Development of an isocentric rotating chair positioner to treat patients of head and neck cancer at upright seated position with multiple nonplanar fields in a fixed carbon-ion beamline.

Purpose: An isocentric rotating chair for a positioner was developed as a nongantry solution to provide multiple nonplanar radiation fields with a maximum tilt of 20 for treating head and neck cancer patients at an upright seated position in a fixed carbon-ion beamline.

Methods: The preclinical validation of the chair was present for this study funded by a grant through the Shanghai Proton and Heavy Ion Center (SPHIC) in Shanghai, China. The chair was installed in SPHIC. A concept of parallel kinematic was adopted to build the chair. Three movement subunits of the chair are a Stewart hexapod platform and two modules for three-dimensional translation and 360 rotation. This chair can position patients with a tilt up to 20 over a continuous 360 rotation. Any weak structures within each subunit were investigated by industrial static/dynamic simulations of used materials. After manufactured subunits were assembled in a factory, a series of executed six degree-of-freedom (DoF) displacements were measured by using a laser-based dynamic tracking system (LDTS) for the initial validation. Deviations between measured and required displacements, referred to as displacement deviation, were used to evaluate the displacement accuracy of the chair. After satisfying the initial validation in the factory, the chair was disassembled and installed in our treatment room. The displacement accuracy of the chair was revalidated by using the LDTS. Then, an integration validation of the chair was conducted to position a head phantom by using our image-guided radiotherapy (IGRT) system. Because the positioning accuracy of our IGRT system achieved a clinical tolerance of 1.0 mm and 1.0 only for a pitch/roll of <5 , the integration validation was conducted on 36 planned fields with a 5 tilt evenly over 360 rotation.

Results: To fulfill the general purpose of positioner, the chair allows the execution of any displacement over a cubic treatment volume with a length of 500 mm. Materials selected by simulations met required strengths under all circumstances of the clinical usage. The displacement accuracy of the chair satisfied the tolerance of 0.3 mm in-translation and 0.3 in-rotation during the initial validation in the factory. After the chair was installed in our institute, a linear displacement deviation of +/-0.6 mm was observed over +/-200 mm displacements in horizontal X/Y axes. After correcting the linear deviation, the displacement deviations of the chair for horizontal and vertical X/Y/Z axes were within 0.5 mm and 0.5 for its revalidation. During the integration validation, the displacement deviation of the chair was 0.8 mm and 0.6 when positioning a head phantom for the 36 fields with a 5 tilt.

Conclusions: The chair achieved the required clinical tolerance for the clinical application. The tilt angle was limited to within 5 to treat patients through a specific treatment workflow with a proper daily quality assurance program during a clinical trial, started in May 2019. An integration validation with a 20 tilt will be conducted in the near future to realize the full potential of the isocentric rotating chair.