## 1 Introduction  
With the technological advancements in 3D printing in recent years, various studies are being carried out. In particular, various studies have been conducted in the field of radiation therapy to evaluate the quality of medical 3D printing and to verify the physical and biological characteristics of radiation treatment by creating a customized shape for each patient [1; 2; 3; 4]. A commercially available phantom with a density similar to that of the human body can be manufactured and compared based on medical factors for each patient using a 3D printer, and it can also be used as an appropriate tool to verify treatment plans through delivery quality assurance (DQA) [5; 6; 7]. Since the CT images used for treatment planning in radiation therapy are mainly acquired by using energy of 50 keV or more, the interaction with radiation is mainly due to Compton scattering and is closely related to the electron density of organs, bones, and tumors [8]. For human tissue materials, data on the electron beam attenuation coefficient can be represented by a single straight line, but for materials such as bone, the attenuation coefficient is different [9]. Currently, commercially available phantoms have disadvantages in terms of high cost, difficulty in realizing the physical properties desired by the user, and the long processing time of the material. In addition, although phantom manufacturers provide standard values for several reference materials, research on materials with various densities for each organ, considering the different characteristics of each individual patient, is necessary [10]. Various studies have been conducted using various methods in the fields of medicine and medical physics using 3D printing technology [11; 12; 13; 14]. Three of the most common 3D printing technologies are stereolithography (SLA), selective laser sintering (SLS), and fused deposition modeling (FDM). SLA uses a photopolymer resin that is cured to form a model layer using an ultraviolet laser. It is possible to create models with various characteristics, such as strength and flexibility, and to fabricate precise models. SLS uses a mixed powder that is cured to form layers using a high-power laser. It can be used layer-by-layer without supports between the layers, and a wider variety of plastics, metals, or glass can be used to form 3D models [15]. These two printing techniques have the advantages of a high printing speed and precise printing; however, they also have disadvantages owing to their very high starting cost and small build size [16]. In addition, they are impractical to implement as the type of usable material is small and the output must be processed again after printing. FDM is the most widely used technology; in this technique, a thermoplastic resin is heated in liquid form, extruded through a nozzle, and cooled on a flatbed to deposit it. The advantage of the use of a filament in FDM is that it is inexpensive and can be used for materials with various properties. In general, the greatest advantage of FDM is that it is possible to control the packing density of the output so that low-density tissues, such as lungs, can be modeled [17; 18]. Studies to evaluate the properties of various materials by analyzing the tissue equivalent and radiological density using the FDM and SLA methods have been conducted [19]. In addition, various metallic and non-metallic 3D printing materials have been printed and comparatively analyzed using various energies, such as kilovoltage computed tomography (kVCT) and megavoltage computed tomography (MVCT) [20]. kVCT is used in clinical practice and can use kilovoltage energy to analyze all the organs in a patient's body, including lungs, dense bone structures, and tissues with densities similar to water. This yields the same results for the powder and resin used in SLS and SLA printers, and notably, both printers have some limitations in terms of low densities [21].