Time-dependent Dose Rate Research Articles

Isothermal thermoluminescence dating of speleothem growth – A case study from Bleßberg cave 2, Germany

Speleothems are a key archive of past climatic and environmental changes. 230Th/U dating is the most commonly used method to determine speleothem ages. However, incorporation of non-radiogenic thorium may hamper 230Th/U dating, and samples older than 600 ka also remain out-of-reach. Calcite exhibits a thermoluminescence (TL) signal at 280 °C with a high characteristic saturation dose, and provides significant potential to date carbonate samples over several million years. Hitherto, the application of TL dating for calcite has mainly been hindered by two factors: 1) a spurious TL signal occurring in the high temperature range, and 2) non-uniform dose rate due to U-series disequilibrium. Here we test an isothermal TL (ITL) dating method on a speleothem sample from Bleßberg cave 2, Germany. We show that the ITL signal measured at 240 °C can completely remove the 280 °C TL peak with a negligible TL contribution from the higher temperature range, thus reducing the influence from the spurious signal. The time-dependent dose rate variation can be simulated using the initial radioactivity of 238U, 234U, 230Th and their decay constants. We use the 230Th/U dating method to provide precise and accurate radiometric ages documenting that the speleothem grew between 425.5 ± 5.4 and 320.5 ± 9.7 ka. The ITL ages (421 ± 23 to 311 ± 23 ka) of four subsamples from the speleothem are consistent with the 230Th/U ages at isochronous sampling positions, showing the general reliability of the ITL dating method. ITL dating provides a pathway to construct chronologies for palaeoclimate reconstructions for speleothems beyond the range of the 230Th/U-method and for samples that are unsuitable for U-series dating methods.

Equivalent uniform RBE-weighted dose in eye plaque brachytherapy.

Brachytherapy for ocular melanoma is based on the application of eye plaques with different spatial dose nonuniformity, time-dependent dose rates and relative biological effectiveness (RBE). We propose a parameter called the equivalent uniform RBE-weighted dose (EUDRBE) that can be used for quantitative characterization of integrated cell survival in radiotherapy modalities with the variable RBE, dose nonuniformity and dose rate. The EUDRBE is applied to brachytherapy with 125I eye plaques designed by the Collaborative Ocular Melanoma Study (COMS). The EUDRBE is defined as the uniform dose distribution with RBE=1 that causes equal cell survival for a given nonuniform dose distribution with the variable RBE>1. The EUDRBE can be used for comparison of cell survival for nonuniform dose distributions with different RBE, because they are compared to the reference dose with RBE=1. The EUDRBE is applied to brachytherapy with 125I COMS eye plaques that are characterized by a steep dose gradient in tumor base-apex direction, protracted irradiation during time intervals of 3-8 days, and variable dose-rate dependent RBE with a maximum of about 1.4. The simulations are based on dose of 85Gy prescribed to the farthest intraocular extent of the tumor (tumor apex). To compute the EUDRBE in eye plaque brachytherapy and correct for protracted irradiation, the distributions of physical dose have been converted to non-uniform distributions of biologically effective dose (BED) to include the biological effects of sublethal cellular repair, Our radiobiological analysis considers the combined effects of different time-dependent dose rates, spatial dose non-uniformity, dose fractionation and different RBE and can be used to derive optimized dose regimens brachytherapy. Our simulations show that the EUDRBE increases with the prescription depths and the maximum increase may achieve 6% for the tumor height of 12mm. This effect stems from a steep dose gradient within the tumor that increases with the prescription depth. The simulations also show that the EUDRBE increase may achieve 12% with increasing the dose rate when implant duration decreases. The combined effect of dose nonuniformity and dose rate may change the EUDRBE up to 18% for the same dose prescription of 85Gy to tumor apex. The absolute dose range of 48-61Gy (RBE) for the EUDRBE computed using 4 or 5 fractions is comparable to the dose prescriptions used in stereotactic body radiation therapy (SBRT) with megavoltage X-rays (RBE=1) for different cancers. The tumor control probabilities in SBRT and eye plaque brachytherapy are very similar at the level of 80% or higher that support the hypothesis that the selected approximations for the EUDRBE are valid. The computed range of the EUDRBE in 125I COMS eye plaque brachytherapy suggests that the selected models and hypotheses are acceptable. The EUDRBE can be useful for analysis of treatment outcomes and comparison of different dose regimens in eye plaque brachytherapy.

