Infill selection for 3D printed radiotherapy immobilisation devices

Abstract

3D printing provides new opportunities to create devices used during radiotherapy treatments, yet little is known about the effect process parameters play on the proposed devices. This study investigates the combined influence of infill pattern, infill density and print orientation on surface dose, as well as on the mechanical properties of 3D printed samples, identifying the optimal infill patterns for use in radiotherapy devices including immobilisation. Fused deposition modelling (FDM) was used to produce sixty samples in two orientations for surface dose measurement, utilising ten different infill patterns. Surface dose testing was performed using a Varian Trubeam linear accelerator with a 6 MV photon beam. A further one hundred and twenty tensile test samples, designed according to ASTM D638 type I standards, were evaluated using a 50 KN Instron 5969. On average, horizontally printed samples had a lower surface dose measurement compared to the vertically orientated samples, with the Stars infill pattern recording the lowest surface dose values in the horizontal orientation, while the Hilbert Curve recorded the lowest surface dose in the edge orientation. Tensile tests revealed the 3D Honeycomb infill pattern to have the highest ultimate tensile strength (UTS) in both horizontal and edge orientations. Overall, the Stars infill pattern exhibited the optimal balance of low surface dose and above average UTS. This study shows how infill patterns can significantly affect dosimetry and mechanical performance of 3D printed radiotherapy devices, and the data can be used by design engineers, clinicians and medical physicists to select the appropriate infill pattern, density and print orientation based on the functional requirements of a radiotherapy device.

Multi‐jet fusion for additive manufacturing of radiotherapy immobilization devices: Effects of color, thickness, and orientation on surface dose and tensile strength

“…As a general trend, this study identified that surface dose increased as material thickness increased, similar to previous studies that found increases in surface dose as the density of material increased. 4 , 23 , 24 For an immobilization device, this may suggest using the lowest printable thickness is ideal. However, this would ignore the impact material thickness plays on the mechanical strength of a part, which is critical when used to restrict patient movement.…”

“… 1 , 2 In order to reduce the toxic side effects of radiation therapy, immobilization devices are used to hold the patient firmly on the treatment couch and minimize exposure of healthy cells to high‐energy radiation beams by keeping the patients in the correct position during the course of treatment. 3 However, these devices can have a bolus effect that increases the radiation dose on the patients’ skin; 4 , 5 , 6 consequently, surface dose measurement is essential to measure the radiation dose on the patient's skin during the course of the treatment. 7 Significant increase in the radiation dose through build‐up effect may lead to painful reddening of the surface tissue.…”

“…One of the challenges with AM is the layer‐by‐layer fabrication process which can result in weakness between layers, and different AM technologies will result in different strength properties. For example, a recent study linked tensile testing of Fused Disposition Modeling (FDM) samples with surface dose testing, finding that samples with maximal ultimate tensile strength (UTS) often recorded a high surface dose, 4 while weak samples likely to be unsuitable for immobilization recorded a low surface dose. Balance is needed between strength and surface dose, with the researchers finding that the stars infill pattern resulted in an optimal strength with low surface dose across different print orientations.…”

“…As a technology capable of high strength parts and fine details, as well as a machine capable of relatively fast production, this may have many applications within clinical contexts, for example, radiotherapy immobilization devices. In order to validate MJF for this application, this research adopted an established methodology 4 to compare the effect of print orientation and thickness on the mechanical and dosimetry properties of samples. Additionally, different colored samples were used to assess whether the color had any influence on the dosimetric and mechanical properties of the printed samples, and identify the optimal color.…”

Development of a patient-specific immobilisation facemask for radiation therapy using additive manufacturing, pressure sensors and topology optimisation

“…As highlighted in Tables 4 and 5, the pitch negative masks were identified as the best designs for immobilising the volunteer based on the collected sensor data. (Asfia et al, 2020a), this infill pattern was not available within the Fortus slicing software, and Hexagram was selected as the closest representation of this infill pattern. The full print parameters are presented in Table 6.…”