Considerations for Upright Particle Therapy Patient Positioning and Associated Image Guidance

Abstract

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.

Keywords: image guidance; particle therapy; seated treatment; treatment chair; upright CT; upright treatment.

Figure 1

Overview over different chair designs targeted for particle therapy found in literature, focusing on those that were constructed as prototype. The color of the time line connectors indicates the chair’s intended use with respect to different treatment sites, as shown on the left. ^aThe chair was installed at the Indiana University Health Proton Therapy Center prior to 2006, as described by Schreuder (30). ^bThe device was installed at the Oklahoma Proton Center as described in (30). The figure shows the couch overlayed on the chair. ^cThe chair system is installed at Northwestern Medicine Chicago Proton Center and was designed by P-Cure¹. Image reprinted with kind permission by Dr. M. Pankuch (Northwestern Medicine Chicago Proton Center). Images ^a,^b, and the Leo Cancer Care Ltd (2021). were reprinted with kind permission by Dr. N. Schreuder (Leo Cancer Care Ltd.). Kamada et al. (1999): Reprinted from Kamada et al. (5) with permission from Elsevier. Buchner et al. (2020): ^©2020 IEEE. Reprinted, with permission, from Buchner et al. (35).

Figure 2

Overview over the required vertical ranges (not to scale) of adjustment for different treatment sites for European adults, computed as the difference between 5 and 95 percentiles from anthropomorphic data in (49). An important constraint for an upright positioning device targeted for a fixed beam line is the fixed height of the isocenter. For pelvis treatments, the patient head can reach up to ~1m above the isocenter.

Figure 3

Schematic overview over different postures. A straight upright posture [e.g (42)., and (31)] has been noted to induce stress on chin (for head&neck patients) and may be uncomfortable for long treatment duration, as indicated by the red areas. More ideal would be ~20° reclined position, where the forward push on the pelvis could be stabilized by a race-car seat posture [e.g (9)], or by adding a knee fixation [e.g (8)] or a belt strap [e.g (35)]. Alternative postures could be forward leaning for head&neck or possibly spine irradiations [e.g (26)] or half-standing, enabling to image and treat sites below the thorax [e.g (8)].