3D printer in dosimetry and mammography – designing and testing an OSL dosemeter holder and a low contrast-detail phantom

Sammanfattning: Introduction: Today, 3D printing is a versatile tool used in a wide range of fields. With the advent of more cost-effective fused deposition modelling (FDM) 3D printing technologies it is time to investi- gate what such a 3D printer has to offer in radiation science applications. This master thesis will inves- tigate if there is a possibility to use a FDM 3D printer to construct an optically stimulated lumines- cence dosemeter holder. Furthermore, the FDM 3D printer will also be used to construct and evaluate a low contrast-detail phantom, used for mammographic quality assurance (QA). There is a wide range of materials used for 3D printing and different printing settings might affect the characteristic of the model. Thus, it is appropriate to benchmark some basic properties like Hounsfield Unit (HU), and effective atomic number (Zeff), depending on both material and print- ing properties. Passive personal dosemeters are commonly used for monitoring occupational exposure. This is a device that is worn at certain body part. Then, at regular intervals, the dosemeter is evaluated and an effective dose is determined. Many such passive dosemeters use either thermoluminescent (TL) or optical stimulated luminescence (OSL) dosemeter materials. NaCl can be used to obtain an OSL signal. NaCl is easily accessible and cheap, however it has a relatively high effective atomic number compared to tissue. Which lead to an overresponse compared to tissue for low energetic photons. The measured quantity in such dosemeters is called the personal dose equivalent, Hp(10), which is defined as the equivalent dose to tissue at a depth of 10 mm. Hp(10) can be calculated by adding filters to the dosemeter holder. Furthermore, the International Electrotechnical Commission (IEC) has specified recommendations of maximum under- and overestimation of the absorbed dose for personal doseme- ters that are commercially available. In mammography, contrast detail phantoms are used for QA of the clinical image. Com- mercially available phantoms are quite expensive, and not easily customisable for various research tasks. Hence, it is of interest to investigate the possible uses of a cost-effective 3D printer to create QA and research phantoms. Methods: The materials that were investigated included PLA (polylactic acid), PMMA (Polymethyl methacrylate), TPU 95A (Thermoplastic polyurethane), Varishore TPU, PVA (Polyvinyl alcohol), and a PLA mixture of wood. Two different filling patterns were explored which result in two different printing patterns. Furthermore, the printing temperature of Varishore TPU and filling density of PMMA was investigated. In total, 18 cubes were printed. The printed cubes were imaged using a CT system to determine the Zeff. A personal dosemeter was designed based on tests of the light transparency of the fila- ments. The dosemeter holder was constructed with four slots, where one of the slots was designed to measure Hp(10). The dosemeter was irradiated at angles of ±45° and 0° at energies of 40 keV and 662 keV for investigating the angular dependence. For mammography, a low contrast detail phantom was designed using the same specifi- cations as a commercially available phantom. The prototype phantom was printed with two different filling methods that were imaged using a clinical mammography system. Contrast to noise ratio (CNR) was calculated and compared to the conventional phantom and the accuracy of the disk diameter was assessed. Results and conclusion: There were no difference in term of HU and Zeff for the two different filling patterns. HU of tough PLA, PVA, PMMA and wooden filaments are within trabecular bone while Varishore TPU has a HU value similar to lung tissue. All 18 cubes had Zeff within the range 6.2≤ Zeff ≤6.8. TPU 95A showed some inconsistency in term of HU that need to be further investigated. The angular dependence of the dosemeter holder was within IEC recommendations. However, Monte Carlo simulations are needed to determine proper filtering for the different slots. The 3D printed low contrast-detail phantom show roughly the same average CNR as the conventional phantom. However, the accuracy of the printer, using the current settings, is limited for disk objects with disk diameters below 1.78 mm. Thus, further benchmarking should be done for improving this accuracy and use a smaller printer nozzle. This MSc covers a broad field which demonstrates the strength of the 3D printer and that it is applicable in a wide range of fields.