Dosimetry characteristics of the Plus and 12i Gammamed PDR [sup 192]Ir sources

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  • Dosimetry characteristics of the Plus and 12i Gammamed PDR 192 Ir sourcesJ. Prez-Calatayud, F. Ballester, M. A. Serrano-Andrs, V. Puchades, J. L. Lluch, Y. Limami, and F. Casal Citation: Medical Physics 28, 2576 (2001); doi: 10.1118/1.1418725 View online: http://dx.doi.org/10.1118/1.1418725 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/28/12?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Dosimetry comparison of 192 Ir Sources Med. Phys. 29, 2239 (2002); 10.1118/1.1508378 Technical note: Monte-Carlo dosimetry of the HDR 12i and Plus 192 Ir sources Med. Phys. 28, 2586 (2001); 10.1118/1.1420398 Beta versus gamma dosimetry close to Ir-192 brachytherapy sources Med. Phys. 28, 1875 (2001); 10.1118/1.1395038 Dosimetry close to an 192 Ir HDR source using N-vinylpyrrolidone based polymer gels and magnetic resonanceimaging Med. Phys. 28, 1416 (2001); 10.1118/1.1382603 Monte Carlo dosimetry of a new 192 Ir high dose rate brachytherapy source Med. Phys. 27, 2521 (2000); 10.1118/1.1315316

    http://scitation.aip.org/content/aapm/journal/medphys?ver=pdfcovhttp://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/test.int.aip.org/adtest/L23/1357685497/x01/AIP/RIT_MPHCovAd_1640x440Banner_2014/OnlineMedphys_Easy_As_Pie_Oct._2014.jpg/4f6b43656e314e392f6534414369774f?xhttp://scitation.aip.org/search?value1=J.+Prez-Calatayud&option1=authorhttp://scitation.aip.org/search?value1=F.+Ballester&option1=authorhttp://scitation.aip.org/search?value1=M.+A.+Serrano-Andrs&option1=authorhttp://scitation.aip.org/search?value1=V.+Puchades&option1=authorhttp://scitation.aip.org/search?value1=J.+L.+Lluch&option1=authorhttp://scitation.aip.org/search?value1=Y.+Limami&option1=authorhttp://scitation.aip.org/search?value1=F.+Casal&option1=authorhttp://scitation.aip.org/content/aapm/journal/medphys?ver=pdfcovhttp://dx.doi.org/10.1118/1.1418725http://scitation.aip.org/content/aapm/journal/medphys/28/12?ver=pdfcovhttp://scitation.aip.org/content/aapm?ver=pdfcovhttp://scitation.aip.org/content/aapm/journal/medphys/29/10/10.1118/1.1508378?ver=pdfcovhttp://scitation.aip.org/content/aapm/journal/medphys/28/12/10.1118/1.1420398?ver=pdfcovhttp://scitation.aip.org/content/aapm/journal/medphys/28/9/10.1118/1.1395038?ver=pdfcovhttp://scitation.aip.org/content/aapm/journal/medphys/28/7/10.1118/1.1382603?ver=pdfcovhttp://scitation.aip.org/content/aapm/journal/medphys/28/7/10.1118/1.1382603?ver=pdfcovhttp://scitation.aip.org/content/aapm/journal/medphys/27/11/10.1118/1.1315316?ver=pdfcov

  • Dosimetry characteristics of the Plus and 12i Gammamed PDR192Ir sources

    J. Perez-CalatayudPhysics Section, Radiation Oncology Department, La Fe University Hospital, Avda. Campanar 21,E-46009 Valencia, Spain

    F. Ballester,a) M. A. Serrano-Andres, V. Puchades, J. L. Lluch, and Y. LimamiDepartment of Atomic, Molecular and Nuclear Physics, University of Valencia, and Instituto de FsicaCorpuscular (IFIC), C/ Dr. Moliner 50, E-46100 Burjassot, Spain

    F. CasalDepartment of Atomic, Molecular and Nuclear Physics, University of Valencia,C/ Dr. Moliner 50,E-46100 Burjassot and Centro Nacional de Dosimetra, Avda. Campanar 21, E-46009 Valencia, Spain

    ~Received 16 March 2000; accepted for publication 26 September 2001!

