Pretreatment intensity-modulated radiotherapy quality assurance is performed using simple rectangular or cylindrical phantoms; thus, the dosimetric errors caused by complex patient-specific anatomy are absent in the evaluation objects. In this study, we construct a system for generating patient-specific three-dimensional (3D)-printed phantoms for radiotherapy dosimetry. An anthropomorphic head phantom containing the bone and hollow of the paranasal sinus is scanned by computed tomography (CT). Based on surface rendering data, a patient-specific phantom is formed using a fused-deposition-modeling-based 3D printer, with a polylactic acid filament as the printing material. Radiophotoluminescence glass dosimeters can be inserted in the 3D-printed phantom. The phantom shape, CT value, and absorbed doses are compared between the actual and 3D-printed phantoms. The shape difference between the actual and printed phantoms is less than 1 mm except in the bottom surface region. The average CT value of the infill region in the 3D-printed phantom is -6 ± 18 Hounsfield units (HU) and that of the vertical shell region is 126 ± 18 HU. When the same plans were irradiated, the dose differences were generally less than 2%. These results demonstrate the feasibility of the 3D-printed phantom for artificial in vivo dosimetry in radiotherapy quality assurance.
Measurement of the precise position of a high activity iridium-192 (192Ir) source during high-dose-rate (HDR) brachytherapy is desired to detect the mispositioning of the source and avoid incidents. Although real-time imaging of the source position using a high-energy gamma camera is a possible method for this purpose, electronics are saturated at small distances from the source because the 192Ir source activity is so high. To solve this problem, we developed a low-sensitivity high-resolution gamma camera. We used a 1-mm-thick cerium-doped yttrium aluminum perovskite (YA1O3: YAP(Ce)) scintillator plate to reduce sensitivity. The developed low-sensitivity gamma camera consists of this thin YAP(Ce) scintillator optically coupled to a flat panel photomultiplier tube (FP-PMT). It is encased in a 20-mm-thick tungsten shield with a 0.5-mm diameter pinhole collimator mounted on its head. The spatial resolution of the gamma camera at 100 mm from the 192Ir source was 3.3 mm FWHM and the sensitivity was 0.52 cps/MBq. The count rate of the camera was ∼180-k cps at 100 mm from the 212.4-GBq 192Ir source and real-time imaging of the 192Ir source was possible. With a 100-mm-thick water phantom positioned between the gamma camera and the 192Ir source, we decreased the count rate to half, but the 192Ir source images could be observed clearly. We conclude that the developed low-sensitivity high-resolution gamma camera system has the potential to be a new real-time imaging system for HDR brachytherapy.
Intraoperative electron radiotherapy (IOERT), which is an accelerated partial breast irradiation method, has been used for early-stage breast cancer treatment. In IOERT, a protective disk is inserted behind the target volume to minimize the dose received by normal tissues. However, to use such a disk, the surgical incision must be larger than the field size because the disk is manufactured from stiff and unyielding materials. In this study, the applicability of newly developed tungsten-based functional paper (TFP) was assessed as an alternative to the existing protective disk. The radiation-shielding performance of the TFP was verified through experimental measurements and Monte Carlo simulations. Percentage depth dose curves and lateral dose profiles with and without TFPs were measured and simulated on a dedicated IOERT accelerator. The number of piled-up TFPs was changed from 1 to 40. In the experimental measurements, the relative doses at the exit plane of the TFPs for 9 MeV were 42.7%, 9.2%, 0.2%, and 0.1% with 10, 20, 30, and 40 TFPs, respectively, whereas those for 12 MeV were 63.6%, 27.1%, 8.6%, and 0.2% with 10, 20, 30, and 40 TFPs, respectively. Slight dose enhancements caused by backscatter radiation from the TFPs were observed at the entrance plane of the TFPs at both beam energies. The results of the Monte Carlo simulation indicated the same tendency as the experimental measurements. Based on the experimental and simulated results, the radiation-shielding performances of 30 TFPs for 9 MeV and 40 TFPs for 12 MeV were confirmed to be acceptable and close to those of the existing protective disk. The findings of this study suggest the feasibility of using TFPs as flexible chest wall protectors in IOERT for breast cancer treatment.
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