Purpose: To evaluate the mechanical accuracy and the robustness of position alignment under x-ray-based image guidance of a treatment chair with six degrees of freedom (6DTC) which was developed for patient treatment in an upright posture at fixed horizontal beam lines in particle (proton, carbon ion, or others) radiotherapy facilities. Method and Material:The positional accuracy including translational and axial rotational accuracy of the 6DTC was evaluated by using a Vicon Motion Capture System (VMCS). Stability of the chair rotation isocenter was determined by a CCD camera with an in-house developed software. The tests were carried out to examine two key motion components of the 6DTC: a floor/rail-mount 360 • -rotating platform and a 6-degree-of-freedom (6DOF) platform. The measurement results were compared to that of a commercial clinical robot couch. The accuracy of position alignment, simulating the actual clinical protocol, through an Image-guided Radiation Therapy (IGRT) system was studied at the pre-treatment position and beam specific treatment position. Results:The translational accuracy was 0.12 mm (SD 0.07 mm) for the 6DOF platform. The rotational accuracy was 0.04 • (SD 0.03 • ) and 0.02 • (SD 0.02 • ) for the 6DOF platform and the 360 • -rotating platform, respectively. The displacement between the chair rotation center and the room isocenter center was no more than 0.18 mm in all three rotational axes. Combined with an x-ray-based IGRT system, the treatment alignment test with a rigid phantom yielded a total positional accuracy of 0.23 mm (SD 0.17 mm) and 0.14 • (SD 0.14 • ) at treatment position. Conclusions:On the basis of the rigid phantom study, the 6DTC showed comparable accuracy to the robot treatment couch. Combining with the IGRT, the 6DTC can provide position alignment with submillimeter accuracy for rigid phantom in upright posture.

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Particle therapy is a rapidly growing field in cancer therapy. Worldwide, over 100 centers are in operation, and more are currently in construction phase. The interest in particle therapy is founded in the superior target dose conformity and healthy tissue sparing achievable through the particles’ inverse depth dose profile. This physical advantage is, however, opposed by increased complexity and cost of particle therapy facilities. Particle therapy, especially with heavier ions, requires large and costly equipment to accelerate the particles to the desired treatment energy and steer the beam to the patient. A significant portion of the cost for a treatment facility is attributed to the gantry, used to enable different beam angles around the patient for optimal healthy tissue sparing. Instead of a gantry, a rotating chair positioning system paired with a fixed horizontal beam line presents a suitable cost-efficient alternative. Chair systems have been used already at the advent of particle therapy, but were soon dismissed due to increased setup uncertainty associated with the upright position stemming from the lack of dedicated image guidance systems. Recently, treatment chairs gained renewed interest due to the improvement in beam delivery, commercial availability of vertical patient CT imaging and improved image guidance systems to mitigate the problem of anatomical motion in seated treatments. In this review, economical and clinical reasons for an upright patient positioning system are discussed. Existing designs targeted for particle therapy are reviewed, and conclusions are drawn on the design and construction of chair systems and associated image guidance. Finally, the different aspects from literature are channeled into recommendations for potential upright treatment layouts, both for retrofitting and new facilities.

PurposeAn 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.MethodsThe 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.ResultsTo 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.ConclusionsThe 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.

PurposeTo verify the practicality and safety of a treatment chair with six degrees of freedom (6DTC) through demonstrating the efficacy of the workflow in clinical settings and analyzing the obtained technical data, including intra-fraction patient movement during the use of the 6DTC.Materials and MethodsA clinical study was designed and conducted to test the clinical treatment workflow and the safety of the 6DTC. Based on the demonstrated dosimetric advantages, fifteen patients with head and neck tumors were selected and treated with the 6DTC. The positional error at the first beam position (PE-B1) and the second beam position (PE-B2) were analyzed and compared with the results from daily quality assurance (QA) procedures of the 6DTC and imaging system performed each day before clinical treatment. The intra-fraction patient movement was derived from the total patient alignment positional error and the QA data based on a Gaussian distribution formulism.ResultsThe QA results showed sub-millimeter mechanical accuracy of the 6DTC over the course of the clinical study. For 150 patient treatment fractions, the mean deviations between PE-B1 and PE-B2 were 0.13mm (SD 0.88mm), 0.25mm (SD 1.17mm), -0.57mm (SD 0.85mm), 0.02° (SD 0.35°), 0.00° (SD 0.37°), and -0.02° (SD 0.37°) in the x, y, z (translational), and u, v, w (rotational) directions, respectively. The calculated intra-fraction patient movement was -0.08mm (SD 0.56mm), 0.71mm (SD 1.12mm), -0.52mm (SD 0.84mm), 0.10° (SD 0.32°), 0.09° (SD 0.36°), and -0.04° (SD 0.36°) in the x, y, z, u, v, w directions, respectively.ConclusionsThe performance stability of the 6DTC was satisfactory. The position accuracy and intra-fraction patient movement in an upright posture with the 6DTC were verified and found adequate for clinical implementation.