1,721,235 research outputs found
Monte Carlo methods in nuclear medicine
No full textMonte Carlo (MC) simulation of the radiation transport and interaction in matter finds an ever-increasing application in nuclear medicine. Starting from a general summary of the principles of MC approach, an overview of the several general-purpose MC codes (GEANT4, MCNP, EGS, PENELOPE, FLUKA), dedicated codes (SIMIND and others) and interfaces specifically developed for medical radiation physics and nuclear medicine applications (GATE, GAMOS and others), are presented. The most relevant fields in which MC simulation has an established role in nuclear medicine are internal dosimetry, instrumentation design and optimization, radiation protection and radionuclide production. Internal dosimetry models, at organ or voxel level, have been successfully developed. MC simulations can be also employed for patient-specific 3D dosimetry based on SPECT-CT and PET-CT data. Concerning imaging devices, the optimization of all the parameters influencing the detector response and image quality, also for innovative scanner architectures, can be deeply investigated. On the other side, existing PET and SPECT tomographs can benefit of MC-derived correction factors. In radiation protection from radionuclides used in diagnostic and therapeutic procedures, MC approach reveals to be a powerful tool for estimating exposures, even in complex environmental conditions, for designing radiation shielding, particularly in the case of high-energy therapeutic beta emitters. Radionuclide production goes through several nuclear reaction processes, induced in most cases by proton or neutron beams. Target assembly, including the choice of target material and shape, and beam features, must be optimized to achieve an optimal production yield and radionuclide purity. Finally, MC simulation shows its potential in supporting the effective teaching of radiation-matter interaction concepts, applied in nuclear medicine
Evaluation of radiation exposure risk from 90Y in the light of internal bremsstrahlung emission
No full textPurpose: Internal bremsstrahlung (IB), occurring after interaction
of electrons with the parent nuclide, is usually neglected when
estimating absorbed dose from exposure to emitters. Nevertheless,
for a set of high energy radionuclides some Authors pointed out
that IB emission intensity can be comparable to (or somehow larger
than) the external bremsstrahlung (EB). By means of Monte Carlo
(MC) simulations and experimental measurements for 90 Y, we
studied the relevance of IB contribution to the total absorbed dose
to the extremities of operators handling real sources.
Materials and Methods: Given the great variety of IB models
existing in literature, and basing on the available experimental
data, we assumed two different hypotheses for IB spectrum and
implemented both of them in MC simulations, in order to estimate
skin and deep absorbed doses for point-like and extended 90 Y sources
(glass and plastic vials). Dose measurements were performed in
different configurations, using a pure 90 Y liquid source in a glass vial
and two portable radiation monitors, calibrated in H*(10).
Results: Evaluation of absorbed doses due to particles, and
consequently to the EB x-rays, was already carried out in a previous
work of ours, using GAMOS, for the same geometrical configurations.
Keeping into account also IB contribution, we now get increased
absorbed doses; depending on the adopted model of IB spectrum,
increments respectively up to 34% and 60% or 15% and 28%, for a
point source and the considered receptacles, are obtained.
Conclusions: This study indicates that IB phenomenon
accompanying -decay can induce relevant contribution in radiation
protection estimations and suggests to extend future theoretical
and experimental studies to other decaying radionuclides.
