Study of upright patient positioning reproducibility in image-guided proton therapy for head and neck cancers.

Purpose: To evaluate the patient's positioning reproducibility during upright treatment with image-guided adaptive proton therapy (IGAPT) for head and neck cancers.

Materials And Methods: 10 head and neck (H&N) patients were treated with gantry-less IGAPT, which includes daily 3D computed tomography (CT) and two 2D kilovoltage radiographs before treatment and weekly 3DCT immediately after irradiation. All procedures were performed in the carbon chair on the 6 degrees of freedom robotic positioner.

Results: Prior to treatment we registered shifts in patient positioning using 3D/3D registration at the imaging isocenter: X  = -0.1 ± 3.9 (mean ± standard deviation) mm, Y = -3.7 ± 3.5 mm, Z = 0.5 ± 6.2 mm. The corresponding vector was applied to the robotic positioner to compensate for the registered shifts, after which the patients were moved to the treatment isocenter and the following shifts were obtained there using 2D/3D registration: X  = -0.31 ± 1.37 mm, Y = -0.02 ± 1.33 mm, Z = 0.59 ± 1.55 mm. Finally, the weekly follow-up 3D/3D registration showed X  = -0.2 ± 1.2 mm, Y = -0.0 ± 1.4 mm, Z = 2.3 ± 2.0 mm.

Conclusions: A novel image-guided gantry-less PT facility showed reliable results in terms of patient positioning for H&N cases during clinical trials. This fact confirmed the suitability of using gantry-less PT for H&N treatment. A small, systematic shift in the vertical direction was detected in the follow-up 3D/3D registration. The effect of this shift will be investigated in further studies with pre/post treatment 2D/3D registration. The next phase of the clinical trial of this facility is dedicated to the thorax region.