Three-dimensional (3D) printing technology is widely used in the field of radiation oncology to make computed tomography (CT)-based patient-specific tools, thereby improving the quality of radiation therapy.1234567) For example, patient-specific compensators for radiotherapy are fabricated for use in areas with severe curvatures, such as the nose, feet, and hands, thus delivering more radiation to a tumor in the area and reducing radiation to the surrounding normal organs.4) For total body irradiation, 3D-printed patient-specific metal compensators reflecting the overall curves of the patient's body shape can deliver more uniform radiation doses to the whole body than conventional methods.6) 3D printing technology has also been used for high dose rate (HDR) brachytherapy to create patientspecific elastic HDR applicators. These deliver accurate doses of radiation to the skin's surface; this technology has been proven to improve treatment delivery accuracy.248) In addition, many attempts have been made to perform quality assurance in radiation therapy by producing patientspecific phantoms using patient CTs and 3D printers.5910111213)

In the 3D printing field, advances have been made not only in printing technology, but also in the printing materials used. Various synthetic 3D printing materials have been developed, including polyethylene terephthalate glycol (PETG) and a unique acrylonitrile butadiene styrene (ABS) plastic blend manufactured by Zortrax (ULTRAT; Zortrax SA, Olsztyn, Poland) which is in a type of thermoplastic polymer. Flexible materials have also been developed, like the thermoplastic polyester elastomer manufactured by Zortrax (FLEX) which had great interlayer adhesion and dimensional tolerance. These materials can be used for a wide range of applications in 3D printing for radiation therapy, in combination with existing 3D printing materials like ABS, polylactic acid (PLA), and high impact polystyrene (HIPS). However, no prior research has evaluated the radiological properties of the various 3D printing materials currently available, knowledge of which is crucial for these material's proper application.

In this study, the radiological properties of newly introduced and existing 3D printing materials were evaluated by measuring their Hounsfield units (HUs). The HU values were evaluated by varying the infill density of each printing material, thus allowing us to assess their applicability to the field of radiation oncology.

This study investigated the radiological properties of six 3D printing materials, to determine how to incorporate a wide range of 3D printing technologies into radiation oncology. The average HU values of the six printing materials were obtained, and the average HU values of HIPS and PLA were analyzed across a range of infill densities.

The time required to print each material at each infill density was also measured. When the infill density was 10%, the average printing time was 32 minutes; when the infill density was 100%, the average time was 12 hours and 45 minutes. Considering that the volume of each prepared cube was 32 m³, a printing time of 12 hours and 45 minutes could be considered quite wasteful. Using 100% infill density should therefore be carefully considered while manufacturing patient-specific human phantoms.111213)

Table 3 shows the HU values for the six printing materials, each at an infill density of 60%. ABS, ULTRAT, HIPS, PETG, PLA, and FLEX had HU values of −535±12, −557±10, −542±7, −508±20, −530±25, and −633±15, respectively. The HU values were similar regardless of the specific density of the materials.

Fig. 3 includes a graph showing HU changing with the infill density of HIPS and PLA. The HU value increased with infill density for each material. The HU values for HIPS and PLA were −131 and 61, respectively, at an infill density of 100%. By comparison, at an infill density of 10%, the HU values for HIPS and PLA were −896 and −904, respectively, In addition, the parameters for fitting the relationship between infill density and HU using Equation (2) are shown in Table 4. In the case of HIPS and PLA, the R² values were 0.994 and 0.976, respectively, indicating that the fitting was successful. Thus, when manufacturing patient-specific phantoms using current materials, HU values for lungs, soft tissues, and fats can be simulated by calculating appropriate infill densities. Matching the HU of bone, however, proved to be difficult when using a material with a high HU value.14) Additional studies should be conducted to find a material that can reflect the HU of bone. In addition, there is a need for a method for overcoming the limitations of the printing material itself (i.e., they cannot produce a sufficiently high HU), including a method for manufacturing a casting mold from the printing material and casting a material with a high HU. An additional study should be conducted to develop such a method.

In this study, HU values were obtained for various 3D printing materials at different infill densities; the radiological characteristics of each material were then analyzed. The results suggest that using optimized infill densities will help improve the quality of radiation therapy by producing customized instruments for each field of radiation therapy.