## 4 Discussion
### Phantom Equivalence
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tissue** & \begin{tabular}{c} **Mass Density** \\ **(kg/m3)** \\ \end{tabular} & \
\begin{tabular}{c} **Tensile Strength** \\ **(MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Young’s Modulus** \\ **(GPa)** \\ \end{tabular} \\ \hline \multicolumn{4}{c}{Material used in study} \\ \hline PLA & 1210–1250 [60] & 60 [61] & 0.35–3.5 [60] \\ \hline ABS & 1040 [61] & 40 [61] & 2.2 [61] \\ \hline Glow-PLA & 1260 [62] & 32 [62] & 2.3 [62] \\ \hline Polyurethane resin & 1050–1250 [63] & 36–52 [63] & 1.2–1.6 [63] \\ \hline \multicolumn{4}{c}{Human Tissue} \\ \hline Lung & 380 [64] & 10.8 [65] & 1.4–6.1 \(\times\) 10\({}^{-3}\)[64] \\ \hline Adipose & 950 [64] & 10–13 [66] & 2.9 \(\times\) 10\({}^{-6}\)[66] \\ \hline Muscle & 1050 [64] & 150.4 [64] & 462.5 [64] \\ \hline Skin & 1090 [67] & 15.6–28 [64] & 0.07–0.168 [64] \\ \hline Bone & 1031–1092 [64] & 165.7 [64] & 4.5–16.7 [64] \\ \hline Blood vessels & 1060 [67] & 2.7 [68] & 1.17 \(\times\) 10\({}^{-3}\)[68] \\ \hline Heart & 1050 [67] & 2.51 [69] & 0.02 [69] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Material properties of phantom compared to human tissues. The phantom tissues showed strong measurement conformance with their patient counterparts (Figure 7), with a mean error of 0.03 cm, corresponding to a 1.2% reduction in anatomical size. This demonstrates the capability of 3D printing to accurately replicate true patient morphologies [29]. The slight reduction in size can likely be attributed to a known limitation of 3D printing technology, where materials shrink during cooling or post-polymerisation [22]. Future studies could enhance the assessment of morphological conformance by comparing the entire surfaces of the phantom and patient STL files [70]. However, this approach may be more demanding, as it requires additional segmentation of the phantom scans. The total cost of the phantom was AUD 527, which is comparable to other 3D-printed chest phantoms costing in the range of AUD 70-110 for single-organ phantoms [71; 72] and AUD 250-5450 for multi-organ chest phantoms. In contrast, commercial phantoms are significantly more expensive, costing upwards of AUD 30,000 [19; 20]. This comparison highlights the value of 3D-printed phantoms to enable more affordable and accessible options for chest phantoms in medical imaging. Furthermore, commercial phantoms have generic dimensions and are not patient specific. For example, the N1-Lungman phantom has a weight of 18 kg, chest girth of 94 cm, a width of 43 cm, and a height of 48 cm [20]. This may not be applicable to specific patients; for example, our phantom represents a real patient with a 22 kg torso, 94 cm chest girth, 37 cm width, and 34 cm height. The printing time of our phantom was 1178.5 h (Table 1), whilst other chest phantoms reported in the literature have taken merely 7-120 h to print [19; 58; 72; 73; 74; 75]. This reflects the time constraint of printing a comprehensive chest phantom with true-to-patient dimensions and opting for single-nozzle printers as opposed to more expensive multi-nozzle and multi-material printers, which speeds up the printing process. Further comparisons of printing time, cost and tissue equivalence are demonstrated in Table 6.