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Monte Carlo for Dose Simulation

 

History and description of the research group

 The members of our research group are F. Salvat (Professor, Universitat de Barcelona), J.M. Fernández-Varea (Associate Professor, UB), J. Sempau (Researcher, Universitat Politècnica de Catalunya) X. Llovet (Research Technician, UB), L. Brualla and F. Al-Dweri (Post-doctoral Fellows, UB) and the following Ph.D Students: D. Bote (UB), A. Badal (UPC), I. Martínez (UPC) and G. González (Uppsala University). F. Salvat is the head researcher of the Maestro Project at UB and L. Brualla’s position is funded by Maestro. The senior members of the group have been working for more than 20 years on theoretical radiation physics.

 

Main activities of the research group

The group develops and maintains the general-purpose Monte Carlo code system PENELOPE, which performs simulations of coupled electron-photon transport in complex material structures. We have also written a geometry package called PENGEOM for modelling complex geometrical systems using a constructive-quadric algorithm. A detailed description of PENELOPE can be downloaded from the following URL: http://www.nea.fr/html/dbprog/peneloperef.html

 From a theoretical point of view, our research work has been devoted to the development of codes and models for first-principles calculations of interaction cross sections for electrons and positrons. The results of our research have been mainly published on Journal of Physics B, Physical Review A, Nuclear Instruments and Methods B, and Computer Physics Communications. We have also performed measurements of cross sections for inner shell ionization of atoms by impact of electrons.

 Our research is also aimed at applications of PENELOPE. These applications include characterisation of ionization chambers (in collaboration with Prof. Pedro Andreo, Karolinska Institute), quantification in electron probe microanalysis, simulation of linear accelerators, and calculation of dose distributions in teletherapy and brachytherapy. The results of this applied research have been published in Physics in Medicine and Biology, Medical Physics, Microchimica Acta, Journal of Applied Physics, Applied Physics Letters,  and other journals.

 During the last ten years we have obtained three research funded projects (3 years each) from the Spanish National Health System, one (3 years) project from the Spanish Ministry of Science and Education, two (3 years) projects from the Spanish National Nuclear Security Council, and one Marie Curie Individual Fellowship, in addition to the Maestro Project of the European Commission. We have also running  consulting agreements with various international companies.

 In collaboration with C.J. Powel (NIST) and A. Jablonski (Polish Academy of Sciences) we have developed a database of cross sections for elastic scattering of low-energy electrons, which has been published by the National Institute of Standards and Technology (USA). We are also working in the development of a  similar data base of cross sections for ionization of inner atomic electron shells by electron and positron impact. F. Salvat served as chairman of the International Commission on Radiation Units and Measurements (ICRU) Committee on “Elastic Scattering of Electrons and Positrons”, which has produced an ICRU report which will be published shortly. J.M. Fernández-Varea is serving as chairman of an ICRU Committee on “Key Data for Dosimetry”; he is also Associated Editor of the journal “Radiation Physics and Chemistry”.

 

Our recent work

1.- Development of PENMAIN, a PENELOPE main program  for the simulation of linear accelerators and other irradiation devices, which uses PENGEOM and implements various variance reduction techniques. PENMAIN automatically generates phase-space files that describe realistic radiation beams, to be used in Monte Carlo simulations of radiotherapy treatments.

2.- Development of PENCT, a main program for the accurate calculation of dose distributions in patients and phantoms described by means of computerized tomography (CT) images.

3.- Formulation of an optimized algorithm for the simulation of radiotherapy treatments. It combines various simplifications of the physical interaction models with a new condensed (class I) simulation algorithm for electron transport.

4.- PENLINAC, a code for easing the task of creating phase-space files of radiation beams from clinical linear accelerators (linacs). The code generates the set of input data files for PENMAIN from only the definition of a few operational parameters that are known to the medical physicist. This tool is aimed at minimizing the risk of user-induced errors in the characterization of linacs.

5.- Modelling and practical generation of phase-space files for various linacs of interest to our partners in the MAESTRO project. Validation of the aforementioned computer codes and algorithms through comparison of simulation results with experimental dose distributions and predictions from commercial planning systems for both real patients and CT phantoms. 

6.- Different theoretical developments in  the following fields:

a) Electron multiple-scattering theory. 

b) Electron and positron interactions.

c) Applications of Monte Carlo simulation to x-ray microanalysis, simulation of x-ray generators, BGO detectors.

Figures and diagrams

Figure 1. Measured and simulated depth–dose distributions for electron beams with the indicated energies impinging normally on a thick silicon slab. Lines are results from PENELOPE and symbols represent the measurements of U. Werner et al. [J. Phys. D: Appl. Phys. 21 (1988) 116].

Figure 2. Details of a 10 x 10 cm2  electron applicator of a Varian 2300 C/D linac, modelled with PENGEOM.

(a)

Figure 3. Multileaf collimator (52 leaves) of a Varian 2300 C/D, modelled with PENGEOM (a),
and impact plot of particle in a phase-space plane just after the MLC showing the effect of leakage through closed-leaf zones (b).
On figure (b), full-red colour corresponds to the multileaf apperture.

Figure 4. View of a Varian 2300 C/D linac in electron mode with a 25´25 cm applicator, modelled with PENGEOM.

 

Figure 5. View of the Varian 2300 C/D linac in photon mode with MLC.

 


(a)                                                                                                     (b)

Figure 6. Central-axis depth dose and lateral profiles in water for electron beams from a Varian 2300 C/D linac with the indicated nominal energies (a). Profile at three depth (15 mm, 10 mm and 25 mm) for 6 MeV incident beam.

Figure 7. Isodose lines of a simulated 6 MV, 5´5 cm2, photon beam on a female patient