Dosimetric properties of a newly developed thermoluminescent sheet-type dosimeter for clinical proton beams.

PURPOSE: This study aimed to evaluate the dosimetric properties of a newly developed thermoluminescent sheet-type dosimeter (TLD-sheet) for clinical proton beams. MATERIALS AND METHODS: The TLD-sheet is composed mainly of manganese doped lithium triborate, with a physical size and thickness of 150 mm × 150 mm and 0.15 mm respectively. It is flexible and can be cut freely for usage. The TLD-sheet has an effective atomic number of 7.3 and tissue-equivalent properties. We tested the reproducibility, fading effect, dose linearity, homogeneity, energy dependence, and water equivalent thickness (WET) of the TLD-sheet for clinical proton beams. We conducted tests with both unmodulated and modulated proton beams at energies of 150 and 210 MeV. RESULTS: The measurement reproducibility was within 4%, which included the inhomogeneity of the TLD-sheet. The fading rates were approximately 20% and 30% after 2 and 7 days respectively. The TLD-sheet showed notable energy dependence in the Bragg peak and distal end of the spread-out Bragg peak regions. However, the dose-response characteristics of the TLD-sheet remained linear up to a physical dose of 10 Gy in this study. This linearity was highly superior to those of commonly used radiochromic film. The thin WET of the TLD-sheet had little effect on the range. CONCLUSION: Although notable energy dependences were observed in Bragg peak region, the response characteristics examined in this study, such as reproducibility, fading effects, dose linearity, dose homogeneity and WET, showed that the TLD-sheet can be a useful and effective dosimetry tool. With its flexible and reusable characteristics, it may also be an excellent in vivo skin dosimetry tool for proton therapy.

End-to-end test to evaluate the comprehensive geometric accuracy of a proton rotating gantry using a cone-shaped scintillator screen detector.

In this study, we aim to evaluate the comprehensive geometric accuracy of proton rotating gantries by performing an end-to-end test using a cone-shaped scintillator screen detector, known as XRV-124. The XRV-124 comprises a cone-shaped sheet-like scintillator and charge-coupled device camera that detects the scintillation light. First, the results of the Winston-Lutz and end-to-end XRV-124 tests performed on a conventional linear accelerator were compared to confirm the reliability of the XRV-124, and the snout position dependency of the geometric accuracy was evaluated for the proton rotating gantry as a pre-verification process. Thereafter, an end-to-end test including computed tomography imaging and irradiation in 30° steps from 0° to 330° for two proton rotating gantries, which have the same specifications, was performed. The results of the pre-verification indicated that sufficient accuracy was obtained for the end-to-end test of the proton rotating gantry. The end-to-end test results showed a peak-to-peak deviation of up to 2 mm for some of the coordinate axes. The two gantries exhibited almost similar results in terms of the absolute quantity; however, a few trends were different. Thus, the beam axis deviations were confirmed to be within the safety margin, as expected in clinical practice. Based on the results of this study, the XRV-124 can be used as a comprehensive end-to-end constancy test tool, as it enables a comparative verification of multiple rotating gantries and geometric accuracy verification of different treatment modalities.

A new concept for verifying the isocentric alignment of the proton-rotational gantry for radiation control.

The purpose of this study was to introduce the modified Winston-Lutz (mWL) test and to evaluate its feasibility. This is a new method to completely absorb the proton beam around the isocenter inside a phantom for radiation control. The mWL test was performed using a 14-cm-diameter acrylic Lucy 3D QA Phantom for a passive-scattering proton beam gantry. The energy of the unmodulated proton beam was adjusted such that the residual range was < 130 mm, and the energy of the proton beam was completely lost around the isocenter. The radiation field was formed with a multi-leaf collimator at 8.6 × 8.6 mm2 in the isocenter plane. The phantom was loaded with a 4-mm-diameter tungsten ball, and the EBT3 was set up at the isocenter. The proton beam was irradiated at gantry angles with 45° steps, and the isocenter deviation of the proton beam was measured and subsequently analyzed. Although the radiation field penumbra was blurred under the influence of scattered radiation due to placement in the phantom compared to the traditional WL test placed in the air, evaluation of the beam axis accuracy was possible. The results confirmed that the maximum total displacement was less than 0.9 mm, and the specifications of the device were met. The mWL test is feasible and effective to reduce the building activation in proton beam treatment facilities. Thus, it can be considered a useful method that sufficiently satisfies the shielding calculation conditions.

Patient-specific quality assurance for proton depth dose distribution using a multi-layer ionization chamber in a single-ring wobbling method.

