Stationary molecules in well-defined internal states are of broad interest for physics and chemistry. In physics, this includes metrology, quantum computing and many-body quantum mechanics, whereas ...in chemistry, state-prepared molecular targets are of interest for uni-molecular reactions with coherent light fields, for quantum-state-selected bi-molecular reactions and for astrochemistry. Here, we demonstrate rotational ground-state cooling of vibrationally and translationally cold MgH+ ions, using a laser-cooling scheme based on excitation of a single rovibrational transition. A nearly 15-fold increase in the rotational ground-state population of the X 1Σ+ electronic ground-state potential has been obtained. The resulting ground-state population of 36.7±1.2% is equivalent to that of a thermal distribution at about 20 K. The obtained cooling results imply that cold molecular-ion experiments can now be carried out at cryogenic temperatures in room-temperature set-ups.
Background
The continued development of new radiotherapy techniques requires dosimetry systems that satisfy increasingly rigorous requirements, such as high sensitivity, wide dose range, and high ...spatial resolution. An emerging requirement is the ability to read out doses in three dimensions (3D) with high precision and spatial resolution. A few dosimetry systems with 3D capabilities are available, but their application in a clinical workflow is limited for various reasons, primarily originating from their chemical nature. The search for a 3D dosimetry system with potential for clinical implementation is thus ongoing.
Purpose
To demonstrate the capabilities of a novel optically‐stimulated‐luminescence (OSL)‐based 3D dosimetry system capable of measuring radiation doses in clinically relevant volumes.
Methods
A laser‐based readout system was used to measure dose distributions delivered by both photons and protons, utilizing the OSL from a 50×50×50$50\times 50\times 50$ mm 3$^3$ YSO:Ce crystal. A homogeneous treatment plan consisting of two opposing photon fields was used to establish an inhomogeneity correction map of the crystal response and demonstrated the accuracy and precision of the system. The crystal was additionally irradiated with a photon treatment plan consisting of three overlapping 10×10$10\times 10$ mm 2$^2$ fields delivered from different angles, and a proton treatment plan consisting of four pencil beams with energies 90 MeV (×2$\times 2$), 115 MeV, and 140 MeV. The system abilities were quantified by comparing the 3D‐resolved measurements to Monte Carlo simulations.
Results
The dose map reproducibility of the system was found to be within 2% including both statistical and systematic errors. The measurements yielded integrated doses from a volume of 50×50×40$50\times 50\times 40$ mm 3$^3$ with voxel volumes of just 0.28×0.28×0.50$0.28\times 0.28\times 0.50$ mm 3$^3$. An excellent agreement between the 3D‐resolved measurements and the simulations was found for both photon‐ and proton‐irradiation.
Conclusions
The capabilities of the devised system for measuring clinically relevant fields of photons and proton pencil beams within a clinically relevant volume were demonstrated. The system poses as a promising candidate for clinical applications, and enables future research in the field of OSL‐based tissue‐equivalent 3D dosimetry.
Most solid-state detectors, including 3D dosimeters, show lower signal in the Bragg peak than expected, a process termed quenching. The purpose of this study was to investigate how variation in ...chemical composition of a recently developed radiochromic, silicone-based 3D dosimeter influences the observed quenching in proton beams. The dependency of dose response on linear energy transfer, as calculated through Monte Carlo simulations of the dosimeter, was investigated in 60 MeV proton beams. We found that the amount of quenching varied with the chemical composition: peak-to-plateau ratios (1 cm into the plateau) ranged from 2.2 to 3.4, compared to 4.3 using an ionization chamber. The dose response, and thereby the quenching, was predominantly influenced by the curing agent concentration, which determined the dosimeter's deformation properties. The dose response was found to be linear at all depths. All chemical compositions of the dosimeter showed dose-rate dependency; however this was not dependent on the linear energy transfer. Track-structure theory was used to explain the observed quenching effects. In conclusion, this study shows that the silicone-based dosimeter has potential for use in measuring 3D-dose-distributions from proton beams.
A new deformable polydimethylsiloxane (PDMS) based dosimeter is proposed that can be cast in an anthropomorphic shape and that can be used for 3D radiation dosimetry of deformable targets. The new ...material has additional favorable characteristics as it is tissue equivalent for high-energy photons, easy to make and is non-toxic. In combination with dual wavelength optical scanning, it is a powerful dosimeter for dose verification of image gated or organ tracked radiotherapy with moving and deforming targets.
Internal organ motion and deformations may cause dose degradations in proton therapy (PT) that are challenging to resolve using conventional image-guidance strategies. This study aimed to investigate ...the potential of
using water-equivalent path length (WEPL) calculations to detect dose degradations occurring in PT.
. Proton ranges were estimated using WEPL calculations. Field-specific isodose surfaces in the planning CT (pCT), from robustly optimised five-field proton plans (opposing lateral and three posterior/posterior oblique beams) for locally advanced prostate cancer patients, were used as starting points. WEPLs to each point on the field-specific isodoses in the pCT were calculated. The corresponding range for each point was found in the repeat CTs (rCTs). The spatial agreement between the resulting surfaces in the rCTs (hereafter referred to as iso-WEPLs) and the isodoses re-calculated in rCTs was evaluated for different dose levels and Hausdorff thresholds (2-5 mm). Finally, the sensitivity and specificity of detecting target dose degradation (V95% < 95%) using spatial agreement measures between the iso-WEPLs and isodoses in the pCT was evaluated.
