## 4 Discussion  
The manufacture of 3D-printed imaging and dosimetry phantoms, as alternatives to commercial phantoms, is an emerging field of research in medical physics. Despite concerns about the calibration and standardization of in-house 3D printing processes, advances in printing techniques and materials support the belief that 3D-printed phantoms may replace commercial phantoms in clinical quality assurance procedures [28]. One of these printing techniques, FDM technology, offers users remarkable opportunities to mimic the radiological properties of most of the biological tissues via 3D printing. Numerous studies have investigated the impact of different printing materials and parameters on the radiological characterization of 3D-printed objects. These endeavors have introduced a wide range of materials and methods, with the goal of reproducing the radiological properties of various tissues. However, a common limitation is reported in the majority of these studies, which is associated with the use of low infill percentages in mimicking low-density tissues such as the lungs [18; 21; 22; 23; 24; 29]. In this study, we investigated and introduced a novel technique for density modulation using a special filament, LW-PLA. Distinguished from regular PLA filaments, LW-PLA exhibits a foaming feature at printing temperatures above 220 \({}^{\circ}\)C. This unique characteristic offers users the opportunity to adjust the density of a printing job at the nozzle exit rather than on the print bed. Consequently, it allows for the production of print jobs with a significantly lower density, even when printed at 100% infill. To the best of our knowledge, there are no published studies on the radiological characterization of this type of filament or its associated printing technique. The framework of this study was established based on the utilization of high printing temperatures to facilitate the expansion of LW-PLA. This phenomenon is expected to become dominant at high temperatures starting from around 220 \({}^{\circ}\)C. Therefore, the choice of high flow rates together with high temperatures may increase the magnitude of expansion experienced by the filament to such a point that it can deteriorate the geometric accuracy of the print job. An illustrative example of this issue is presented in Figure 2, showcasing a sample printed at 250 \({}^{\circ}\)C with a 100% flow rate. Therefore, to prevent overexpansion and maintain the geometric accuracy of the printed samples, the maximum applied flow rate was reduced by 10% each time the printing temperature was increased by 10 \({}^{\circ}\)C.  
The findings of the LW-PLA samples are illustrated in Table 2. The density of the cylindrical samples varied between 0.36 g/cm\({}^{3}\) and 1.21 g/cm\({}^{3}\). The lowest density was observed for the sample printed at 250 \({}^{\circ}\)C with a 20% flow rate, while the highest density was observed for the sample printed at 210 \({}^{\circ}\)C with a 100% flow rate. These results demonstrate a reduction in the density of up to 70% for the same printing material with a constant infill of 100%. As expected, strong linear correlations were observed between the flow rate and density at all printing temperatures (R\({}^{2}\) > 0.99). Additionally, the findings from the regular PLA samples, printed at 230 \({}^{\circ}\)C with different flow rates between 100% and 20%, showed very similar densities, as given in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Diameter (cm)** & **Mass (g)** & **Density (g/cm\({}^{3}\))** & **Mean HU** & **STD\({}_{\text{xy}}\)** & **STD\({}_{\text{z}}\)** \\ \hline
1.5 & 1.53 & 0.43 & \(-\)589.1 & 9.2 & 2.5 \\ \hline
3.5 & 8.33 & 0.43 & \(-\)587.4 & 7.3 & 3.7 \\ \hline
5.5 & 20.6 & 0.43 & \(-\)581.4 & 6.1 & 2.7 \\ \hline \end{tabular}
\end{table} 
Table 6: Mass, density, mean HU, STD\({}_{\text{xy}}\), and STD\({}_{\text{z}}\) findings of cylindrical LW-PLA samples printed at 27 mm/s print speed with different diameters.