This study used a self-contained industrial orthovoltage X-ray system (output voltage rage 32–320 kV, tube current range 3–30 mA; Philips Medical Systems, Best, Netherlands) that is generally used for small animal radiotherapy. The unit has several applicator cones, a field size of φ4.0 cm and a treatment distance of 50 cm.
Monte Carlo simulation for orthovoltage radiotherapy
Monte Carlo orthovoltage dose distributions were simulated with EGSnrc-based BEAMnrc and DOSXYZnrc codes using MATLAB from the NRCC group [14, 15]. The orthovoltage apparatus consists of a target, X-ray tube window, primary collimator, and mirror. The geometric dimensions and materials of each component module were provided by vendors and obtained from reference tables [16, 17]. The incident electron energy was set to 300 kV, with an added filtration of 2.0 mm aluminum and 0.5 mm copper. The percentage dose depth was measured at 10, 20, 30, 40, 50 and 80 mm with standard PTW 30013 0.6 cc waterproof Farmer ionization chambers (PTW, Freiburg, Germany) in a WP1D water phantom (IBA Dosimetry, Schwarzenbruck, Germany), and these physical measurements in the water phantom were used as comparators for the simulated doses. The diagonal off-axis ratio for each depth was measured with the Gafchromic EBT3 film (Ashland Advanced Materials, Bridgewater, NJ, USA) and a tough water phantom (Kyoto Kagaku, Kyoto, Japan) to model the distribution spread for the Monte Carlo simulation.
Evaluation of dose distribution between kV energy and MV energy
The dose distributions in the model dog CT image were compared to those in the Monte Carlo simulations for this study’s orthovoltage radiotherapy and those for an Agility MV voltage linear accelerator (Elekta AB, Stockholm, Sweden) using previously published models [9].
We assessed the effects of energy differences on bone by creating a heterogeneous bone and tissue phantom consisting of a 1.0 cm thick layer of water above a 1.0 cm thick layer of cortical bone (physical density ρ = 1.40 g/cm3) and a 10 cm layer of water. Dose profiles were compared between orthovoltage radiotherapy (energy 300 KeV and a field size of 4.0 × 4.0 cm2) and the Agility MV voltage linear accelerator (6 MV and a field size of 4.0 × 4.0 cm2).
Printing of the dog skull water phantom
The 3D printed dog skull water phantom was modeled on a medium-sized Shiba Inu (~ 12 kg) suitable for treatment with orthovoltage or linear accelerator radiotherapy. Using a 3D workstation (Ziostation-2; Ziosoft/AMIN, Tokyo, Japan), 3D CT raw images of the dog were processed for erosion. Structures other than the 3 mm surface were removed from the image after subtraction processing, and a STL image of only the 3 mm thick surface was created. Next, using the CAD STL data editing software GeoMagic Design X (3D Systems, Rock Hill, SC USA) and SOLIDWORKS 3D CAD Premium 2020 (Dassault Systèmes, Vélizy-Villacoublay, France), a dog-shaped water phantom was designed that included insertion points for the Farmer chamber dosimeters and two Gafchromic films. A RaFaEl II plus 300C-HT 3E printer (Aspect Co., Tokyo, Japan) was used to create the phantom with polyamide 12 nylon [18] in order to make it impermeable to water, and an acrylic paint was applied to the surface to suppress gas permeability (Table 1).
Table 1 Material and geometry information for a dog skull water phantom
Dog skull water phantom
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3D printing
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Additive manufacturing techniques
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Material
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Polyamide 12 nylon
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Specific gravity
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1.03
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Particle size (μm)
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45 ± 5
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Joining
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Cemedine PPX
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Surface coating
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Acrylic paint
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Distance from the center of the Farmer chamber to the surface (mm)
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16.0
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Distance from Gafchromic film (1st) to surface (mm)
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33.2
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Distance from Gafchromic film (1st) to nose surface (mm)
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15.0
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