TLD efficiency calculations with heavy ions

Thermoluminescence dosimeters (TLDs) are solid state detectors widely used in conventional radiation detection and dose verification. The development of ion beam cancer therapy, and the research in radioprotection in space, stimulated the use of the TLDs for heavy ions dosimetry. The main advantages of this kind of detector, compared, e.g., to ionization chambers, are the small dimensions, ease of handling, no interference on the radiation field and the usability in solid state phantoms. However the response of TLDs with dose is non-linear. It can be supralinear and is affected by saturation effects as well. The response of these detectors when irradiated with particle beams depends also strongly on the quality of the radiation. For this reason, in order to use TLDs with particle beams, and specifically to get a prediction of their response in a treatment plan, a model that can reproduce the behavior of these detectors in different conditions is needed. In literature, several models describing the TLDs behavior already exist and in this work we start briefly introducing some of them. In particular, we focus on an extension of `local effect model' (LEM). This model stems from an amorphous track structure model and assumes the knowledge of both the radial dose distribution around heavy ion trajectories and the detector response to reference radiation. Even though the LEM was originally developed for predicting the response of biological systems following ion irradiation, it can also be extended for efficiency calculations of solid state detectors, such as TLD. In this context a new, simple and completely analytical algorithm for the calculation of the efficiency dependence on ion charge Z and energy E has been developed. The response of the whole detector has been evaluated starting from the response to a single ion of the beam. The dose contributions coming from neighbouring tracks are assumed to add up linearly. This approach is realistic in a low dose approximation, but we nevertheless analized its limits of validity. Its main advantage is that, being fully analytical, it is computationally fast and can be efficiently integrated in treatment planning verification tools. Furthermore, it is robust against modifications of the radial dose distribution of a single ion in the detector, as well as to different detector response models as we critically evaluated in our work. The calculated values of the efficiency have been compared with experimental data, as well as to other calculated values provided by different approaches. Moreover, after implementing our model data in the treatment planning code TRiP98, we performed signal calculations on macroscopic target irradiated with an extended Carbon ion field. Also these results were compared with recent experimental measurements as well as with alternative calculations. The results, both for pure efficiency calculations and for their propagation in macroscopic dose response prediction, achieve a level of accuracy which is comparable to previous calculations, while needing a much lighter computational effort

Nanoscale insights on hypoxia radiosensitization with ion beams

Tumors with a nonuniform oxygen distribution show also an inhomogeneous radiosensitivity. In particular, the hypoxic regions results to be more radioresistant, limiting the efficacy of radiotherapy. It has been observed that high linear energy transfer, LET, radiation can counteract this effect to a certain extent, suggesting ion beam therapy as one of the most promising strategies to treat hypoxic tumors. On the nanoscale, the oxygen effect is assumed to be related to the indirect action of radiation. Several theories exist that aim to provide an explanation of the nature of this effect and its LET dependence, on the radiation chemistry. However, a mechanistic description is still missing and little is known about the indirect action and the chemical processes taking place along an ion track. In this work, the Monte Carlo particle track structure code TRAX has been extended to the pre-chemical and chemical stage of the radiation effect and is now able to simulate the chemical evolution of the most important products of water radiolysis under different irradiation conditions and target oxygenation levels. The validity of the model has been verified by comparing the calculated time and LET-dependent yields of the different radiolytic species to experimental data and other simulation approaches. As an example of the application of the newly implemented TRAX-CHEM code, a study on the dose enhancement effect and radical enhancement effect of gold nanoparticles has been performed under varying irradiation conditions and oxygenation levels. This will contribute to the basic understanding of still unsolved mechanisms for nanoparticle sensitization

Dose Limits and Countermeasures for Mitigating Radiation Risk in Moon and Mars Exploration

After decades of research on low-Earth orbit, national space agencies and private entrepreneurs are investing in exploration of the Solar system. The main health risk for human space exploration is late toxicity caused by exposure to cosmic rays. On Earth, the exposure of radiation workers is regulated by dose limits and mitigated by shielding and reducing exposure times. For space travel, different international space agencies adopt different limits, recently modified as reviewed in this paper. Shielding and reduced transit time are currently the only practical solutions to maintain acceptable risks in deep space missions

