Effects of ionization chamber orientation in photon and neutron beams

Boron neutron capture therapy (BNCT) is a biologically targeted radiotherapy modality that utilizes a neutron beam. Ionization chambers are used in neutron beam dosimetry which is part of the commissioning process and periodic quality assurance. Ionization chambers used in BNCT are a bit larger in size compared to conventional external beam radiotherapy counterparts and a fixed horizontal beam direction poses its challenges among other considerations. Because of this, more flexible ionization chamber positioning would be preferred even in a large water phantom. In this work, we explain the effects and uncertainties that depend on chamber orientation. We compared depth dose curves measured with Exradin T2 and M2 ion chambers with two radiation sources, 6X photon and epithermal neutron beam, and in two orientations; one where the ionization chamber rod was positioned in perpendicular and the other where it was positioned in parallel orientation compared to the beam central axis. The perpendicular orientation is the reference orientation used in calibration. However, the parallel orientation allowed us to measure depth dose closer to the surface and the measurements had smaller systematic uncertainties because of a more straightforward alignment process. We can convert the data between the two different orientations with two simple correction parameters; correction factor which adjusted amplitude (k) and shift which adjusted the change of the effective point position in depth direction (Δ). All correction factor results were within 1.000±0.015 and all shift results were smaller or the same order of magnitude compared to the positioning accuracy, 0.05 mm - 0.5 mm. Even without the usage of corrections, the measurements were already almost in agreement. Because of this, we would recommend taking into consideration 2 % additional uncertainty if measurements are performed in parallel orientation and no such correction parameters are used.