Characterization of Radiation Shielding Capabilities of High Concentration PLA-W Composite for 3D Printing of Radiation Therapy Collimators

Abstract

This work evaluates the radiation shielding capabilities of the PLA-W composite for MV energy photons emitted by a linear accelerator and the feasibility of manufacturing a clinically-used collimator grid in spatially fractionated radiotherapy (SFRT) using the material extrusion (MEX) 3D printing technique. The PLA-W filament used has a W concentration of 93% w/w and a green density of 7.51 g/cm³, characteristics that make it suitable for this purpose. Relevant parameters such as the density and homogeneity distribution of W in the manufactured samples determine the mass attenuation coefficient, directly affecting the radiation shielding capacities, so different printing parameters were evaluated, such as layer height, deposition speed, nozzle temperature, and infill, to improve the protection performance of the samples. Additionally, physical and mechanical tests were conducted to ensure structural stability and spatial variability over time, which are critical to ensure precise spatial modulation of radiation. Finally, a complete collimator grid measuring 9.3 × 9.3 × 7.1 cm³ (consisting of 39 conical collimators with a diameter of 0.92 cm and center-to-center spacing of 1.42 cm) was manufactured and experimentally evaluated on a clinical linear accelerator to measure the radiation shielding and dosimetric parameters such as mass attenuation coefficient, half-value layer (HVL), dosimetric collimator field size, and inter-collimator transmission using radiochromic films and 2D diode array detectors, obtaining values of 0.04692 cm²/g, 2.138 cm, 1.40 cm, and 15.6%, respectively, for the parameters in the study. This shows the viability of constructing a clinically-used collimator grid through 3D printing.

Keywords: PLA-W; PLA-metal composites; SFGRT; grid therapy.

Figure 1

Samples of 10 pieces of filament of 2 cm length were used for filament density measurement.

Figure 2

Samples from the 12 PLA-W slabs of dimensions 20 × 20 × 1.8 mm³.

Figure 3

1.0 × 1.0 × 1.0 cm³ cubes of PLA-W in ascending order of infill percentage: 15%, 50%, and 100% (from left to right).

Figure 4

PLA-W specimens with a variable layer thickness (0.15, 0.30, and 0.45 mm) and PLA specimen samples for tensile evaluation (left). PLA-W and PLA in mechanical tensile stress evaluation (center and right).

Figure 5

Samples of 12 slabs prepared with variation of printing parameters distributed in the order indicated in Table 1 (lower left corner), together with copper, aluminum, and lead filters (right) arranged in digital chassis for evaluation of kilovoltage radiography (left image) and 6 MV scintigraphy (right image).

Figure 6

Experimental setup for HVL determination with a UNIQUE linear accelerator and PLA-W slabs in solid water phantom with an ionization chamber (top). Enlarged image of setup with 3.6 × 3.6 cm² irradiation field (left). PLA-W filter distribution (right).

Figure 7

(Left) A grid made of PLA-W with honeycomb pattern holes mounted on a UNIQUE linear accelerator tray and (Right) grid inserted in linear accelerator head (seen from below).

Figure 8

Stress-strain curves of PLA (above) and PLA-W (below), where curves 1 (red line), 2 (burgundy line), and 3 (green line) correspond to layer heights 0.15, 0.3, and 0.45 mm, respectively.

Figure 8

Stress-strain curves of PLA (above) and PLA-W (below), where curves 1 (red line), 2 (burgundy line), and 3 (green line) correspond to layer heights 0.15, 0.3, and 0.45 mm, respectively.

Figure 9

Submillimeter image of the PLA-W manufactured layers showing the discontinuities of the PLA fibers in the structure.

Figure 10

Experimental attenuation with different PLA-W thicknesses.

Figure 11

Images of grid measurements with EBT3 radiochromic films. Each image set of the grid is in order: at the grid exit (A,C) and a distance of 35.1 cm from the grid at the isocenter (B,D). (A,B) show the hexagonal array to measure distances while (C,D) shows the calculation of the divergence of the central hole. On the right (E), an image of the radiation fields on the central axis of the holes with their respective divergences at the isocenter with a field aperture of 13 × 13 cm².

Figure 12

EBT3 radiochromic film irradiated with a 6 MV beam from the UNIQUE linear accelerator, at SSD: 95 cm, field size 10 × 10 cm² and 5 cm depth. Square symbols representing the evaluated ROIs.

Figure 13

Relative dose distribution of the grid with MapCheck array for a 10 × 10 cm² field size.