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Pedro Andreo - One of the best experts on this subject based on the ideXlab platform.
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cema based formalism for the determination of Absorbed Dose for high energy photon beams
Medical Physics, 2021Co-Authors: Gunther H Hartmann, Pedro Andreo, Ralfpeter Kapsch, Klemens ZinkAbstract:PURPOSE Determination of Absorbed Dose is well established in many dosimetry protocols and considered to be highly reliable using ionization chambers under reference conditions. If dosimetry is performed under other conditions or using other detectors, however, open questions still remain. Such questions frequently refer to appropriate correction factors. A converted energy per mass (cema)-based approach to formulate such correction factors offers a good understanding of the specific response of a detector for dosimetry under various measuring conditions and thus an estimate of pros and cons of its application. METHODS Determination of Absorbed Dose requires the knowledge of the beam quality correction factor kQ,Qo , where Q denotes the quality of a user beam and Qo is the quality of the radiation used for calibration. In modern Monte Carlo (MC)-based methods, kQ,Qo is directly derived from the MC-calculated Dose conversion factor, which is the ratio between the Absorbed Dose at a point of interest in water and the mean Absorbed Dose in the sensitive volume of an ion chamber. In this work, Absorbed Dose is approximated by the fundamental quantity cema. This approximation allows the Dose conversion factor to be substituted by the cema conversion factor. Subsequently, this factor is decomposed into a product of cema ratios. They are identified as the stopping power ratio water to the material in the sensitive detector volume, and as the correction factor for the fluence perturbation of the secondary charged particles in the detector cavity caused by the presence of the detector. This correction factor is further decomposed with respect to the perturbation caused by the detector cavity and that caused by external detector properties. The cema-based formalism was subsequently tested by MC calculations of the spectral fluence of the secondary charged particles (electrons and positrons) under various conditions. RESULTS MC calculations demonstrate that considerable fluence perturbation may occur particularly under non-reference conditions. Cema-based correction factors to be applied in a 6-MV beam were obtained for a number of ionization chambers and for three solid-state detectors. Feasibility was shown at field sizes of 4 × 4 and 0.5 cm × 0.5 cm. Values of the cema ratios resulting from the decomposition of the Dose conversion factor can be well correlated with detector response. Under the small field conditions, the internal fluence correction factor of ionization chambers is considerably dependent on volume averaging and thus on the shape and size of the cavity volume. CONCLUSIONS The cema approach is particularly useful at non-reference conditions including when solid-state detectors are used. Perturbation correction factors can be expressed and evaluated by cema ratios in a comprehensive manner. The cema approach can serve to understand the specific response of a detector for dosimetry to be dependent on (a) radiation quality, (b) detector properties, and (c) electron fluence changes caused by the detector. This understanding may also help to decide which detector is best suited for a specific measurement situation.
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advances in the determination of Absorbed Dose to water in clinical high energy photon and electron beams using ionization chambers
Physics in Medicine and Biology, 2004Co-Authors: Pedro AndreoAbstract:During the last two decades, Absorbed Dose to water in clinical photon and electron beams was determined using dosimetry protocols and codes of practice based on radiation metrology standards of air kerma. It is now recommended that clinical reference dosimetry be based on standards of Absorbed Dose to water. Newer protocols for the dosimetry of radiotherapy beams, based on the use of an ionization chamber calibrated in terms of Absorbed Dose to water, ND,w, in a standards laboratory's reference quality beam, have been published by several national or regional scientific societies and international organizations. Since the publication of these protocols multiple theoretical and experimental dosimetry comparisons between the various ND,w based recommendations, and between the ND,w and the former air kerma (NK) based protocols, have been published. This paper provides a comprehensive review of the dosimetry protocols based on these standards and of the intercomparisons of the different protocols published in the literature, discussing the reasons for the observed discrepancies between them. A summary of the various types of standards of Absorbed Dose to water, together with an analysis of the uncertainties along the various steps of the dosimetry chain for the two types of formalism, is also included. It is emphasized that the NK–ND,air and ND,w formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of Absorbed Dose to water.