The use of a multi-layer ionization chamber, Zebra, in patient-specific quality assurance (QA) for proton depth dose distributions in a single-ring wobbling method is investigated. The depth dose distributions measured using Zebra are compared with those calculated using the treatment planning system (TPS), XiO-M, and measured using an ionization chamber with a motorized water phantom system. Because the TPS only provides point doses, the average doses are calculated using in-house software. The detector size-corrected depth dose distributions are obtained by determining the average of the dose distributions from the TPS over a cylindrical region similar to the size of the Zebra detectors. The calculated depth dose distributions from the cases with a simple compensator shape are in good agreement with those obtained from the TPS without performing volume averaging; however, a 15% difference was shown when compared with those from the cases with a complex compensator shape. Then, the measurements are compared with the detector size-corrected depth dose distributions, showing an improved agreement within 3% for the highly steep dose gradient regions. Although there are some field size limitations, the Zebra system is a useful device for the fast measurement of patient-specific QA for depth dose distributions in wobbled proton beams. However, careful consideration is required for complex dose distribution fields, because the measurements obtained using Zebra cannot be directly compared to the depth dose distributions from the TPS owing to the finite detector size of the Zebra chamber.

New isoreticular metal-organic framework materials for high hydrogen storage capacity.

We propose new isoreticular metal-organic framework (IRMOF) materials to increase the hydrogen storage capacity at room temperature. Based on the potential-energy surface of hydrogen molecules on IRMOF linkers and the interaction energy between hydrogen molecules, we estimate the saturation value of hydrogen sorption capacity at room temperature. We discuss design criteria and propose new IRMOF materials that have high gravimetric and volumetric hydrogen storage densities. These new IRMOF materials may have gravimetric storage density up to 6.5 wt % and volumetric storage density up to 40 kg H2/m3 at room temperature.

Binding energies of hydrogen molecules to isoreticular metal-organic framework materials.

Recently, several novel isoreticular metal-organic framework (IRMOF) structures have been fabricated and tested for hydrogen storage applications. To improve our understanding of these materials, and to promote quantitative calculations and simulations, the binding energies of hydrogen molecules to the MOF have been studied. High-quality second-order Moller-Plesset (MP2) calculations using the resolution of the identity approximation and the quadruple zeta QZVPP basis set were used. These calculations use terminated molecular fragments from the MOF materials. For H2 on the zinc oxide corners, the MP2 binding energy using Zn4O(HCO2)6 molecule is 6.28 kJ/mol. For H2 on the linkers, the binding energy is calculated using lithium-terminated molecular fragments. The MP2 results with coupled-cluster singles and doubles and noniterative triples method corrections and charge-transfer corrections are 4.16 kJ/mol for IRMOF-1, 4.72 kJ/mol for IRMOF-3, 4.86 kJ/mol for IRMOF-6, 4.54 kJ/mol for IRMOF-8, 5.50 and 4.90 kJ/mol for IRMOF-12, 4.87 and 4.84 kJ/mol for IRMOF-14, 5.42 kJ/mol for IRMOF-18, and 4.97 and 4.66 kJ/mol for IRMOF-993. The larger linkers are all able to bind multiple hydrogen molecules per side. The linkers of IRMOF-12, IRMOF-993, and IRMOF-14 can bind two to three, three, and four hydrogen molecules per side, respectively. In general, the larger linkers have the largest binding energies, and, together with the enhanced surface area available for binding, will provide increased hydrogen storage. We also find that adding up NH2 or CH3 groups to each linker can provide up to a 33% increase in the binding energy.

Computational study of hydrogen binding by metal-organic framework-5.

We report the results of quantum chemistry calculations on H(2) binding by the metal-organic framework-5 (MOF)-5. Density functional theory calculations were used to calculate the atomic positions, lattice constant, and effective atomic charges from the electrostatic potential for the MOF-5 crystal structure. Second-order Møller-Plesset perturbation theory was used to calculate the binding energy of H(2) to benzene and H(2)-1,4-benzenedicarboxylate-H(2). To achieve the necessary accuracy, the large Dunning basis sets aug-cc-pVTZ, and aug-cc-pVQZ were used, and the results were extrapolated to the basis set limit. The binding energy results were 4.77 kJ/mol for benzene, 5.27 kJ/mol for H(2)-1,4-benzenedicarboxylate-H(2). We also estimate binding of 5.38 kJ/mol for Li-1,4-benzenedicarboxylate-Li and 6.86 kJ/mol at the zinc oxide corners using second-order Møller-Plesset perturbation theory. In order to compare our theoretical calculations to the experimental hydrogen storage results, grand canonical Monte Carlo calculations were performed. The Monte Carlo simulations identify a high energy binding site at the corners that quickly saturated with 1.27 H(2) molecules at 78 K. At 300 K, a broad range of binding sites are observed.