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literature, only a few physical phantoms of pregnant women have been developed to evaluate fetal dose during radiological examinations or radiotherapy( Hoerner et al., 2018; Shirkhani et al., 2020; Matsunaga et al., 2021; Kopačin et al., 2023). In the current study, 3D printed“ bellies” of pregnant woman has been developed to be added on an ATOM ® woman phantom, mimicking different stages of pregnancy and enabling fetal dose estimation.
3D printing technology has been used to produce individualized phantoms, quality assurance phantoms, compensators, brachytherapy applicators as well as equipment supporting patient immobilization( Gear et al., 2014; Su et al., 2014; Arenas et al., 2017; Rooney et al., 2020). For these applications, materials such as thermoplastics or light-cured resins are commonly used. However, special consideration must be given to the accuracy of the printed materials, particularly regarding printing precision and tissue equivalence.
The present study aims to describe a method of 3D modeling and printing bellies which will be added to the ATOM ® phantom. Additionally, the study aims to evaluate the dosimetric water equivalence of the VeroWhite resin for radiotherapy beam energies.
2 Materials and methods
2.1 ATOM ® phantom
The anthropomorphic adult female ATOM ® phantom from Computerized Imaging Reference Systems( CIRS) consists of three parts: the head with the cervical spine( plates 1-10), thorax( plates 11-23) and pelvis( plates 24-38) 1, the plates being made of materials reproducing the attenuation of human tissues. The compositions and nominal physical densities of the materials used in the phantom ranged from( 0.9 to 1.1 g / cm 3) 2. The height and the weight of the phantom were, respectively, 160 cm and 55 kg, leading to body mass index( BMI) of 21.5 kg / m 2.
2.2 3D-printing process to model the bellies for different stages of pregnancy
First, the ATOM ® phantom was scanned to obtain its external contours( GE Discovery RT, 120 kV, X-ray tube current from 148 to 379 mA and slice thickness of 0.625 mm). Then, the DICOM images were imported into 3D Slicer software to extract the external contours in( 0.3 dcm) format( Fedorov et al., 2012; Ashenafi et al., 2023). These were then imported into Rhinoceros ® 3D software 3. The Non-Uniform Rational Basics Spline( NURBS) format was chosen. The NURBS format was used for phantom modeling, because of their flexibility and accuracy for the various transformation operations. The bellies for different stages of pregnancy were modeled to fit perfectly on the ATOM ® phantom and to respect
1 https:// www. cirsinc. com / wp-content / uploads / 2019 / 05 / 701-706- ATOM-PB-120418. pdf. 2 https:// pdf. medicalexpo. com / pdf / cirs / model-701-706 /
95901-152992- _ 4. html. 3 https:// www. rhino3d. com / fr /. the fundal height, as described in the next section. The bellies represent the volumes to be fixed on the pelvis section of the ATOM ® phantom. Then, the NURBS surfaces were converted to mesh using the Stereo Lithographic file format(*. stl) which can be easily read by the 3D printer. Finally, the 3D modeled volumes were printed using 3D Printer( Stratasys), and VeroWhite resin( Polyjet Opaque Resin) 4. This technique uses photosensitive polymer liquids to create 3D printed objects. The liquid undergoes photopolymerization meaning it is solidified by UV light, layer by layer, to create 3D prints.
2.3 Modeling, geometry of the bellies and definition of points of interest for fetal dose measurements
The symphysis fundal height is an important clinical indicator to assess fetal growth and development and is used primarily to detect fetal intrauterine growth restriction( Robert Peter et al., 2015). It represents the distance, in centimeters, from the mother’ s pubic bone as a reference point to the top of the uterus( fundus). In practice, from the 4th to the 7th month, to obtain the reference measurement based on the pregnancy term, the number of months is multiplied by 4. From the 8th month until term, the number of months is multiplied by 4, and the resulting number is then subtracted by 2 cm( Ayet, 2014).
Three stages of pregnancy were considered for this study: first, 2 nd, and 3 rd trimesters. The previous formula was used to determine the fundal heights for the 2nd, 6th and 8-months of pregnancy, corresponding to the three stages. To define the 3D bellies’ s dimensions, the theoretical fundal height, and the dimensions of the ATOM ® phantom in x and y axis were used as fixed values, then the bellies dimensions were manually optimized in respect of these values and to be realistic. In collaboration with a gynecologist, the position of the uterus on a lateral section of the ATOM ® phantom with bellies, was determined in the form of ellipses.
Different points inside these elliptic uterus were chosen for measurements: at the level of the fundus and cervix corresponding either to the fetus’ s gonads or its head, one point was selected to measure the maximum dose due to scattered radiation from the patient( a point deep closest to the woman’ s head and in the uterus) and additional points were defined in the uterus to assess the average fetal dose( Fig. 1).
Because a part of the uterus is located within the ATOM ® phantom, two specific ATOM ® phantom plates were replaced by printed plates to enable ionization chamber point measurements. This was achieved by creating holes in them.
2.4 Characterization of the VeroWhite resin
VeroWhite resin was initially motivated in this study by the following reasons: resin-based printing helps to minimize air gaps between layers unlike filament-based methods, does not require manual post-printing treatment, and its density is relatively close to that of water compared to other available resins. The VeroWhite resin has already been used for the design of thorax phantom with mobile tumor for radiation
4 https:// www. sculpteo. com / fr / materiaux / materiau-resine-polyjet / materiau-verowhite /.