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 ( ) 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 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 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 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 . The effective measurement depth was determined to be . 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 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 was found to be suitable for use as a dosimetric tool in MRI-guided proton therapy.