TRAX-CHEMxt: Towards the Homogeneous Chemical Stage of Radiation Damage

The indirect effect of radiation plays an important role in radio-induced biological damages. Monte Carlo codes have been widely used in recent years to study the chemical evolution of particle tracks. However, due to the large computational efforts required, their applicability is typically limited to simulations in pure water targets and to temporal scales up to the µs. In this work, a new extension of TRAX-CHEM is presented, namely TRAX-CHEMxt, able to predict the chemical yields at longer times, with the capability of exploring the homogeneous biochemical stage. Based on the species coordinates produced around one track, the set of reaction–diffusion equations is solved numerically with a computationally light approach based on concentration distributions. In the overlapping time scale (500 ns–1 µs), a very good agreement to standard TRAX-CHEM is found, with deviations below 6% for different beam qualities and oxygenations. Moreover, an improvement in the computational speed by more than three orders of magnitude is achieved. The results of this work are also compared with those from another Monte Carlo-based algorithm and a fully homogeneous code (Kinetiscope). TRAX-CHEMxt will allow for studying the variation in chemical endpoints at longer timescales with the introduction, as the next step, of biomolecules, for more realistic assessments of biological response under different radiation and environmental conditions

Range margin reduction in carbon ion therapy: potential benefits of using radioactive ion beams

Radiotherapy with heavy ions, in particular, 12C beams, is one of the most advanced forms of cancer treatment. Sharp dose gradients and high biological effectiveness in the target region make them an ideal tool to treat deep-seated and radioresistant tumors, however, at the same time, sensitive to small errors in the range prediction. Safety margins are added to the tumor volume to mitigate these uncertainties and ensure its uniform coverage, but during the irradiation they lead to unavoidable damage to the surrounding healthy tissue. To fully exploit the benefits of a sharp Bragg peak, a large effort is put into establishing precise range verification methods for the so-called image-guided radiotherapy. Despite positron emission tomography being widely in use for this purpose in 12C ion therapy, the low count rates, biological washout, and broad shape of the activity distribution still limit its precision to a few millimeters. Instead, radioactive beams used directly for treatment would yield an improved signal and a closer match with the dose fall-off, potentially enabling precise in vivo beam range monitoring. We have performed a treatment planning study to estimate the possible impact of the reduced range uncertainties, enabled by radioactive 11C beams treatments, on sparing critical organs in the tumor proximity. We demonstrate that (i) annihilation maps for 11C ions can in principle reflect even millimeter shifts in dose distributions in the patient, (ii) outcomes of treatment planning with 11C beams are significantly improved in terms of meeting the constraints for the organs at risk compared to 12C plans, and (iii) less severe toxicities for serial and parallel critical organs can be expected following 11C treatment with reduced range uncertainties, compared to 12C treatments

Observation of dose-rate dependence in a Fricke dosimeter irradiated at low dose rates with monoenergetic X-rays

<p>Absolute measurements of the radiolytic yield of Fe3+ in a ferrous sulphate dosimeter formulation (6 mM Fe2+), with a 20 keV x-ray monoenergetic beam, are reported. Dose-rate suppression of the radiolytic yield was observed at dose rates lower than and different in nature to those previously reported with x-rays. We present evidence that this effect is most likely to be due to recombination of free radicals radiolytically produced from water. The method used to make these measurements is also new and it provides radiolytic yields which are directly traceable to the SI standards system. The data presented provides new and exacting tests of radiation chemistry codes.</p

In-situ estimation of ice crystal properties at the South Pole using LED calibration data from the IceCube Neutrino Observatory

The IceCube Neutrino Observatory instruments about 1 km3 of deep, glacial ice at the geographic South Pole using 5160 photomultipliers to detect Cherenkov light emitted by charged relativistic particles. A unexpected light propagation effect observed by the experiment is an anisotropic attenuation, which is aligned with the local flow direction of the ice. Birefringent light propagation has been examined as a possible explanation for this effect. The predictions of a first-principles birefringence model developed for this purpose, in particular curved light trajectories resulting from asymmetric diffusion, provide a qualitatively good match to the main features of the data. This in turn allows us to deduce ice crystal properties. Since the wavelength of the detected light is short compared to the crystal size, these crystal properties do not only include the crystal orientation fabric, but also the average crystal size and shape, as a function of depth. By adding small empirical corrections to this first-principles model, a quantitatively accurate description of the optical properties of the IceCube glacial ice is obtained. In this paper, we present the experimental signature of ice optical anisotropy observed in IceCube LED calibration data, the theory and parametrization of the birefringence effect, the fitting procedures of these parameterizations to experimental data as well as the inferred crystal properties.</p

