title of the paper: centered, uppercase, 14 point times new roman, on

IX Latin American IRPA Regional Congress on Radiation Protection and Safety - IRPA 2013
Rio de Janeiro, RJ, Brazil, April 15-19, 2013
SOCIEDADE BRASILEIRA DE PROTEÇÃO RADIOLÓGICA - SBPR
TWENTY YEARS OF VISUAL MONTE CARLO
John G. Hunt1, Bernardo M. Dantas1, Denison Santos1, Adelaide G.F. Azeredo1 and
Albérico Blohem2
1
Instituto de Radioproteção e Dosimetria – CNEN
Av. Salvador Allende s/n - Rio de Janeiro- RJ
CEP 22780-160. [email protected]
2
Department of Physics, Federal University of Sergipe,
Rod. Marechal Rondon s/n, Jardim Rosa Elze,
CEP 49100-000 São Cristóvão-SE, Brazil
[email protected]
ABSTRACT
The development of Visual Monte Carlo (VMC) started in 1993 at the Instituto de Radioproteção e Dosimetria
(IRD). The twenty years of development has produced a robust, well documented and fully benchmarked Monte
Carlo program. VMC has been applied to the calculation of gamma spectrometry systems and for dose
calculations involving the transport of photons, alpha particles and protons. VMC is written in the Visual Basic
language which provides a friendly graphics interface with the user. VMC developed into two main softwares:
VMC in-vivo for simulation of whole body counter laboratories, and VMC dose calculation, for dose
calculations due to exposure to radionuclides or X-rays. The two main VMC programs are freely available for
download at the site http://www.vmcsoftware.com/Index.html. VMC can be considered a “specialized” Monte
Carlo program as it is designed specifically to solve radiation protection problems, and the main geometry used
is the voxel matrix. However, VMC can be programmed to perform calculations which would be difficult to
execute using more “generic” Monte Carlo programs such as MCNP or GEANT. For example, matrices
containing two or more anthropomorphic phantoms and special geometries for source tissues, such as the
endosteum/cortical bone interface may be simulated. The development of VMC continues, and the next twenty
years shows much promise.
1. INTRODUCTION
The development of Visual Monte Carlo (VMC) [1] started in 1992 at the Instituto de
Radioproteção e Dosimetria. VMC began as a solution to the problem of photon transport
through voxel phantoms representing the human body, followed by the detection of the
photons leaving the human body. Twenty years ago, the Monte Carlo programs available,
MCNP [2], FLUKA [3], GEANT [4] and EGS [5] for example, did not accept large voxel
matrix geometries, so the option was to write a new Monte Carlo program from the
beginning. The idea seemed ambitious. However the problem to be solved represented a
special case, with a limited number of materials and repetitive voxel geometry for the
phantom and cylindrical geometries for the detectors.
The language chosen was Visual Basic 6, which allows the programmer to establish a userfriendly graphics interface. The main debugging method chosen was to represent on the video
screen the phantom, the detector, and the complete history of each photon. If the photon was
emitted in one direction, and the resulting interaction with the material happened in the
exactly opposite direction, then there was clearly a serious problem with the program.
There are many parts to a Monte Carlo program for photon transport; the pseudorandom
number generator, the algorithms to calculate the mass attenuation coefficients, the Compton
scattering calculations with energy deposition and direction cosines. There are the detector
routines, the line intersection algorithms with planes and cylinders, the calculations of the
distance between interactions, and so on. Today, with the VB6 code resources that the internet
affords, this work is considerably easier.
VMC evolved from its photon transport origins for calculating calibration factors for whole
body counters to a program which now transports electrons, protons and alpha particles
through many types of geometries. VMC developed into two main softwares: VMC in-vivo
for simulation of Whole Body Counter laboratories, and VMC dose calculation, for dose
calculations due to exposure to radionuclides or X-rays. The two main VMC programs are
freely available for download at the site http://www.vmcsoftware.com/Index.html.
2. VMC APPLICATIONS
2.1. VMC applied to whole body counting laboratories
To evaluate the activity of a radionuclide deposited in a body organ, such as the lung, through
direct bioanalysis methods, it is necessary to calibrate the counting geometry. In the
laboratory routine this calibration is performed through the use of physical phantoms
containing known activities of the radionuclide under investigation. Measuring the net cps in
a given photopeak for the contaminated person and comparing it with the net cps for the
phantom, it is possible to estimate the activity deposited in the contaminated person.