Keywords: Internal bremsstrahlung; external exposure; yttrium-90;
Monte Carlo
Technical note: The contribution of internal bremsstrahlung to the 90Y dose point kernel
No full textBackground: Internal dosimetry has an increasing role in the planning and verification of nuclear medicine therapies with radiopharmaceuticals. Dose Point Kernels (DPKs), quantifying the energy deposition all around a point source, in a homogenous medium, are extensively used for 3D dosimetry and nowadays are mostly evaluated by Monte Carlo (MC) simulation. To our knowledge, DPK for beta emitters is estimated neglecting the continuous photon emission due to the Internal Bremsstrahlung (IB), whose contribution to the absorbed dose can be relevant beyond the maximum range of betas, as evidenced in recent works. Purpose: Aim of this study was to investigate and quantify, by means of MC simulations, the contribution of IB photons to DPK calculated for 90Y and provide the updated 90Y DPK. Methods: The overall radiation due to the decay of a 90Y point source, placed at the centre of concentric water shells of increasing radii from 0.02 cm to 20 cm, was simulated with GAMOS, including the IB source term whose spectral distribution was described by an analytical model. Energy deposition was scored in the shells as a function of the distance from the source, R, and DPK was estimated in terms of the scaled absorbed dose fraction, F(R/X90), where X90 is the range within which the beta particles deposit 90% of their energy. Results: A comparison between the two simulated absorbed dose distributions, calculated with or without IB, clearly shows that the latter (incomplete) choice is consistent with the findings of other Authors and systematically underestimates the absorbed dose imparted to the tissue. 90Y DPK values currently used are underestimated by 20%-34% for R>2X90. Conclusions: The revised values provided in this work suggest that the inclusion of IB emission in DPK evaluations is advisable for pure beta emitters
An analytic model to calculate voxel s-values for 177Lu
No full textObjective. 177Lu is one of the most employed isotopes in targeted radionuclide therapies and theranostics, and 3D internal dosimetry for such procedures has great importance. Voxel S-Values (VSVs) approach is widely used for this purpose, but VSVs are available for a limited number of voxel dimensions. The aim of this work is to develop an analytic model for the calculation of 177Lu-VSVs in any cubic voxelized geometry of practical interest. Approach. Monte Carlo (MC) simulations were implemented with the toolkit GAMOS to evaluate VSVs in voxelized geometries of soft tissue from a source of 177Lu homogeneously distributed in the central voxel. Nine geometric setups, containing 15 × 15 × 15 cubic voxels of sides l ranging from 2 mm to 6 mm, in steps of 0.5 mm, were considered. For each l, the VSVs computed as a function of the ‘normalized radius’, R n = R/l (with R = distance from the center of the source voxel), were fitted with a parametric function. The dependencies of the parameters as a function of l were then fitted with appropriate functions, in order to implement the model for deducing 177Lu-VSVs for any l within the aforementioned range. Main results. The MC-derived VSVs were satisfactorily compared with literature data for validation, and the VSVs computed with the analytic model agree with the MC ones within 2% for R n ≤ 2 and within 6% for R n > 2. Significance. The proposed model enables the easy and fast calculation, with a simple spreadsheet, of 177Lu-VSVs in any cubic voxelized geometry of practical interest, avoiding the necessity of implementing ad-hoc MC simulations to estimate VSVs for specific voxel dimensions not available in literature data
Internal Bremsstrahlung, the missing process in beta decay Monte Carlo simulation: The relevance in 32P Dose-Point-Kernel estimation
No full textPurpose: In nuclear medicine, Dose Point Kernels (DPKs), representing the energy deposited all around a point isotropic source, are extensively used for dosimetry and are usually obtained by Monte Carlo (MC) simulations. For beta-decaying nuclides, DPK is usually estimated neglecting Internal Bremsstrahlung (IB) emission, a process always accompanying the beta decay and consisting in the emission of photons having a continuous spectral distribution. This work aims to study the significance of IB emission for DPK estimation in the case of 32P and provide DPK values corrected for the IB photon contribution. Methods: DPK, in terms of the scaled absorbed dose fraction, F(R/X90), was first estimated by GAMOS MC simulation using the standard beta decay spectrum of 32P, Fβ(R/X90). Subsequently, an additional source term accounting for IB photons and their spectral distribution was defined and used for a further MC simulation, thus evaluating the contribution of IB emission to DPK values, Fβ+IB(R/X90). The relative percent difference, δ, between the DPKs obtained by the two approaches, Fβ+IB vs. Fβ, was studied as a function of the radial distance, R. Results: As far as the energy deposition is mainly due to the beta particles, IB photons does not significantly contribute to DPK; conversely, for larger R, Fβ+IB values are higher by 30–40% than Fβ. Conclusions: The inclusion of IB emission in the MC simulations for DPK estimations is recommended, as well as the use of the DPK values corrected for IB photons, here provided
