## 4 Discussion  
We developed the FDM 3D printing filament material with similar electron density to real human organs and bones and analyzed its characteristics in this study. As shown in Table 3, by controlling the infill density of commercially produced filament materials, it was possible to create a 3D printing material corresponding to the electron density of a real human organ. As shown in Table 3, by controlling the infill density of commercially produced filament materials, it was possible to create a 3D printing material corresponding to the electron density of a real human organ. In addition, as shown in Table 2, by varying the ratios of BaSO4, PBAT, and PLA, bone filament materials with various electron densities, from trabecular bone to dense bone, were developed. [PERSON] et al. conducted a study with an electron density similar to that of human tissue using ABS, PLA, WoodFill,  
Figure 6: Comparison of uniformity of 3D printing materials (ABS, PLA, TPU) for (**a**) infill density 10%, (**b**) infill density 30%, (**c**) infill density 50%, (**d**) infill density 70%, (**e**) infill density 90%, (**f**) infill density 100%. CopperFill, and BronzeFill [20]. The infill density of ABS and PLA was adjusted to find the density corresponding to human organs such as lung, liver, and adipose, and the results were similar to those of this study. On the other hand, in their study, the bone material was manufactured using commercially available metal materials, and the electron density was 2-3 times higher than that of actual human bone. [PERSON] et al. used stone filament to develop a filament material with a density corresponding to that of bone, but the uniformity was low [29]. On the other hand, we were able to produce a filament with an electron density very similar to that of human bone, as shown in Table 3, by mixing a non-metal material rather than a metallic material. The use of 3D printing with the bone density obtained in our study can provide a more uniform and high-quality bone density. As a result of our comparative analysis of the infill density of 3D printed materials, a larger standard deviation was observed in the region with a lower internal filling value, and a low-density volume was achieved by mixing large air voxels with PLA filament voxels [30]. The error bar in the graph shown in Figure 4 indicates that it has a low error value at a relative density of 70% or more. [PERSON] et al. also recommended 60% more infill density based on an analysis of the electron density of polystyrene filaments, so that the deviation value of the material may be improved through mutual comparison [18]. This shows that the use of the DQA phantoms in the low-density range is limited, and the accuracy of the dose calculation seems limited in terms of dosimetry and quality assurance. Furthermore, when irradiating low-density phantoms from all directions rather than a single beam in one direction, the voids between multiple layers cannot be reliably reproduced when a 3D printed phantom with an infill ratio of less than 70% is used. Phantom dosimetry is very sensitive to the settings of a small stereotactic irradiation surface with dimensions similar to the air gap; therefore, it is difficult to guarantee the quality of dosimetry in the case of a very low-density 3D manufactured phantom [31]. Filaments mixed with a metal exhibit other effects, such as a photoelectric effect at kV energy, which affects the linearity of the curve and results in a high deviation. It was also demonstrated in the studies of [PERSON] et al. and [PERSON] et al. that the CT values of the metallic objects themselves were erroneously expressed in the kVCT image [20; 21]. In the kilovoltage peak images, which are energies generally used for patients, it can be observed that the CT values of metallic materials exhibit significantly high off-axis fluctuations and errors in the linear graph.