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Monte carlo calculation of dose delivered to patients in radiotherapy treatments
829
105
106
ALGORITHMSFOR FAST MONTECARLO ELECTRON BEAMTREATMENTPLANNING
MONTE CARLO CALCULAnON OF DOSE DELIVERED TO PAnENTS IN RADIOTHERAPY TREATMENTS.
HansNeuenschwander, Dept of Medical Radiation Physics, University of Berne,Berne, Switzerland
I.Jtj1Jllo dI Fi8ictI Sperl_tole, ilia P.Gillrio, 1- ToriIIO .It4ly
Although hardwareperformance has increased enormously duringthe last decade, conventional general purpose Monte Carlo (MC) codes likeEGS and ETRAN/ITS are stilltoo slow to be of practical use in routine radiotherapy treatment planning (RTP). Fortunately, in RTP the range of materials and energies to be considered in an electron dose calculation is very limited, and one is primarily interested in the dose deposited in a voxel-based geometry. Therefore, a lot of approximations (eg. neglect or positrons, global treatment of bremsstrahlung etc.) can be made without compromising the high accuracy that has to be achieved in a clinical dose calculation. This fact has allowed the development of simpler and faster MC algorithms dedicated to and optimized for electron beam treatment planning. One class of algorithms utilizes various possible approximations while still retaining some 'microscopic' characterization of electronpaths through the absorber. Anotherclass of algorithms are the so-called 'local-to-global' MC methods. These algorithms rely on precalculated and tabulated results of electron transport calculations in a well-defined 'local'geometry. Based on these precalculated distributions, an absorber-specific 'global' calculation is performed by transporting electronsin 'macroscopic' steps throughthe absorber. As an example of a 'local-to-global' algorithm, the Macro Monte Carlo (MMC) methodand its implementation on a parallel computerwillbe discussed in some detail. Its high performance in terms of speed and accuracy will be demonstrated for typical clinical situations. In conclusion, the increase in hardware performance together with algorithmic approaches to speed up electron MC have lead us to a stage where MC methods become a feasible alternative to today's commercially implemented electrondose calculation algorithms.
A ZaniDi
C. Manfredotti UNastui O~S.GiOWlll1liA.S .• iliae-.
31-ToriltD.ItaIy
IstilJllo NlIZiotIak di FisiaJNIIC1etn. ilia P.Gillrio, I - Torino. Italy
A Monte Cado user codeca1Ied SHAPE andbased on EGS4 c:ode basbeen developed. It is based on I tbreHimensioIII geometry in orderto siudate electron or photon beams geoerated by I lineIr ae:celerator and interacting with patient's CT slices. A geometry of planes peraI1el to x,y,z exes is used beeIuse it defines the same volume elements (VOXELS) of CT image. The IuF DIIDIbcr of voxels is conveniently reduced by an originII IIgoritbm called UNION wbicb coupledto I definition of one or more high resolution zones seIccted by the user and to I fist identificlltion of acluaI particle region, allows I strongly reduction of CPU time. In practice, SHAPE works simultaneously both in high and low resolution geometry. giving accurate results only in regions wbich are interesting from I radiotbaapic point of view (central slice, aitical organs at risk, tumour volume, etc.). It is important to note that UNION a1goritlnn does not affect the resolution of heterogeneities boundaries in low resolution zones. Conceming iDcidClld photon beams, it is important to note that the eDeIBY spec:trum of photon canying out liom I LINAC simulated in great details in all its components, basbeenused. The results are good and compare \'U)' well with experimaUI dIta obtIined both in I water pbmtom and in an anthropomorphic phantom. Treatment pIanninp of held and tboru: are pneented. Cbecb on CIIcuIation time are curied out both with and without PRESTA algorithm. Finally, an evaluation of the cfI'ective neutron dose delMnd to the patientduring I tre8tIIIed bas been curied out by using MCNP c:ode and(r,n) cross section dataliom literature.
107
108 Improving the accuracy of fast 3D photondosecalculations with the aid of a superposition algorithm
Monte-Carlo developments for radiation oncology: the PEREGRINE program C. Hartmann Siantar, W.P. Chandler. l.R. Rathkopf, M.M. Svatos, M.B. Chadwick. LJ. Cox. D.A. Resler. R.M. White (Livermore, USA)
M. M. Asprada.kis(l), A. T. Redpath(l), O. A. Sauer(2) 1.
2.