Steady-State X-Ray Radiation-Induced Attenuation in Canonical Optical Fibers

The so-called canonical optical fibers (OFs) are samples especially designed to highlight the impact of some manufacturing process parameters on the radiation responses. Thanks to the results obtained on these samples, it is thus possible to define new procedures to better control the behaviors of OFs in radiation environments. In this article, we characterized the responses, under steady-state X-rays, of canonical samples representative of the most common fiber types differing by their core-dopants: pure silica, Ge, Al, and P. Their radiation-induced attenuation (RIA) spectra were measured online at both room temperature (RT) and liquid nitrogen temperature (LNT), in the energy range [~0.6-~3.0] eV (~2100-~410 nm), highlighting the RIA growth kinetics during the fiber exposure up to an accumulated dose of ~200 Gy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) at a constant dose rate of 100 mGy/s at RT. At LNT, the deposited doses varied between 100 and 180 Gy, with a time-dependent dose rate. In order to understand the origin of the excess losses and the difference between the RIA spectral shapes observed at the two temperatures, spectral decomposition of the optical losses has been performed using a set of Gaussian absorption bands related to the already known point defects. As a result, if the RIA in the visible domain is quite well understood, the knowledge of the RIA origin in the near-IR remains incomplete, justifying new and deeper studies to clarify the response of the fibers under steady-state irradiation.

Change is the only constant - time-dependent dose rates in luminescence dating

Abstract Time-dependent changes in dose rates generally need to be considered in luminescence dating due to the dynamic character of the geosphere. Nevertheless, this aspect is rather marginally treated in the literature. The present paper describes the methodology of including temporal variations of overburden, moisture content and radionuclide activity into the calculation of dating results. Consideration of possible occurrences of variable dose rates has consequences for fieldwork procedures as well as for the choice of appropriate methods of radionuclide determination. A program code for age calculation by luminescence methods called ADELE v2017 is presented which implements several examples of temporal dose rate variations and layered structures with inhomogeneous distribution of dose rate relevant elements. The impact of variable dose rates on age data is illustrated by applying ADELE v2017 to several case studies.

Biological-Effect Modeling of Radioimmunotherapy for Non-Hodgkins Lymphoma: Determination of Model Parameters

Treatment with Tositumomab and 131I tositumomab anti-CD20 radioimmunotherapy (Bexxar) yields a nonradioactive antibody antitumor response (the so-called cold effect) and a radiation response. Numerical parameter determination by least-squares (LS) fitting was implemented for more accurate parameter estimates in equivalent biological-effect calculations. One hundred thirty-two tumors in 37 patients were followed using five or six SPECT/CT studies per patient, three each (typical) post-tracer (0.2 GBq) and post-therapy (∼3 GBq) injections. The SPECT/CT data were used to calculate position- and time-dependent dose rates and antibody concentrations for each tumor. CT-defined tumor volumes were used to track tumor volume changes. Combined biological-effect and cell-clearance models were fit to tumor volume changes. Optimized parameter values determined using LS fitting were compared to previous fitted values that were determined by matching calculated to measured tumor volume changes using visual assessment. Absorbed dose sensitivity (α) and cold-effect sensitivity (λp) parameters were the primary fitted parameters, yielding equivalent biological-effect (E) values. Individual parameter uncertainties were approximately 10% and 30% for α and λp, respectively. LS versus previously fit parameter values were highly correlated, although the averaged α value decreased and the averaged λp value increased for the LS fits compared to the previous fits. Correlation of E with 2-month tumor shrinkage data was similar for the two fitting techniques. The LS fitting yielded improved fit quality and likely improved parameter estimation.