. The spatial agreement between the iso-WEPLs and isodoses in the rCTs depended on the Hausdorff threshold. The agreement was 65%-88% for a 2 mm threshold, 83%-96% for 3 mm, 90%-99% for 4 mm, and 94%-99% for 5 mm, across all fields and isodose levels. Minor differences were observed between the different isodose levels investigated. Target dose degradations were detected with 82%-100% sensitivity and 75%-80% specificity using a 2 mm Hausdorff threshold for the lateral fields.
. Iso-WEPLs were comparable to isodoses re-calculated in the rCTs. The proposed strategy could detect target dose degradations occurring in the rCTs and could be an alternative to a fully-fledged dose re-calculation to detect anatomical variations severely influencing the proton range.
Abstract
Background. The increasing complexity of radiotherapy (RT) has motivated research into three-dimensional (3D) dosimetry. In this study we investigate the use of 3D dosimetry with ...polymerizing gels and optical computed tomography (optical CT) as a verification tool for complex RT: dose painting and target tracking. Materials and Methods. For the dose painting studies, two dosimeters were irradiated with a seven-field intensity modulated radiotherapy (IMRT) plan with and without dose prescription based on a hypoxia image dataset of a head and neck patient. In the tracking experiments, two dosimeters were irradiated with a volumetric modulated arc therapy (VMAT) plan with and without clinically measured prostate motion and a third with both motion and target tracking. To assess the performance, 3D gamma analyses were performed between measured and calculated stationary dose distributions. Results. Gamma pass-rates of 95.3% and 97.3% were achieved for the standard and dose-painted IMRT plans. Gamma pass-rates of 91.4% and 54.4% were obtained for the stationary and moving dosimeter, respectively, while tracking increased the pass-rate for the moving dosimeter to 90.4%. Conclusions. This study has shown that the 3D dosimetry system can reproduce and thus verify complex dose distributions, also when influenced by motion.
Purpose:
Both temporal and thermal dependencies of the dose response have been observed in radiochromic dosimeters. As these dependencies may be influenced by the dose level, the present study ...investigates the temperature dependence during irradiation and the temporal change of the optical response following irradiation of radiochromic dosimeters at a range of doses.
Methods:
Cuvette samples of the PRESAGE™ radiochromic dosimeter were irradiated within a dose range of 0–10 Gy at irradiation temperatures within 5–35 °C and postirradiation storage within 6–30 °C. The optical response due to irradiation was measured using a standard spectrophotometer and the data were analyzed in terms of thermal and temporal change.
Results:
The initial dose response was linear over the applied dose range independent of irradiation temperature. However, the optical response to a specific dose increased exponentially with irradiation temperature corresponding to an activation energy of 0.114 ± 0.007 eV. The temporal change in dose response after irradiation consisted of an offset, an auto-oxidation rate with activation energy 0.84 ± 0.03 eV, and an initial exponential increase in optical response (1.6 ± 0.2 eV) followed by an exponential decrease in optical response (0.98 ± 0.08 eV). These contributions depended on both storage temperature and the dose given, leading to a nonlinear dose response with time at low storage temperatures and a high auto-oxidation rate at high storage temperatures.
Conclusions:
Thermal equilibration is important to the radiochromic dosimeter investigated due to an exponential change in dose response with irradiation temperature and a considerable postirradiation temporal change in response. For the dosimeter version investigated in this study, a compromise in storage temperature has to be made between increasing the nonlinearity of the dose response with time and inducing a high auto-oxidation rate.
Purpose:
The dose response of radiochromic dosimeters is based on radiation-induced chemical reactions and is thus likely to be thermally influenced. In this study we have therefore investigated the ...temperature dependence of the dose response for such dosimeters, regarding both irradiation and storage conditions.
Methods:
Dosimeter samples in cuvettes were irradiated to 5 Gy. The temperature for the different cuvettes during irradiation and post-irradiation storage was varied in the range of 3–30 °C in order to quantify the temperature dependence of the dosimeter response. The optical properties of the dosimeter samples were measured using a spectrophotometer before irradiation as well as at several times after irradiation to quantify the temporal variation of dose response (expressed as the optical density change induced by irradiation) as a function of storage temperature.
Results:
The measurements show considerable temperature dependencies of dose response both during irradiation and storage. Fit to an Arrhenius equation revealed an activation energy of 1.4 ± 0.2 eV for the variation in irradiation temperature, indicating a contribution from a thermally activated process. Variation in dose response at different storage temperatures showed an exponential increase with time followed by a decrease in optical density. Exponential Arrhenius fits to rate constants gave activation energies of 1.7 ± 0.2 eV for the increase in dose response and 2.3 ± 0.5 eV for the subsequent decrease, in this case dominated by thermally activated processes.
Conclusions:
Due to the exponential dependencies, stabilization of the dosimeter during irradiation at low temperatures (e.g., 5 °C) is preferable in clinical use to optimize the accuracy of the dose response. In addition, a low storage temperature is recommended in order to minimize the post-irradiation temporal change in dose response and thereby increase the post-irradiation stability of the dosimeter. The measurements in this study show that if the observed temperature and temporal dependencies are not considered, this could potentially deteriorate the accuracy of the dosimeter.
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