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calculation of Absorbed Dose and biological effectiveness from photonuclear reactions in a bremsstrahlung beam of end point 50 mev
Physics in Medicine and Biology, 1999Co-Authors: Irena Gudowska, Anders Brahme, Pedro Andreo, W Gudowski, J KierkegaardAbstract:The Absorbed Dose due to photonuclear reactions in soft tissue, lung, breast, adipose tissue and cortical bone has been evaluated for a scanned bremsstrahlung beam of end point 50 MeV from a racetrack accelerator. The Monte Carlo code MCNP4B was used to determine the photon source spectrum from the bremsstrahlung target and to simulate the transport of photons through the treatment head and the patient. Photonuclear particle production in tissue was calculated numerically using the energy distributions of photons derived from the Monte Carlo simulations. The transport of photoneutrons in the patient and the photoneutron Absorbed Dose to tissue were determined using MCNP4B; the Absorbed Dose due to charged photonuclear particles was calculated numerically assuming total energy absorption in tissue voxels of 1 cm3. The photonuclear Absorbed Dose to soft tissue, lung, breast and adipose tissue is about (0.11-0.12)±0.05% of the maximum photon Dose at a depth of 5.5 cm. The Absorbed Dose to cortical bone is about 45% larger than that to soft tissue. If the contributions from all photoparticles (n, p, 3He and 4He particles and recoils of the residual nuclei) produced in the soft tissue and the accelerator, and from positron radiation and gammas due to induced radioactivity and excited states of the nuclei, are taken into account the total photonuclear Absorbed Dose delivered to soft tissue is about 0.15±0.08% of the maximum photon Dose. It has been estimated that the RBE of the photon beam of 50 MV acceleration potential is approximately 2% higher than that of conventional 60Co radiation.
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Absorbed Dose beam quality factors for the dosimetry of high energy photon beams
Physics in Medicine and Biology, 1992Co-Authors: Pedro AndreoAbstract:The parallelism before existing air kerma formalisms based on the 'Absorbed Dose to air' chamber factor, ND (or Ngas), and the more simple approach based on calibrations in terms of Absorbed Dose to water, Nw, is described. The importance of avoiding steps performed by the user that introduce avoidable uncertainties in the dosimetric procedure, is emphasized. Radiation beam quality factors normalized to 60 Co gamma -rays have been calculated from a comparison between the two formalisms, and sets of tables produced for the different ionization chambers included in the 1987 IAEA Code of Practice. The calculated set of data is compared with existing experimental determinations at Primary Standard Dosimetry Laboratories, showing agreement within estimated uncertainties.
R F Wimmerschweingruber - One of the best experts on this subject based on the ideXlab platform.
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galactic cosmic ray induced Absorbed Dose rate in deep space accounting for detector size shape material as well as for the solar modulation
Journal of Space Weather and Space Climate, 2019Co-Authors: Sasa Banjac, L Berger, S Burmeister, B Heber, K Herbst, R F WimmerschweingruberAbstract:Depending on the radiation field, the Absorbed Dose rate can depend significantly upon the size of the detectors or the phantom used in the models. In deep space (interplanetary medium) the radiation field is on avarage dominated by Galactic Cosmic Ray (GCR) nuclei. Here, the deep space Dose rate that a typical small silicon slab detector measures is compared to a larger phantom corresponding to an ICRU sphere with a 15 cm radius composed of water. To separate and understand respective effects from the composition, size and shape differences in the detectors, this comparison is implemented in several steps. For each phantom, the Absorbed Dose rate due to GCR nuclei up to Z = 28, as a function of solar modulation conditions, is calculated.The main components of the GCR flux are protons, followed by helium nuclei and electrons, with Z > 2 nuclei accounting for approximately 1% of the total number of particles. Among the light nuclei with Z > 2, most abundant ones are C, N and O. In this study, we use the GEANT4 model to calculate the Absorbed Dose (energy deposited as ionization, divided by mass) due to the GCR flux provided by the Badhwar-O’Neill 2010 (BON-10) model. Furthermore, we investigate how the determined Absorbed Dose rate changes throughout the solar cycle by varying the GCR models from solar minimum to solar maximum conditions. The developed model is validated against the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) microdosimeter measurements. In our current approach, we do not consider the effects of shielding, which will always be present under realistic scenarios.A second goal of this study is to quantify the contribution of each Z = 1, …, 28 GCR nuclei to Absorbed Dose rate, in relation to the phantom characteristics. For each Z we determine the most relevant energy range in the GCR spectra for Absorbed Dose rate estimations. Furthermore, we calculate a solar modulation dependent conversion factor to convert Absorbed Dose rate measured in silicon to Absorbed Dose rate in water. This information will improve our understanding of the radiation environment due to GCR in the near-Earth deep space and also benefit further modeling efforts by limiting the number and energy range of primary particle species that have to be considered.
Emiliano Spezi - One of the best experts on this subject based on the ideXlab platform.