Effect of variable dose rate on biologically effective dose

Purpose: To investigate the effect of variable dose rate on biologically effective dose (BED).Materials and methods: By using the linear-quadratic (LQ) model with bi-exponential repair, we analytically determine the time-dependent dose rate which minimizes the effective protraction factor (Geff) and BED under the condition of fixed fraction time and dose per fraction. Because normal tissue complication probability (NTCP) monotonically decreases with decreasing BED, the dose rate also minimizes NTCP.Results: The dependences of Geff , BED and NTCP on fraction time were determined for different radiobiological parameters and two different dose rates: constant dose rate R0 and varying dose rate .Conclusion: The results of this study indicate that under certain conditions the reduction in BED for late-responding tissues due to increased fraction time can be significantly greater than the reduction in BED for tumors. Our analysis also indicates that dose rate optimization can be radiobiologically beneficial because of the resulting decrease in NTCP.

Radiation measurements around X-ray cabinet systems

Security personnel who operate X-ray units for the control of hand luggage and personal items at airports are generally not under dosimetric surveillance. A significant increase in the number of inspected items per passenger, due to rigorous air traffic security measures, raises a question of extended exposure of these workers to scattered X-ray radiation. A new approach to investigating directions of breaches of scattered X-ray radiation in the area near to an X-ray cabinet system, which is based on using active electronic dosemeters is presented. Influence of the increase in the number of inspected items in time on the dose rate is described. Time-dependent dose rates have showed a very good correlation with passengers undergoing security control prior to boarding an airplane. Measurements confirmed that an increase in the dose rate, coinciding with rush hours, was caused by scattered radiation passing through incompletely closed lead curtains. It is found that the doses at the entrance to the inspection tunnel are 50% higher than those at the exit, which is a consequence of inherent operational characteristics of X-ray cabinet systems.

Estimates of Radiation Exposures for Human Crews in Deep Space from the January 15, 2005, Solar Energetic Particle Event Using the Earth-Moon-Mars Radiation Environment Module

The Earth-Moon-Mars Radiation Environment Module (EMMREM) is a numerical model for characterizing the time-dependent radiation environment in the Earth-Moon-Mars and interplanetary space environments. In this work we demonstrate the capabilities of the module for performing analyses of time-dependent exposures from solar energetic particle (SEP) events near Earth and Mars by calculating time-dependent dose rates, dose equivalent rates, and accumulated dose and accumulated dose equivalents for surrogates of the skin and the blood forming organs (BFOs) of crew members shielded by as much as 10 g/cm2 of aluminum shielding for the January 15, 2005, SEP event. The motivation for the development of EMMREM is the need to better understand the radiation hazards in deep space and near Earth and other planetary bodies, in near real time in support of possible future space exploration by manned and unmanned spacecraft. Characterizing the radiation environment for different locations on and close to Earth for SEP events is fairly well developed. However, estimating the probable radiation environment near Mars and other locations throughout the solar system is not currently supported for SEP events. Such capability is critical for future human exploration of the Moon and Mars in the upcoming decades. The calculated doses for the skin and BFO surrogates are compared with the National Aeronautics and Space Administration’s short-term permissible exposure limits.

Grundlagen der patientenspezifischen Dosimetrie bei Radionuklidtherapien

A prerequisite for the risk assessment in targeted radiotherapy is the knowledge of the patient-specific absorbed dose in critical organs as the biological effect is determined by the absorbed dose. In addition, one needs to take potential inhomogeneous spatial activity distributions, time-dependent dose rates and the energies and emission probabilities of the emitted particles into account. Radiation biology effects should not be neglected for therapy planning. Combining all these factors leads to a reliable dose-effect relationship which is mandatory for the further development of targeted radiotherapy. In the past the absorbed dose was calculated based upon the administered activity, the biokinetics and upon simplified human models. Today, nuclear medicine imaging provides the opportunity to determine, for many organs, the biokinetics of radioactive labelled pharmaceuticals patient-specifically. Recently, many patient-specific methods have been developed for determining the absorbed dose to critical organs. In the present work several methods for the assessment and the critical appraisal of the absorbed dose as well as radio-biological concepts such as the biological effective dose (BED) for therapy planning will be introduced and discussed.

