Monte Carlo study of voxel S factor dependence on tissue density and atomic composition
Introduction
The continuous improvement of the multi-modality tomographic imaging devices in nuclear medicine has triggered the development of more sophisticated approaches to internal dosimetry, such as the three-dimensional dosimetry at the voxel level.
The voxel S factors approach to three-dimensional dosimetry, introduced in Ref. [1], has been more widely used than other methods (such as dose point-kernel convolution and direct Monte Carlo computation methods), due to its recognized simplicity and reliability.
Recalling the updated MIRD nomenclature [2], the average dose to the target voxel can be calculated as:where is the time-integrated activity in the generic source voxel, and is defined as:where is the mean energy emitted as radiation i per decay, is the absorbed fraction in t of the radiation i emitted in s, and mt is the mass of the target voxel.
In the MIRD Pamphlet no. 17 [1], voxel S factors were given for five radionuclides (32P, 89Sr, 90Y, 99mTc and 131I) and two cubic voxel sizes (3 and 6 mm).
The need to generalize the approach to different beta–gamma emitting radionuclides and voxel sizes led to computation methods exploiting direct Monte Carlo computations or convolution of dose point kernels.
In particular, Lanconelli et al. [3] presented a website providing voxel S factors for seven radionuclides and several voxel sizes in soft or compact bone tissues; data available from such a repository were obtained starting from the simulation, through the EGSnrc code, of monoenergetic sources of electrons and photons. The obtained voxel S factors, for each radionuclide and voxel size, underwent to a quality check through a comparison with the results of other Monte Carlo codes, before being published on the website.
Furthermore, a method to calculate voxel S factors for a generic voxel size was introduced by Dieudonné et al. [4]. They used a resampling procedure that started from nuclide-specific fine-resolution voxel S factors, previously calculated through Monte Carlo simulation.
In a previous work [5], we proposed an analytical calculation method for voxel S factors for a generic beta–gamma emitting radionuclide, valid in a model of soft tissue with 1.04 g cm−3 density. There, we pointed out the inaccuracies which arise when using, in bone tissue, voxel S factors evaluated for soft tissue.
In the present work, we aimed to extend our analytical approach to tissue materials of different density and atomic composition. Such generalization is of potential clinical relevance, since it solves the systematic biases affecting the three-dimensional dosimetric approach at the voxel level in tissues whose density and atomic composition is significantly different from that of the soft tissue. The proposed methodology allows to obtain data for several tissues and voxel sizes in a simple way, without direct Monte Carlo calculations.
Section snippets
Materials and methods
In order to study the dependence of voxel S factors on tissue density and atomic composition, we used a Monte Carlo simulation in GEANT4 [6], a simulation toolkit originally developed for high energy physics, currently applied also in the field of medical radiation physics [7], [8], [9], [10]. We adapted a simulation code previously developed for calculating dose factors for electrons and photons in soft tissue [11], [12], [13], [14]. We considered an infinite volume of homogeneous absorbing
Results and discussion
Fig. 1, Fig. 2 report the average energy deposition per event (monoenergetic photon or electron, respectively) emitted in the central voxel, for the eight materials studied. The analysis of photon data reveals that density influences the steepness of Edep(Rn) curves for all energies, while materials with the same or similar densities and different atomic composition show differences in the Edep(Rn) curves which decrease as the energy increases.
Electron data series show the usual trend composed
Conclusions
In this work, we developed a Monte Carlo simulation in GEANT4 which allowed us to study the dependence of voxel S factors on tissue density and atomic composition. We simulated eight standard materials and applied a previously proposed analytical formulation in order to fit Edep(Rn) curves for monoenergetic electrons and photons. Following the approach introduced in Ref. [5], the knowledge of the fit parameters allows to calculate voxel S factors for a generic beta–gamma emitting radionuclide
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2019, Physica MedicaCitation Excerpt :Both methods, however, can suffer from two limitations potentially leading to some degree of inaccuracy. First, usually they assume an uniform (often unitary) material density, neglecting the remarkable differences between human tissues, such as inflated lungs, soft tissue and bone [10,11]. Nevertheless, some Authors developed convolution methods in heterogeneous media [12].
A methodological approach to a realistic evaluation of skin absorbed doses during manipulation of radioactive sources by means of GAMOS Monte Carlo simulations
2018, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCitation Excerpt :To this aim, we developed a Monte Carlo simulation adopting geometries as close as possible to the real manipulation scenarios encountered in diagnostic or therapeutic nuclear medicine. Our analysis was carried out by means of GAMOS [6], a friendly interface to GEANT4 [7], that is a simulation toolkit originally conceived in the framework of High Energy Physics and then extended to Medical Physics applications [8–13]. GAMOS makes GEANT4 easier to use in the medical field.
Radiation protection from external exposure to radionuclides: A Monte Carlo data handbook
2018, Physica MedicaCitation Excerpt :A further limitation of the data contained in Ref. [3] concerns the restricted number of geometric configurations for which skin and deep doses are evaluated. On the other hand, the availability of Monte Carlo codes for the simulation of radiation transport and interaction in matter allows to get realistic evaluations of radiation absorbed doses in complex geometries [11–16]. In this paper, we selected a set of radionuclides for which the superficial (70 μm depth) and deep (10 mm depth) dose equivalents in tissue were evaluated by means of Monte Carlo simulations in GEANT4-based Architecture for Medicine-Oriented Simulation, GAMOS, version 5.1.0 [17].
Influence of voxel S factors on three-dimensional internal dosimetry calculations
2016, Physica MedicaCitation Excerpt :They can register serial quantitative emission tomographies, such as SPECT or PET, using CT scans as volume reference, and calculate time-integrated activities at the voxel level. These data are then convolved with radionuclide-specific dosimetric factors at the voxel level (voxel S factors) to obtain the radiation absorbed dose distribution in the volumes of interest [9–12]. The present study was designed to analyse the influence of voxel S factors on the final dosimetric data.