Updating 90Y Voxel S-Values including internal Bremsstrahlung: Monte Carlo study and development of an analytical model
No full textPurpose: Internal Bremsstrahlung (IB) is a process accompanying β-decay but neglected in Voxel S-Values (VSVs) calculation. Aims of this work were to calculate, through Monte Carlo (MC) simulation, updated 90Y-VSVs including IB, and to develop an analytical model to evaluate 90Y-VSVs for any voxel size of practical interest. Methods: GATE (Geant4 Application for Tomographic Emission) was employed for simulating voxelized geometries of soft tissue, with voxels sides l ranging from 2 to 6 mm, in steps of 0.5 mm. The central voxel was set as a homogeneous source of 90Y when IB photons are not modelled. For each l, the VSVs were computed for 90Y decays alone and for 90Y + IB. The analytical model was then built through fitting procedures of the VSVs including IB contribution. Results: Comparing GATE-VSVs with and without IB, differences between + 25% and + 30% were found for distances from the central voxel larger than the maximum β-range. The analytical model showed an agreement with MC simulations within ± 5% in the central voxel and in the Bremsstrahlung tails, for any l value examined, and relative differences lower than ± 40%, for other distances from the source. Conclusions: The presented 90Y-VSVs include for the first time the contribution due to IB, thus providing a more accurate set of dosimetric factors for three-dimensional internal dosimetry of 90Y-labelled radiopharmaceuticals and medical devices. Furthermore, the analytical model constitutes an easy and fast alternative approach for 90Y-VSVs estimation for non-standard voxel dimensions
A novel database of 90Y voxel S-Values including Internal Bremsstrahlung and an analytical model extending the calculation to any voxel size
No full textIn this paper we summarize our work aimed at producing via Monte Carlo (MC) simulations an updated database of Voxel S-Values (VSVs) for the radioisotope 90Y, widely used in nuclear medicine therapies. The usually neglected contribution due to Internal Bremsstrahlung accompanying β-decay was included in the computation of the VSVs, increasing their accuracy with respect to pre-existing databases. An analytical model enabling to extend the VSVs calculation to any voxel size of interest was additionally developed, to overcome the limitation due to the finite number of sizes directly evaluable via MC
Monte Carlo Simulations Corroborate PET-Measured Discrepancies in Activity Assessments of Commercial 90Y Vials
No full textIn a recent multicenter study, discrepancies between PET/CT-measured activity and vendor-calibrated activity for 90Y glass and resin microspheres were found. In the present work, the origin of these discrepancies was investigated by Monte Carlo (MC) simulations. Methods: Three vial configurations, containing 90Y-chloride, 90Y-labeled glass microspheres, and 90Y-labeled resin microspheres, were modeled with GAMOS, and the electric signal generated in an activity meter was simulated. Energy deposition was scored in the activity meter–active regions and converted into electric current per unit activity. Internal bremsstrahlung (IB) photons, always accompanying b-decay, were simulated in addition to 90Y decays. The electric current per source activity obtained for 90Y glass and resin microspheres, Iglass and Iresin, was compared in terms of relative percentage difference with that of 90Y-chloride («glass and «resin) and each other (d). The findings of this work were compared with the ones obtained through PET measurements in the multicenter study. Results: With the inclusion of IB photons as primary particles in MC simulations, the «glass and «resin results were 24.6% 6 3.9% and 215.0% 6 2.2%, respectively, whereas d was 46.5% 6 1.9%, in very good agreement with the values reported in the multicenter study. Conclusion: The MC simulations performed in this study indicate that the discrepancies recently found between PET/CT-measured activity and vendor-calibrated activity for 90Y glass and resin microspheres can be attributed to differences in the geometry of the respective commercial vials and to the metrologic approach adopted for activity meter calibration with a 90Y-chloride liquid source. Furthermore, IB photons were shown to play a relevant role in determining the electric current in the activity meter
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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