## 4 Discussion
### Three-Dimensional-Printing Thoracic Organs

Table 5: _Cont._aerated lung tissue (\(<\)\(-\)1000 HU [32; 73]), achieving an average radiodensity of \(-\)610 HU (\(-\)417 to \(-\)933 HU). Underlying this challenge is the requirement for 3D constructs to have a printing scaffold and to maintain structural integrity, which limits the reduction of infill rates and the presence of large air gaps [32]. Furthermore, minimum attenuations are ascribed to the intrinsic properties of the base material as revealed by [PERSON] et al. 's [46] paper-based lung model, which was unable to replicate aerated lung densities. PLA with 10% infill produced the closest approximation to aerated lung tissue (\(-\)933 HU) [32]. Similarly, studies faced challenges in replicating the higher attenuations of bone (>1000 HU [58]), as the raw materials used typically fall within the soft tissue density range [32; 38; 51]. PLA doped with 50% iron achieved the highest attenuations, closest to dense cortical bone [41]. The high atomic number and electron density of iron make it an ideal additive for increasing the attenuation of PLA composite materials, primarily due to the enhanced occurrence of the photoelectric effect [52; 55]. Stone-filled filaments as well as radiopaque substances were additionally employed; however, they achieved relatively lower attenuations, likely due to lower densities and malabsorption of contrast [34; 51]. Similarly, [PERSON] et al. [74] used a Bismuth-doped ABS filament in their 3D-printed nasocranial phantom, achieving radiodensity between 1000 and 3000 HU. Thus, the incorporation of filaments with mixed metallic and high-density additives shows promise for improving the replication of bone-like attenuations in thoracic phantoms [32; 55]. However, over time, dense metal particles can abrade the printer nozzle, leading to imperfections in the 3D object with different attenuations and geometries [55]. Studies that printed lung lesions included between 1 and 12 nodules, created using pearl-powder solution, PLA, Silicone foam, Nylon, Acrylonitrile Styrene Acrylonate (ASA), Acrylonitrile Butadiene Styrene (ABS), and Polyethylene terephthalate glycol (PETG) of varying infill percentages [32; 33; 36; 38; 39; 45; 46; 48]. These studies achieved radiodensities between \(-\)909 and 227 HU, representing sub-solid and solid nodules, employing printing methods including SLA, FDM, Binder Jetting, and SLS. The selection of SLA, SLS, and binder jetting over FDM in some studies likely aimed to achieve finer details due to their higher printing resolution, despite the associated higher costs of these techniques [36; 45]. Printed lung nodules in phantoms served multiple purposes, including feasibility assessment for creating tissue equivalent radiodensities [38; 39], validation of imaging algorithms [28], quality analysis of X-ray images [36], and comparison of the detection sensitivity of paediatric lung nodules using different image reconstruction methods [45]. However, no study utilised these phantoms for optimising low-dose protocols for lung cancer screening, such as modifying kVp and mAs acquisition parameters, revealing a potential avenue for further research. Furthermore, this review underscores that the use of 3D-printed thoracic phantoms for optimizing low-dose protocols has predominantly been explored in the cardiovascular field [18; 28; 29], indicating a need to expand such investigations into the realm of pulmonary imaging and screening protocols. Another limitation of these 3D-printed chest phantoms is their inability to simulate physiological conditions such as dynamic cardiovascular systems with haemodynamic flow, heartbeat, and lung movements during breathing. This has implications for image quality for example by creating movement artifacts and distributing dose differently in moving tissues [75]. Although challenging, addressing these tasks in future studies is worthwhile.