However, physical phantoms are expensive and do not cover all the cases of radionuclides
and contaminated organs that may arise. If a physical phantom is not available, it is possible
to calculate the calibration factor using VMC in-vivo. The software VMC in-vivo has been
extensively benchmarked in international intercomparisons against known activities in
physical phantoms. The three main intercomparisons were the IAEA 2002 WBC
intercomparison [6] and the EURADOS intercomparisons, one on the University of
Cincinnati knee containing 241Am [7] and second with enriched uranium in the Lawrence
Livermore National Laboratory (LLNL) lung phantom [8]. Figure 1 shows the simulated
geometry of the second EURADOS intercomparison, with four germanium detectors placed
over a voxel phantom of the lung
Figure 1. Counting geometry showing the position
of the four germanium detectors and the voxel
thorax with lungs containing enriched uranium.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
The agreement between the VMC in-vivo calculated values and the reference values in all
cases has been very good. Figure 2 shows the comparison between the VMC in-vivo
spectrum and the reference spectrum for the lung counting geometry shown in Figure 1.
Figure 2. Intercomparison results, the red line
represents the reference spectrum and the blue
line the VMC in-vivo results. 185 keV is the main
photopeak for 235U.
The VMC in-vivo library contains a number of mathematical phantoms such as BOMAB,
lung, knee, head, and 5, 10, 15 year old [9] and the ICRP adult male and female phantoms
[10]. A multi-channel analyzer is simulated which reproduces the simulated energy spectrum
as would be seen with the GENIE 2000 software from Canberra.
2.2. VMC dose calculation
VMC dose calculation allows Monte Carlo calculations to be performed for exposure to
photon fields generated by radionuclides or X-ray equipment. The program is especially
useful for calculating doses in the case of accidents where high activity sources are placed
close to the body, for example in a pocket, for a certain time. The program calculates the
Tissue equivalent dose to each radiosensitive organ as defined in the ICRP 103 [11]
recommendation and also allows isodose curves to be established in the region close to the
source. A typical calculation is shown in Figure 3.
Figure 3. Axial slice of ICRP phantom showing
position of 2.4 TBq source of Ir-192 which was
kept in the back pocket for one hour.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
The Visual Monte Carlo code and the female voxel phantom FAX [12] were used to calculate
organ and effective doses delivered by target–source irradiation geometries associated with
radioiodine therapy treatments (Figure 4)[13]. Specific situations were considered: when a
patient was accompanied during hospitalization, when a patient was accompanied on return
to his or her residence, and when a patient received daily care at home. This simulation study
showed that, in the 3 situations considered, the total effective dose to an individual in normal
contact with the patient was less than 0.85 mSv for up to 11.1 GBq (300 mCi) of
administered activity. The results of this study suggest that for these patients receiving
radioiodine therapy, radiation protection procedures after hospital discharge are unnecessary.
Figure 4. Simulation geometry which the patient homecoming in a car.
Other VMC work compared the dose to an individual due to exposure from a radioactive
patient using three models (point, line and volume), for three therapeutic regimens
(hyperthyroidism, thyroid cancer and non-Hodgkin´s lymphoma) (Figure 5) [14]. For the
volume source calculations, Monte Carlo simulations employing the VMC code and the
voxel phantom FAX were used. For hyperthyroid patients, the point, line, and volume source
models predicted doses to exposed individuals of 54, 24 and 14 mSv, respectively, at a
distance of 0.3 m and 4.8, 4.0 and 3.3 mSv at a distance of 1 m. For thyroid cancer patients,
the dose values were 85, 38, and 18 mSv at 0.3 m and 7.6, 6.4, and 4.4 mSv at 1 m,
respectively. For non-Hodgkins lymphoma subjects, the doses were 230, 103, and 36 mSv at
0.3 m and 21, 17, and 10 mSv at 1 m. These results show that patient release based on point
source calculations include unnecessary conservatism.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Figure 5. Simulation geometry, thyroid source,
source in the whole body, source in the abdominal
region (Non-Hodgkin’s lymphoma).