Deputm••" of ...dic;.J, Pb,.i... load Clink,,} ODeolol1. The 'tl'.u._lIitJ of adiablll'll., adiabar,h. Scotlqd, UK: Uai"onh"·Pnuell.ldiaik, Stn.hleliabtei1uDI, W1i.nbllr" Gum••,
Abstract notreceived The superposition model [1], [2], can be considered to bridge the gap between fast, but approximate, photon dose calculation algorithms and Monte Carlo simulations. Although faster than Monte Carlo ca.\culations, the method is currently computationally too intensive for clinical use. This work utilizes a superposition code in conjunction with a conventional, fast (simplistic) photon dose calculation a.\gorithm in order to improve the accuracy of the latter within clinically acceptable times. In 3D radiotherapytreatment planning, patient density information is obtained from CT-images. Conventional algorithmscan be implemented in three dimensions in a voxel by voxel calculation, even while usingtwo dimensional densityinformation to accountfor the inhomogeneous nature of the patient. The dose from the inaccurate inhomogeneity corrections is modified by further applyinga correctionfactor derived fromsuperposition. This approach is essentially a three step procedure. First dose values are obtainedin 3Don a finerectangulargrid usinga fast dosecalculation method such as the power-law (Batho). Next, dose values from a superposition calculation are obtainedon a coarsematrix. The ratio of dosevaluesfromthe twoseparatecalculations defines a correction factor, values of which are interpolatedfor all pointson the finegrid and finally applied to the dose values derived from the simplistic method. The performance ofthis approachis tested fordifferent heterogeneous phantoms.
(II
1'1
A., A.-ch~. P., Ih. . . . . A.. Oalc'lllatioD _do .pplica'ioD oi pOUlt .pread faBc, tio.1 for tr•• tme.t plaaam, witk hi,b. ...... 7 photOD b..... , Acta Oll.co1o,ica 2&, Iii?
.A.aII-JO.
Paa»-~.
N., Mackie, T. R. o "eapr-WeB., C ••
a.......
M. 1.".I,i.alioD of th.
coa'tolulioD m.thod. for poly'.u•• tic: .peel,., Wed. Pt.,•. 20(&), 1113
105
106
ALGORITHMSFOR FAST MONTECARLO ELECTRON BEAMTREATMENTPLANNING
MONTE CARLO CALCULAnON OF DOSE DELIVERED TO PAnENTS IN RADIOTHERAPY TREATMENTS.
HansNeuenschwander, Dept of Medical Radiation Physics, University of Berne,Berne, Switzerland
I.Jtj1Jllo dI Fi8ictI Sperl_tole, ilia P.Gillrio, 1- ToriIIO .It4ly
Although hardwareperformance has increased enormously duringthe last decade, conventional general purpose Monte Carlo (MC) codes likeEGS and ETRAN/ITS are stilltoo slow to be of practical use in routine radiotherapy treatment planning (RTP). Fortunately, in RTP the range of materials and energies to be considered in an electron dose calculation is very limited, and one is primarily interested in the dose deposited in a voxel-based geometry. Therefore, a lot of approximations (eg. neglect or positrons, global treatment of bremsstrahlung etc.) can be made without compromising the high accuracy that has to be achieved in a clinical dose calculation. This fact has allowed the development of simpler and faster MC algorithms dedicated to and optimized for electron beam treatment planning. One class of algorithms utilizes various possible approximations while still retaining some 'microscopic' characterization of electronpaths through the absorber. Anotherclass of algorithms are the so-called 'local-to-global' MC methods. These algorithms rely on precalculated and tabulated results of electron transport calculations in a well-defined 'local'geometry. Based on these precalculated distributions, an absorber-specific 'global' calculation is performed by transporting electronsin 'macroscopic' steps throughthe absorber. As an example of a 'local-to-global' algorithm, the Macro Monte Carlo (MMC) methodand its implementation on a parallel computerwillbe discussed in some detail. Its high performance in terms of speed and accuracy will be demonstrated for typical clinical situations. In conclusion, the increase in hardware performance together with algorithmic approaches to speed up electron MC have lead us to a stage where MC methods become a feasible alternative to today's commercially implemented electrondose calculation algorithms.
