Estimation of shifts of therapeutic carbon-ion beams owing to cavities in a polyethylene target by measuring prompt X-ray images

We evaluated an estimation ability of shifts of therapeutic carbon-ion beams owing to cavities in a polyethylene target by measuring prompt X-rays emitted from beam trajectories. Carbon-12 beams having the energy of 241.5 MeV u−1 were irradiated on a polyethylene target. The target had a square-prism-shaped cavity in it. The thickness of the cavity was changed from 3.0 to 0.0 cm with 0.3 cm steps. For each setup of the cavity, 7.5 × 1010 carbon ions were irradiated. A pinhole-type X-ray camera was placed beside the target and utilized to acquire the beam images. The beam trajectory and a gap on the trajectory clearly appeared in the acquired images. The actual beam shifts well coincided with the estimated beam shifts from the acquired images. The maximum fluctuation of the estimated shifts was approximately 0.2 cm. It was confirmed that the internal cavity can be imaged and the range can be accurately evaluated.