Recently, VMC dose calculation has been applied to the doses resulting from fluoroscopy
procedures. In this case, the dose to the patient and the dose to the attending medical staff is
calculated, see Figure 6.
Figure 6. Axial slice of patient and saggital slice of
doctor with the patient being irradiated in PA
geometry with an 87 kV X-ray equipment.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
VMC dose calculation has been extensively benchmarked against irradiations of physical
phantoms containing TLD‟s [6] and against the calculations of other Monte Carlo programs
such as MCNP, GEANT and EGS NRC.
2.3. Alpha transport through highly detailed bone structures
VMC has been adapted to transport alpha particles through detailed micro voxel structures of
the bone. The objective of the calculation is to evaluate the absorbed fraction of the energy
deposited in each bone tissue due to alpha emitting bone seeking radionuclides such as Pu239. The voxel bone phantoms are of 23 bone sites, and the cubic voxel side is of 50 µm. The
small dimension of the bone voxels is due to the fact that alpha particles have a short range in
tissues. A 6 MeV alpha particle will travel around 50 µm in tissue.
A slice through the trabecular bone structure of the femur is shown in Figure 7 below. The
white voxels represent the cortical (hard) bone, the yellow voxels represent the inactive
marrow (adipose marrow) and the blue voxels represent the active marrow. The endosteum,
or bone surface, is represented by a 50 µm layer of active or inactive marrow cells which are
immediately in contact with the cortical bone. The endosteum is the organ which may
develop osteosarcomas.
Figure 7. Slice through femur trabecular bone.
The white voxels represent the cortical (hard)
bone, the yellow voxels represent the inactive
marrow (adipose marrow) and the blue voxels
represent the active marrow.
Recently, MCNP has also been used to transport alpha particles through micro voxel bone
structures. The MCNP code is more limited than the VMC code, but it can be seen that
MCNP agrees with the VMC results as to the absorbed fractions calculation, see Figure 8.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Figure 8. Calculation of absorbed fractions of
energy (MeV/MeV) in the Os Coxae bone site as a
function of alpha energy. The absorbed fraction is
for alpha emitters in the active marrow (AM)
irradiating the active marrow (AM) as target.
2.4. Proton transport through voxel structures
VMC was adapted to transport protons through voxel structures. Proton transport is
complicated by the fact that protons straggle, they suffer nuclear interactions, and they
scatter. Proton transport up to 188 MeV was implemented in VMC and the calculated results
were benchmarked against results from the literature and through the QUADOS
intercomparison of 2003 [15]. The intercomparison involved dose calculations for proton
therapy to the eye: a 50 MeV modulated proton beam incident on an eye water phantom. The
benchmarking results are shown in Figures 9 and 10 below.
Figure 9. The VMC proton dose as a function of
distance and the reference values of the QUADOS
intercomparison. The doses are normalized with
respect to the maximum dose.
IRPA 2013, Rio de Janeiro, RJ, Brazil.
Figure 10. Slice through voxel structure of the
skull showing proton beam isodoses for proton eye
therapy.
3. CONCLUSIONS
Over the last twenty years, VMC has proved to be a useful tool for dose calculations and for
the establishment of calibration factors in gamma spectrometry. The development work
continues, and includes the calculation of dose rates as measured by portable dose rate meters
near adults and children internally contaminated with gamma emitters, such as would
possibly happen after a major release during a nuclear power plant accident. Electron
transport will also be added to VMC which will allow for more accurate specific absorbed
fraction calculations. An update of the programming language to Visual Basic 10 is also
foreseen.
ACKNOWLEDGMENTS
I would like to thank all of those who have helped and contributed in some way to VMC.
Apart from the authors, these include Chris Watchman, David Broggio, Dunstana Melo, Eder
Lucena, Francisco Cesar da Silva, Gary Kramer, Gianfranco Gualdrini, Henry Spitz, Irena
Malátová, José Gomez-Ros, Joyce Lipsztein, Choosnik Lee, Maria Christina Lourenço, Paulo
Becker, Peter Dimbylow, Rodolfo Cruz-Suarez, Vladimir Kutkov and Wesley Bolch.
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