Dose Limits and Countermeasures for Mitigating Radiation Risk in Moon and Mars Exploration

After decades of research on low-Earth orbit, national space agencies and private entrepreneurs are investing in exploration of the Solar system. The main health risk for human space exploration is late toxicity caused by exposure to cosmic rays. On Earth, the exposure of radiation workers is regulated by dose limits and mitigated by shielding and reducing exposure times. For space travel, different international space agencies adopt different limits, recently modified as reviewed in this paper. Shielding and reduced transit time are currently the only practical solutions to maintain acceptable risks in deep space missions

Radioactive Beams for Image-Guided Particle Therapy : The BARB Experiment at GSI

Several techniques are under development for image-guidance in particle therapy. Positron (beta(+)) emission tomography (PET) is in use since many years, because accelerated ions generate positron-emitting isotopes by nuclear fragmentation in the human body. In heavy ion therapy, a major part of the PET signals is produced by beta(+)-emitters generated via projectile fragmentation. A much higher intensity for the PET signal can be obtained using beta(+)-radioactive beams directly for treatment. This idea has always been hampered by the low intensity of the secondary beams, produced by fragmentation of the primary, stable beams. With the intensity upgrade of the SIS-18 synchrotron and the isotopic separation with the fragment separator FRS in the FAIR-phase-0 in Darmstadt, it is now possible to reach radioactive ion beams with sufficient intensity to treat a tumor in small animals. This was the motivation of the BARB (Biomedical Applications of Radioactive ion Beams) experiment that is ongoing at GSI in Darmstadt. This paper will present the plans and instruments developed by the BARB collaboration for testing the use of radioactive beams in cancer therapy.Peer reviewe

Impact of Target Oxygenation on the Chemical Track Evolution of Ion and Electron Radiation

The radiosensitivity of biological systems is strongly affected by the system oxygenation. On the nanoscopic scale and molecular level, this effect is considered to be strongly related to the indirect damage of radiation. Even though particle track radiolysis has been the object of several studies, still little is known about the nanoscopic impact of target oxygenation on the radical yields. Here we present an extension of the chemical module of the Monte Carlo particle track structure code TRAX, taking into account the presence of dissolved molecular oxygen in the target material. The impact of the target oxygenation level on the chemical track evolution and the yields of all the relevant chemical species are studied in water under different irradiation conditions: different linear energy transfer (LET) values, different oxygenation levels, and different particle types. Especially for low LET radiation, a large production of two highly toxic species (HO2• and O•− 2), which is not produced in anoxic conditions, is predicted and quantified in oxygenated solutions. The remarkable correlation between the HO2• and O•− 2 production yield and the oxygen enhancement ratio observed in biological systems suggests a direct or indirect involvement of HO2• and O•− 2 in the oxygen sensitization effect. The results are in agreement with available experimental data and previous computational approaches. An analysis of the oxygen depletion rate in different radiation conditions is also reported. The radiosensitivity of biological systems is strongly affected by the system oxygenation. On the nanoscopic scale and molecular level, this effect is considered to be strongly related to the indirect damage of radiation. Even though particle track radiolysis has been the object of several studies, still little is known about the nanoscopic impact of target oxygenation on the radical yields. Here we present an extension of the chemical module of the Monte Carlo particle track structure code TRAX, taking into account the presence of dissolved molecular oxygen in the target material. The impact of the target oxygenation level on the chemical track evolution and the yields of all the relevant chemical species are studied in water under different irradiation conditions: different linear energy transfer (LET) values, different oxygenation levels, and different particle types. Especially for low LET radiation, a large production of two highly toxic species (HO2• and O•− 2), which is not produced in anoxic conditions, is predicted and quantified in oxygenated solutions. The remarkable correlation between the HO2• and O•− 2 production yield and the oxygen enhancement ratio observed in biological systems suggests a direct or indirect involvement of HO2• and O•− 2 in the oxygen sensitization effect. The results are in agreement with available experimental data and previous computational approaches. An analysis of the oxygen depletion rate in different radiation conditions is also reported