A ZaniDi
C. Manfredotti UNastui O~S.GiOWlll1liA.S .• iliae-.
31-ToriltD.ItaIy
IstilJllo NlIZiotIak di FisiaJNIIC1etn. ilia P.Gillrio, I - Torino. Italy
A Monte Cado user codeca1Ied SHAPE andbased on EGS4 c:ode basbeen developed. It is based on I tbreHimensioIII geometry in orderto siudate electron or photon beams geoerated by I lineIr ae:celerator and interacting with patient's CT slices. A geometry of planes peraI1el to x,y,z exes is used beeIuse it defines the same volume elements (VOXELS) of CT image. The IuF DIIDIbcr of voxels is conveniently reduced by an originII IIgoritbm called UNION wbicb coupledto I definition of one or more high resolution zones seIccted by the user and to I fist identificlltion of acluaI particle region, allows I strongly reduction of CPU time. In practice, SHAPE works simultaneously both in high and low resolution geometry. giving accurate results only in regions wbich are interesting from I radiotbaapic point of view (central slice, aitical organs at risk, tumour volume, etc.). It is important to note that UNION a1goritlnn does not affect the resolution of heterogeneities boundaries in low resolution zones. Conceming iDcidClld photon beams, it is important to note that the eDeIBY spec:trum of photon canying out liom I LINAC simulated in great details in all its components, basbeenused. The results are good and compare \'U)' well with experimaUI dIta obtIined both in I water pbmtom and in an anthropomorphic phantom. Treatment pIanninp of held and tboru: are pneented. Cbecb on CIIcuIation time are curied out both with and without PRESTA algorithm. Finally, an evaluation of the cfI'ective neutron dose delMnd to the patientduring I tre8tIIIed bas been curied out by using MCNP c:ode and(r,n) cross section dataliom literature.
107
108 Improving the accuracy of fast 3D photondosecalculations with the aid of a superposition algorithm
Monte-Carlo developments for radiation oncology: the PEREGRINE program C. Hartmann Siantar, W.P. Chandler. l.R. Rathkopf, M.M. Svatos, M.B. Chadwick. LJ. Cox. D.A. Resler. R.M. White (Livermore, USA)
M. M. Asprada.kis(l), A. T. Redpath(l), O. A. Sauer(2) 1.
2.
Deputm••" of ...dic;.J, Pb,.i... load Clink,,} ODeolol1. The 'tl'.u._lIitJ of adiablll'll., adiabar,h. Scotlqd, UK: Uai"onh"·Pnuell.ldiaik, Stn.hleliabtei1uDI, W1i.nbllr" Gum••,
Abstract notreceived The superposition model [1], [2], can be considered to bridge the gap between fast, but approximate, photon dose calculation algorithms and Monte Carlo simulations. Although faster than Monte Carlo ca.\culations, the method is currently computationally too intensive for clinical use. This work utilizes a superposition code in conjunction with a conventional, fast (simplistic) photon dose calculation a.\gorithm in order to improve the accuracy of the latter within clinically acceptable times. In 3D radiotherapytreatment planning, patient density information is obtained from CT-images. Conventional algorithmscan be implemented in three dimensions in a voxel by voxel calculation, even while usingtwo dimensional densityinformation to accountfor the inhomogeneous nature of the patient. The dose from the inaccurate inhomogeneity corrections is modified by further applyinga correctionfactor derived fromsuperposition. This approach is essentially a three step procedure. First dose values are obtainedin 3Don a finerectangulargrid usinga fast dosecalculation method such as the power-law (Batho). Next, dose values from a superposition calculation are obtainedon a coarsematrix. The ratio of dosevaluesfromthe twoseparatecalculations defines a correction factor, values of which are interpolatedfor all pointson the finegrid and finally applied to the dose values derived from the simplistic method. The performance ofthis approachis tested fordifferent heterogeneous phantoms.
(II
1'1
A., A.-ch~. P., Ih. . . . . A.. Oalc'lllatioD _do .pplica'ioD oi pOUlt .pread faBc, tio.1 for tr•• tme.t plaaam, witk hi,b. ...... 7 photOD b..... , Acta Oll.co1o,ica 2&, Iii?
.A.aII-JO.
Paa»-~.
N., Mackie, T. R. o "eapr-WeB., C ••
a.......
M. 1.".I,i.alioD of th.
coa'tolulioD m.thod. for poly'.u•• tic: .peel,., Wed. Pt.,•. 20(&), 1113