## 4 Discussion
### Three-Dimensional-Printing Methods and Materials  
Most studies printed thoracic models using FDM, involving the additive layering of melted thermoplastics extruded through a heated nozzle onto a printing bed [11]. The popularity of FDM technology can be attributed to the wide availability of commercially available thermoplastic printing materials [8] as well as the growing body of evidence investigating different additives and composite materials in attempts to broaden the profile of radiodensities they can mimic [10; 54; 55]. Furthermore, FDM printers are cheaper and more widely available compared to other printing technologies [28; 56]. Studies in this review utilised FDM through three primary methodologies: 1. adjusting infill ratios to tailor radiodensities for specific tissue types [32; 33; 34; 38; 39; 40; 41], 2. Modifying the volume of extruded filament and adjusting extrusion rates per voxel [42; 43; 44], and 3. crafting skin and external organ shells to encase filler materials of dedicated densities [18; 30]. Manipulating the infill ratio is advantageous because it allows for the use of fewer material types. Some studies opt for a single material, simplifying the process and reducing costs [32; 34; 41]. However, this challenged the achievement of radiation equivalence, requiring higher atomic additives for better HU replication [32]. The pixel-by-pixel method introduces a unique approach to 3D printing by removing the requirement to segment DICOM images [42; 44]. Instead, CT intensities are directly translated into G-code representing printer instructions of varying extrusion volumes or speeds, allowing for heterogenous densities, with a wider range of attenuations [42; 43; 44]. Regardless, printing times were longer for the pixel-by-pixel method, and the G-code is proprietary, with one study demonstrating poor methodological quality [44]. This was due to the absence of statistical analysis, lack of detailed information including costs and scanning parameters, measurement bias involving a single assessor, and concluding statements that extended beyond the scope of the study (Table 4). However, the direct conversion from DICOM image to printer instructions likely improves spatial resolution, due to avoiding the subjective contouring and inaccuracies of manual thresholding during segmentation and associated partial volume effects [57; 58]. FDM was critiqued by the literature for causing spatial mismatches between patient and phantom replicas because of post-polymerisation shrinkage and small build platforms requiring assembly of printed parts [35; 36; 40]. This is an already established drawback of FDM polymer materials, whereby warping and cracking of the material accrues after cooling, leading to rough surface finishes [8]. Potential oozing of heated remnant material from the nozzle onto the printed surface can exacerbate geometrical errors [42]. Moreover, FDM applies thicker layers of printed material, resulting in a z-axis resolution typically ranging between 0.1 and 0.5 mm [49], which can produce stair-step deformities [38; 59]. Consequently, FDM printers exhibit lower resolution compared to other printing methods, such as Material Jetting (Multi/PolyJet), stereolithography (SLA), and Selective Laser Sintering (SLS), which offer comparative resolutions in the range of 0.02 mm and create smoother finishes [49; 60]. FDM prints are also limited by shell artifacts, whereby sudden transitions in attenuation at the rim of the printed parts limit the realism of homogenous tissue backgrounds. Furthermore, an infill percentage below 40% results in visible and unrealistic print patterns on CT [22]. Material Jetting uses an inkjet head to successively eject droplets of photopolymers that are selectively cured using ultraviolet light to build a 3D construct. SLA selectively cures a vat of photocurable resin [8], while SLS employs a laser to selectively fuse regions of a powder bed [59].