Commissioning and initial experience with the first clinical gantry-mounted proton therapy system

Abstract

The purpose of this study is to describe the comprehensive commissioning process and initial clinical experience of the Mevion S250 proton therapy system, a gantry-mounted, single-room proton therapy platform clinically implemented in the S. Lee Kling Proton Therapy Center at Barnes-Jewish Hospital in St. Louis, MO, USA. The Mevion S250 system integrates a compact synchrocyclotron with a C-inner gantry, an image guidance system and a 6D robotic couch into a beam delivery platform. We present our commissioning process and initial clinical experience, including i) CT calibration; ii) beam data acquisition and machine characteristics; iii) dosimetric commissioning of the treatment planning system; iv) validation through the Imaging and Radiation Oncology Core credentialing process, including irradiations on the spine, prostate, brain, and lung phantoms; v) evaluation of localization accuracy of the image guidance system; and vi) initial clinical experience. Clinically, the system operates well and has provided an excellent platform for the treatment of diseases with protons.

Figure 1

The dimensions of the moving parts that hold the chamber holder. There is a pair of the moving parts on both sides of the water tank. The dashed line indicates the water surface in our setup.

Figure 2

Variations in the stopping power ratios (a) of the CIRS phantom from four proton institutes. The error bars indicated the absolute range of the variations and the digits under the error bars were the percentage variations with respect to the mean values. (b) CT calibration from our institute (red) plotted against the prediction from IROC. (c) CT calibration generated with historic annual QA data from 2008 to 2013.

Figure 3

Measurements were plotted against fitting for (a) effective SAD; (b) effective source size; (c) virtual SAD.

Figure 4

Depth‐dose curves (a) of all 24 options; (b) widths of the pristine Bragg peaks plotted against fitting; (c) distal penumbra plotted against fitting.

Figure 5

Examples of SOBP measurement plotted against TPS modeling for options 1, 13, and 18; each are options with largest range in large, deep, and small groups.

Figure 6

Comparison of crossline profiles of divergent aperture, nondivergent aperture, and treatment planning at various depths. Noticeable differences were observed along the field edges, both inside and outside of the fields. The differences vanished with increased depth in water.

Figure 7

Angular dependency of output in large (red), deep (green), and small (blue) options.

Figure 8

Distribution of discrepancies between our MU prediction model and measurements.

Figure 9

The heterogeneous phantom (a) used for validation of dose distribution. The thickness of the bone slab was 2 cm and the stopping power ratio was 1.63. The measured crossline profile (b) was plotted against prediction from TPS. The maximum discrepancy was measured 4.7%.

Figure 10

Differences between Verity applied 6D couch corrections and physically measured (a) translations and (b) rotations; results show accuracy is less than 1 mm and 0.2°. Differences between Verity suggested shifts/rotations and known CT (c) translations and (d) rotations; results show accuracy is less than 1 mm and 0.2°.

Figure 11

Dedicated star shot phantom kV radiographs (left) and resulting radiochromic couch star shot used for calculating radiation isocenter precision ($<1\hspace{0.17em}\text{mm}$) and distance between imaging and radiation isocenters ($<1\hspace{0.17em}\text{mm}$).