SU-GG-T-409: Equivalent Uniform Dose Calculation for I-131 Tositumomab Therapy Including the Cold Protein Effect

Purpose: Individualized treatment planning may benefit from equivalent uniform dose (EUD) techniques to help correlate ‘dose’ with objective patient outcome such as time to recurrence or response rate. Our objective is to illustrate methods used to perform EUD calculations, specifically to incorporate the cold protein effect for non-Hodgkins lymphoma (NHL) patients treated with I-131 tositumomab. Method and Materials: Data from NHL patients treated with I-131 tositumomab were imaged multiple times for tracer and therapy studies using a SPECT/CT system. Dose rate calculations were performed by Monte Carlo technique for each image set. A 3D time dependent dose rate description was developed using measured uptake curves for the whole body and tumor volume(s). Tumor subunits were connected between time points using center-of-mass alignment and radial deformation. The , where 〈S〉min is the minimum average relative cell survival. , where the biological equivalent dose for each subvolume; D is the dose; and the λt and λp represent the effects of proliferation and cold protein (P). Results: Parameter fits were performed to match changes in tumor size. The tracer study was used to set λp based on patient-specific tumor response. The λt parameter was set by typical time to progression-free recurrence rates. The linear-quadratic parameters were set consistent with hypersensitivity at low dose rates (α=0.7 Gy−1). EUD was calculated contrasting patients for which there was no cold protein effect with substantial cold protein effect. A common parameter set was used to fit the data, only varying by the cold effect parameter. Conclusion: The cold protein effect can be substantial and is being included in the EUD calculation to help correlate delivered dose with patient outcome. Research Support: NIH 2R01 EB001994.

Optically stimulated luminescence dating of sublittoral and intertidal sediments from Dubai, UAE: Radioactive disequilibria in the uranium decay series

Shallow marine and intertidal shell rich sediments from the low coastal plain of Dubai were dated by optically stimulated luminescence. Combined high-resolution gamma spectrometry and ICP–MS analyses revealed significant disequilibria in the 238U decay series. Time dependent dose rates were modelled for two scenarios including a closed system and an open system with linear U and Ra uptake. OSL ages that were calculated using the linear uptake model are in good agreement with archaeological evidence and radiocarbon datings on shells and wood. The presented results illustrate that luminescence-dating techniques are suitable to set up a chronostratigraphic frame for calcareous coastal environments if an appropriate model is applied to consider the influence of radioactive disequilibria and time dependent dose rates. A comprehensive radionuclide analysis of the sediments is thought necessary to perform geochemically supported dose rate simulations.

A diversity of responses displayed by a stochastic model of radiation carcinogenesis allowing for cell death

A stochastic model is presented of carcinogenesis induced by irradiation with arbitrary time-dependent dose rate. The key feature of the model is that it allows for radiation-induced cell killing to compete with the process of tumor promotion. Two versions of the model arise when considering target tissues with slow and rapid replacement of damaged cells. These versions show dissimilar shapes of the dose-response curves in the case of short-term exposure. The model provides a natural explanation of the basic experimental findings documented in the radiobiological literature.

Biological repair with time-dependent irradiation.

Recent experiments have provided new data that explore the effectiveness of biological repair in assessing damage due to exposures from ionizing radiation. These data are mainly from experiments conducted at constant dose rates, to study the effectiveness per unit dose of different dose rates. Here, we develop new formulae to estimate the effectiveness of an arbitrary time-dependent dose rate exposure.

Comparison of ESR ages of corals from marine terraces with 14C and 230Th/ 234U ages

The ages of corals from marine terraces in the Ryukyu Islands of Japan have been determined by electron spin resonance (ESR). ESR ages deduced from the total dosage of natural radiation taking the time-dependent dose rate due to 238 U-series disequilibrium into account agreed fairly well with those determined by 14 C and 230 Th/ 234 U dating. The ESR age can be extended, well beyond the limits of 14 C dating (50,000 years) and the 230 Th/ 234 U method (300,000 years), to a million years.