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effect of image registration on 3d Absorbed Dose calculations in 177lu dotatoc peptide receptor radionuclide therapy
Physica Medica, 2018Co-Authors: Elisa Grassi, Federica Fioroni, Salvatore Berenato, Nick Patterson, Valentina Ferri, Luca Braglia, Angelina Filice, Annibale Versari, Mauro Iori, Emiliano SpeziAbstract:Peptide receptor radionuclide therapy (PRRT) is an effective MRT (molecular radiotherapy) treatment, which consists of multiple administrations of a radiopharmaceutical labelled with 177Lu or 90Y. Through sequential functional imaging a patient specific 3D dosimetry can be derived. Multiple scans should be previously co-registered to allow accurate Absorbed Dose calculations. The purpose of this study is to evaluate the impact of image registration algorithms on 3D Absorbed Dose calculation. A cohort of patients was extracted from the database of a clinical trial in PRRT. They were administered with a single administration of 177Lu-DOTATOC. All patients underwent 5 SPECT/CT sequential scans at 1 h, 4 h, 24 h, 40 h, 70 h post-injection that were subsequently registered using rigid and deformable algorithms. A similarity index was calculated to compare rigid and deformable registration algorithms. 3D Absorbed Dose calculation was carried out with the RayDose Monte Carlo code. The similarity analysis demonstrated the superiority of the deformable registrations (p < .001). Average Absorbed Dose to the kidneys calculated using rigid image registration was consistently lower than the average Absorbed Dose calculated using the deformable algorithm (90% of cases), with percentage differences in the range [-19; +4]%. Absorbed Dose to lesions were also consistently lower (90% of cases) when calculated with rigid image registration with Absorbed Dose differences in the range [-67.2; 100.7]%. Deformable image registration had a significant role in calculating 3D Absorbed Dose to organs or lesions with volumes smaller than 100 mL. Image based 3D dosimetry for 177Lu-DOTATOC PRRT is significantly affected by the type of algorithm used to register sequential SPECT/CT scans.
A Sarfehnia - One of the best experts on this subject based on the ideXlab platform.
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water calorimetry in mr linac direct measurement of Absorbed Dose and determination of chamber
Medical Physics, 2020Co-Authors: Mark Dsouza, Humza Nusrat, Viktor Iakovenko, B Keller, Arjun Sahgal, J Renaud, A SarfehniaAbstract:PURPOSE To use a portable 4°C cooled MR-compatible water calorimeter to measure Absorbed Dose in a magnetic resonance-guided radiation therapy (MRgRT) system. Furthermore, to use the calorimetric Dose results and direct cross-calibration to experimentally measure the combined beam quality and magnetic field correction factor ( kQmag ) of a clinically used reference-class ionization chamber placed under the same radiation field. METHODS An Elekta Unity MR-linac (7 MV FFF, B = 1.5 T) was used in this study. Measurements were taken using the in-house designed and built water calorimeter. Following preparation and cooling of the system, the MR-compatible calorimeter was positioned using a combination of MR and EPID imaging and the Dose to water was measured by monitoring the radiation-induced temperature change. Immediately after the calorimetric measurements, an A1SL ionization chamber was placed inside the calorimeter for direct cross-calibration. The results allowed for a direct and absolute experimental measurement of kQmag for this chamber and comparison against existing Monte Carlo values. RESULTS The calorimeter was successfully positioned using imaging in under an hour. The 1-hour setup time is from the time the calorimeter leaves storage to the first calorimetric measurement. Absorbed Dose was successfully measured with a relative combined standard uncertainty of 0.71 % (k = 1). Through a cross-calibration, the kQmag for an Exradin A1SL ionization chamber, set up perpendicular to the incident photon beam and opposite to the direction of the Lorentz force, was directly determined in water in absolute terms to be 0.977 ± 0.010. The currently published kQmag results, obtained via Monte Carlo calculations, agree with experimental measurements in this work within combined uncertainties. CONCLUSIONS A novel design of an MR-compatible water calorimeter was successfully used to measure Absorbed Dose in an MR-linac and determine an experimental value of kQmag for a clinically used ionization chamber.
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direct Absorbed Dose to water determination based on water calorimetry in scanning proton beam delivery
Medical Physics, 2010Co-Authors: A Sarfehnia, B Clasie, E Chung, J Flanz, E Cascio, Martijn Engelsman, H Paganetti, J SeuntjensAbstract:Purpose: The aim of this manuscript is to describe the direct measurement of absolute Absorbed Dose to water in a scanned proton radiotherapy beam using a water calorimeter primary standard. Methods: The McGill water calorimeter, which has been validated in photon and electron beams as well as in HDR {sup 192}Ir brachytherapy, was used to measure the Absorbed Dose to water in double scattering and scanning proton irradiations. The measurements were made at the Massachusetts General Hospital proton radiotherapy facility. The correction factors in water calorimetry were numerically calculated and various parameters affecting their magnitude and uncertainty were studied. The Absorbed Dose to water was compared to that obtained using an Exradin T1 Chamber based on the IAEA TRS-398 protocol. Results: The overall 1-sigma uncertainty on Absorbed Dose to water amounts to 0.4% and 0.6% in scattered and scanned proton water calorimetry, respectively. This compares to an overall uncertainty of 1.9% for currently accepted IAEA TRS-398 reference Absorbed Dose measurement protocol. The Absorbed Dose from water calorimetry agrees with the results from TRS-398 well to within 1-sigma uncertainty. Conclusions: This work demonstrates that a primary Absorbed Dose standard based on water calorimetry is feasible in scattered and scanned proton beams.
