Retrospective analysis of treatment-positioning accuracy and dose error in boron neutron capture therapy using a sitting-position treatment system for head and neck cancer

The neutron beam in boron neutron capture therapy (BNCT) exhibits poor directionality and significantly decreasing neutron flux with increasing distance. Therefore, the treatment site must be close to the irradiation aperture. Some patients with head and neck cancer may benefit from a sitting-position setup. The study aim was to evaluate the treatment-positioning accuracy and dose error in sitting patients receiving BNCT. Thirty-two patients with head and neck cancer who underwent sitting-position BNCT at Southern Tohoku BNCT Research Center were included in the study. Horizontal (Delta X) and vertical (Delta Y) errors were defined as the displacement between the treatment planning system (TPS) digital reconstructed radiograph and the pre-treatment X-ray image. Using in-house software, image matching was performed. The beam-axial directional (Delta Z) error was compared with the parameters entered into the TPS and the actual pre-treatment measured values. The translational-position error was reflected in the TPS's patient coordinate system with respect to the reference plan. Re-dose calculations were performed to evaluate the effect of positional error on tumor and normal-tissue doses. The [Delta X, Delta Y, Delta Z] (DRR-CR) mean +/- 1SD were - 0.40 +/- 2.0, 0.30 +/- 2.3, and - 1.4 +/- 1.5 mm, respectively. The D-mean and D-98% tumor-dose errors were 1.22 % +/- 1.44 % and 0.99 % +/- 1.63 %, respectively. The D-2% pharyngeal and oral mucosal-dose errors were 0.98 % +/- 1.91 % and 1.21 % +/- 1.78 %, respectively. The tumor- and normal-tissue dose errors were typically < 5 %. High-precision treatment was feasible in sitting-positioned BNCT.