1. References :
[1] arxiv.org/abs/1211.5960
[2] Josefsson A, Forssell-Aronsson E. Microdosimetric analysis of 211At
in thyroid models for man, rat and mouse. EJNMMI Res. 2012;2(1):29.
Small scale dosimetry for 211At distribution in thyroid follicle models
using AlfaMC Monte Carlo simulations
Romiani A, Chakarova R, Forssell-Aronsson E
Department of Radiation Physics, Institute of clinical Sciences, Sahlgrenska Cancer Center,
Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden
Background
Radionuclide therapy, using radionuclides attached to tumor targeting
carriers, is an increasing therapy method well suited for metastatic tumor
diseases.
The alpha-emitter 211At has been proposed for therapy both as free
radionuclide and attached to tumor-seeking carriers. Regardless of the choice
of chemical form, it is necessary to estimate the distribution of free 211At,
and corresponding absorbed dose to potential risk organs, since 211At can be
released from the carrier molecule in vivo. The thyroid gland is considered to
be one of the vital risk organs, in case of internal contamination of free
211At.
Monte Carlo (MC) technique is an appropriate statistical method for
dosimetric calculations and is well suited to master the stochastic nature of
radiation transport. AlfaMC is a relatively new MC code that was introduced
in 2012 by Luis Peralta and Alina Louro at the Science faculty of the
University of Lisbon [1]. Comparison with other MC codes has shown good
agreement for alpha particles with energies between 1 MeV and 12 MeV [1].
Purpose
To determine microdosimetric quantities for homogeneous and
heterogeneous 211At distribution within different thyroid follicle models
using Monte Carlo code AlfaMC and compare the results with previously
published results from similar studies using MC code MCNPX [2] and
results from MC code GEANT4.
Methods
A single- and a multiple-thyroid follicle model were developed, for man, rat
and mouse [2]. The single-thyroid follicle model (Figure 1a) represents a
single sphere-shaped thyroid follicle and consists of a spherical follicle
lumen, a layer of follicle cells (shaped as a shell around the follicle lumen)
and six spherical follicle cell nuclei distributed symmetrically at equal
distances from the center of the follicle lumen.
The multiple-thyroid follicle model represents the case where the thyroid
follicle is surrounded by neighboring follicles (Figure 1 b). The follicle cells
from the neighboring follicles form a spherical shell around the follicle cells
in the center follicle and around their merged lumens.
Four different locations and distributions of 211At were simulated: 211At was
homogeneously distributed 1) within the follicle lumen, 2) within the follicle
cells, 3) within the follicle cell nuclei, and 4) on the surface of concentric
spheres, with their center located at the center of the follicle lumen
(representing heterogeneous distribution of 211At). In all simulations the
targets were the six follicle cell nuclei.
Conclusion
The overall agreement between results from AlfaMC and MCNPX was
very good. AlfaMC is a faster MC code that showed to be suitable for
these types of simulations.
Results
The mean specific energy, 𝑧 , and the single-hit mean specific energy, 𝑧1 ,
are shown in Table for the single follicle model. The single-hit mean specific
energy distribution, f(z1) is given in Figure 2 for 211At in the follicle cells.
𝑧1 for the human multiple follicle model is shown in Figure 3.
Table. The mean specific energy, 𝒛 , and the single-hit mean specific energy, 〈𝒛 𝟏〉, in the
follicle cell nuclei calculated by AlfaMC, MCNPX [2] and GEANT4 for single thyroid follicle
models, for mouse, rat and man. 211At was homogenously distributed in the follicle lumen.
Data is given as mean (SD). The percentage difference between results from the codes is
presented in the last three rows.
Figure 2. Single-hit mean specific energy distribution, f(z1), in the thyroid follicle cell
nuclei with the follicle cells as the source, simulated with AlfaMC and MCNPX [2]. The
curves simulated with AlfaMC were normalized such that the area under each curve is equal to
1. Due to different normalizations the scale on y-axes differ.
Figure 1. Schematic figures of the a) single- and b) multiple-thyroid follicle
models [2]. The follicle cells, the follicle lumen and the follicle cell nuclei are green, blue
and red, respectively. The multiple-thyroid follicle model is presented to the right, the light
green color represents the neighboring follicle cells. Each component had the density of
liquid water (1.0 g/cm3).
Figure 3. Single-hit mean specific energy as a function of the radius of the 211At
surface source for a human multiple follicle model, simulated with AlfaMC and
MCNPX [2]. The two follicle cell layers are shown by dotted lines. SD is presented as error
bars, but in general smaller than data symbols.
a) b)