## 4 Discussion
### Three-Dimensional-Printing Thoracic Organs  
The current literature has mostly investigated the creation of discrete thoracic organs with limited consideration of comprehensive chest phantoms. For example, [PERSON] et al. [30] printed a single heart, [PERSON] et al. [18] developed 3D-printed coronary arteries, [PERSON] et al. [39] produced an aorta, and [PERSON] et al. [31] created pulmonary arteries. Likewise, [PERSON] et al. [34; 35] solely investigated the bony thorax, without the inclusion of other thoracic structures. Additionally, skin, subcutaneous fat, and muscle structures tend to not be delineated into their sub-structures but rather printed as a single soft-tissue structure with homogenous radiography [36; 37; 40; 41]. [PERSON] et al. [39] produced the most comprehensive model of all studies, incorporating seven thoracic structures: skin, fat, muscle, bone, heart, lung, and parenchymal lesions. Despite achieving radiation equivalence, the radiation-attenuating properties of the heart were not evaluated, and the phantom merely represented an axial slice rather than comprising the entire torso. [PERSON] et al. [32] produced a comprehensive thoracic model built with a single material (PLA); however, the phantom did not achieve radiation or geometrical equivalence. Tissue attenuations are impacted by surrounding tissues and structures due to beam hardening, thus limiting the application and generalisability of these single-organ studies and phantoms with unrealistic tissue backgrounds [52]. This warrants further studies investigating comprehensive, holistic, and more realistic thoracic models. Thoracic phantoms described in the literature predominantly consist of lung replicas, created using a variety of materials, including PLA (infill rates of \(10\%,30\%\), \(46\%\), and \(100\%\)), ABS (\(50\%\)), TPU (\(50\%\)), Nylon, low-density paper, and foamed silicone gels [32; 36; 38; 39; 40; 41; 42; 44]. Lung phantoms mostly achieved radiation equivalence within the norms of pulmonary parenchyma, which ranges between \(-700\) and \(-900\) HU [73]. However, most of the models did not include blood vessels and struggled to match the low radiodensity of \begin{table}
\begin{tabular}{l l l} \hline \hline
**Material (Printing Method)** & **Advantages** & **Disadvantages** \\ \hline Visjlet EX200 (Multi-Jet) & Very tough and durable [65] & May cause skin irritation [69] \\  & Transparent—allows visualisation of & Slight odour [69] \\  & internal structures [66] & Requires supporting material for \\  & High resolution—enables smooth & overhanging structures [64] \\  & curves or sharp edges [67] & Expensive printer [37] \\  & Biocompatible [68] & \\ \hline Gypsum Powder (Binder Jetting) & Low cost and accessible [70] & Low strength [70] \\  & High Density of \(1.57\) g/cm\({}^{3}\), gives & Porous [70] \\  & radiodensities between \(372\) and & \\  & \(995\) HU, similar to bone [10; 36] & \\ \hline PMMA (FDM) & Transparent—allows visualisation of & Shrinks and warps without a heated \\  & internal structures [71] & printing bed [71] \\  & Strong and durable [71] & Harmful gasses emitted during \\  & Resistance to UV and other weather & printing—requires good ventilation \\  & exposures [71] & \\  & Density of \(1.12\) g/cm\({}^{3}\)[10]. Suitable & \\  & radiodensity for glandular tissue at & \\  & \(-95\) HU [35] & \\ \hline Silicone of the FlexFoam-IT series (casting) [72] & Expandable and durable, suitable & Pot life of only 1 min after opening \\  & densities for representing skin and & [39] \\  & lung parenchyma according to & Requires a silicone-releasing agent in \\  & expansion factor [39] & order to remove the mould [39] \\  & Silicone of the FlexFoam-IT series has & Requires a completely sealed mould \\  & short curing time of less than \(2\) h [34]. & in order to avoid leaking into \\  & Silicone Dragon Skin has a long & neighbouring areas [35] \\  & shelf-life and fast curing time & \\  & (\(<\)16 h) [36] & \\ \hline \hline \end{tabular}
\end{table}