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development of a water calorimetry based standard for Absorbed Dose to water in hdr 192ir brachytherapy
Medical Physics, 2010Co-Authors: A Sarfehnia, J SeuntjensAbstract:Purpose: The aim of this article is to develop and evaluate a primary standard for HDR I 192 r brachytherapy based on 4 ° C stagnant watercalorimetry. Methods: The absolute Absorbed Dose to water was directly measured for several different Nucletron microSelectron I 192 r sources of air kerma strength ranging between 21 000 and 38 000 U and for source-to-detector separations ranging between 25 and 70 mm. The COMSOL MULTIPHYSICS™ software was used to accurately calculate the heat transport in a detailed model geometry. Through a coupling of the “conduction and convection” module with the “Navier–Stokes incompressible fluid” module in the software, both the conductive and convective effects were modeled. Results: A detailed uncertainty analysis resulted in an overall uncertainty in the Absorbed Dose of 1.90% ( 1 σ ) . However, this includes a 1.5% uncertainty associated with a nonlinear predrift correction which can be substantially reduced if sufficient time is provided for the system to come to a new equilibrium in between successive calorimetric runs, an opportunity not available to the authors in their clinical setting due to time constraints on the machine. An average normalized Dose rate of 361 ± 7 μ Gy / ( h U ) at a source-to-detector separation of 55 mm was measured for the microSelectron I 192 r source based on watercalorimetry. The measured Absorbed Dose per air kerma strength agreed to better than 0.8% ( 1 σ ) with independent ionization chamber and EBT-1 Gafchromic film reference dosimetry as well as with the currently accepted AAPM TG-43 protocol measurements. Conclusions: This work paves the way toward a primary Absorbed Dose to water standard in I 192 r brachytherapy.
Sasa Banjac - One of the best experts on this subject based on the ideXlab platform.
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galactic cosmic ray induced Absorbed Dose rate in deep space accounting for detector size shape material as well as for the solar modulation
Journal of Space Weather and Space Climate, 2019Co-Authors: Sasa Banjac, L Berger, S Burmeister, B Heber, K Herbst, R F WimmerschweingruberAbstract:Depending on the radiation field, the Absorbed Dose rate can depend significantly upon the size of the detectors or the phantom used in the models. In deep space (interplanetary medium) the radiation field is on avarage dominated by Galactic Cosmic Ray (GCR) nuclei. Here, the deep space Dose rate that a typical small silicon slab detector measures is compared to a larger phantom corresponding to an ICRU sphere with a 15 cm radius composed of water. To separate and understand respective effects from the composition, size and shape differences in the detectors, this comparison is implemented in several steps. For each phantom, the Absorbed Dose rate due to GCR nuclei up to Z = 28, as a function of solar modulation conditions, is calculated.The main components of the GCR flux are protons, followed by helium nuclei and electrons, with Z > 2 nuclei accounting for approximately 1% of the total number of particles. Among the light nuclei with Z > 2, most abundant ones are C, N and O. In this study, we use the GEANT4 model to calculate the Absorbed Dose (energy deposited as ionization, divided by mass) due to the GCR flux provided by the Badhwar-O’Neill 2010 (BON-10) model. Furthermore, we investigate how the determined Absorbed Dose rate changes throughout the solar cycle by varying the GCR models from solar minimum to solar maximum conditions. The developed model is validated against the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) microdosimeter measurements. In our current approach, we do not consider the effects of shielding, which will always be present under realistic scenarios.A second goal of this study is to quantify the contribution of each Z = 1, …, 28 GCR nuclei to Absorbed Dose rate, in relation to the phantom characteristics. For each Z we determine the most relevant energy range in the GCR spectra for Absorbed Dose rate estimations. Furthermore, we calculate a solar modulation dependent conversion factor to convert Absorbed Dose rate measured in silicon to Absorbed Dose rate in water. This information will improve our understanding of the radiation environment due to GCR in the near-Earth deep space and also benefit further modeling efforts by limiting the number and energy range of primary particle species that have to be considered.
