proton energies as well as measurements of a two-field patient treatment plan at different water equivalent depths.

Results: The dose rate response was found to be linear up to $80\text{Gy/min}$ . The effective measurement depth was determined to be $8.1\pm 0.2\mathrm{mm}$ . The response homogeneity was found to be suitable for the verification of proton treatment plans. The patient-specific QA test without magnetic field was satisfactory and also the measurements inside the $0.32T$ in-beam MR scanner provided reasonable results. Their comparison allowed an assessment of the magnetic field effects on the dose distributions.

Conclusions: Concluding from these tests, the Octavius 1500 ${}^{MR}$ was found to be suitable for use as a dosimetric tool in MRI-guided proton therapy.