Modeling and Validation of a Proton Therapy Couch Top

Abstract

Purpose: This paper describes the modeling of the kVue One Proton couch top (CQ Medical, Avondale, PA) for use in treatment planning system (TPS), and the verification of the calculated effect of the model in the TPS against measured radiological properties.

Material and methods: The kVue One Proton couch top consists of a higher-density shell surrounding a lower-density core. The radiological properties of the kVue One Proton couch top, as assessed by range pullback of direct and oblique incidence proton beams, were measured and used to evaluate different density overrides in a model of the couch top base created in the TPS, RayStation 12ASP1 (RaySearch Laboratories, Stockholm, Sweden). A similar approach was followed for MCsquare implementation.

Results: The average value (of water equivalent thickness) for all water tank measurements in lateral and longitudinal positions in the uniform area of 2 protons kVue One Proton couch top (except the couch ramp) was 6.46 mm. The posterior HU profiles showed a ramp area more extended longitudinally than the 7 cm physical shape of the kVue One Proton Couch Top. This was confirmed with differences in WET measurements and HU profiles observed up to 14 cm inferiorly from the superior part of the couch top, the remaining area being uniform in WET.

Conclusion: A 0.1 g/cm³ carbon fiber density for both the shell and core showed a maximum difference of 1.5 mm between measurements with a direct beam and the treatment planning model, over the full length of the kVue One Proton Couch Top. Similar results were obtained using oblique incidence proton beams.

Keywords: Commissioning; Couch top; Proton therapy.

Figure 1

CQ MEDICAL kVue One Proton couch top shell thicknesses: red: 1.1 mm, green: 1.5 mm, blue: 2.7 mm, yellow 1.8 mm.

Figure 2

PTW water tank and Bragg peak chambers BPC34070-SN324 and BPC34080-SN137 positioned on the Varian Multiroom ProBeam360° rotating floor (left), IBA-dosimetry multilayer ion chamber device Zebra with a rotating device between the Zebra and an en-face beam (right).

Figure 3

Left-right (A), postero-anterior (B), and supero-inferior (longitudinal) HU profiles of the posterior (C) and anterior (D) carbon fiber shell, acquired in the middle of the kVue One Proton couch top.

Figure 4

Difference in WET introduced by the kVue One Proton couch top measured by using PTW water tank and Bragg peak chambers, in the Ohio State Varian Multiroom ProBeam360⁰ (Gantry 1-water tank [G1], Gantry 2-water tank [G2]), measured with MLIC (MLIC-G2) and calculated in TPS for 2 material/density shell/core combinations. The vertical green line represents the physical ramp limit. Abbreviations: MLIC, multilayer ionization chamber; WET, water equivalent thickness.

Figure 5

Water equivalent thickness (WET) of a 1-piece (left) and 2-pieces (right) couch top model calculated by the treatment planning system (TPS) and compared to the measured WET (displayed in red).