    In this study a complete set of dosimetric data for the Plus and 12i Gammamed PDR~pulsed doserate! 192Ir sources is presented. These data have been calculated using the Monte Carlo simulationcodeGEANT3. Absolute dose rate distributions in water around these sources were calculated andare presented in form of conventional two dimensional~2D! Cartesian look-up tables. All dosim-etric quantities recommended by the AAPM Task Group 43 report have been also calculated. Thesequantities are dose rate constant, radial dose function, anisotropy function and anisotropy factor.The dose rate distribution of the 12i source was compared with the corresponding data for themicroselectron PDR source showing large differences between both sources. 2001 AmericanAssociation of Physicists in Medicine.@DOI: 10.1118/1.1418725#

    Key words: Ir-192, PDR sources, brachytherapy, dosimetry, Monte Carlo, treatment-planningsystems

    I. INTRODUCTION

    It is well known that the clinical use of brachytherapysources requires an extensive base of dosimetric data foreach source type. As it is indicated in the TG56 AAPMreport of brachytherapy1 accurate dose rate distributiontables based on realistic geometry and mechanical character-istic of each source type used in clinical practice areneeded to verify treatment planning system calculations, oras input data to them. Monte Carlo simulation technique is amethod chosen to estimate the dose rate distribution aroundbrachytherapy sources that can significantly reduce the ex-perimental uncertainties and provide the required accuratecalculations.

    The motivation to present in this study a complete dosi-metric evaluation of the Plus and 12i Gammamed PDR192Irsources was the lack of data for these sources in the litera-ture. TheGEANT3 Monte Carlo particle transport code2 wasused to calculate the 2D absolute dose rate distributions inwater for distances up to 15 cm from the source. The data arepresented as conventional 2D look-up tables~away-and-along! and in terms of the dosimetric ratios recommendedby the AAPM Task Group 43.3 The two dimensional doserate distribution of the 12i source was compared with thecorresponding data of the microselectron PDR source4 thathas a similar design and dimensions.

    II. MATERIAL AND METHODS

    A. The radioactive sources

    The Plus and 12i Gammamed PDR192Ir sources, accord-ing to the material composition and the construction designprovided by Gammamed,5 consist of a cylindrical active iri-dium core~70% Ir and 30% Pt,r521.76 g/cm3! of 0.6-mmdiameter and 0.5-mm length~Fig. 1!. The active iridium isuniformly distributed in this core, which is encapsulated in astainless steel wire~No 1.4404!. The active core in bothsources is filtered in the extreme end by a cylindrical piece ofaluminum ~No 3.1645! and a cap of stainless steel~No.1.4404!. The extreme end of this cap was approximated inthe Monte Carlo geometry by a cone 0.2-mm length for the12i source and 0.12-mm length for the Plus source. In thelower part of the source, the radiation is filtered by 0.5-mmstainless steel in the 12i source and by 0.3-mm stainless steelin the Plus source. Dimensions of the different parts of bothsources are included in the diagram shown in Fig. 1.

    These PDR sources are used in single-stepping source re-mote afterloaders. The source welded to the end of cableadvances through one catheter after another, sequentiallytreating each programmed dwell position. The dose rate dis-tribution delivered to the patient is then calculated by addingthe dose rate distribution delivered in each dwell position. Ineach dwell position the source and the cable orientation

    2576 2576Med. Phys. 28 12, December 2001 0094-2405 20012812257610$18.00 2001 Am. Assoc. Phys. Med.

  • changes and the calculation of the total dose rate distributiondelivered must take into account the presence of the cable. Inorder to provide realistic dose rate distributions data of thePDR sources, in this study the extension of the proximal endof the cable was modeled as 6 cm long and an effectivedensity5 of r55.6 g/cm3.

    B. Monte Carlo simulation code

    The Monte Carlo method has been used to calculate air-kerma strength and absolute dose rate values to water inwater around the Plus and 12i Gammamed PDR192Irsources. The calculations have been performed by means ofthe GEANT3 Monte Carlo code.2 Methods, approximationsand restrictions about the physical processes can be found inthe GEANT3 user manual.6

    All physical processes for low energy photons are imple-mented inGEANT3: photoelectric effect, Rayleigh and Comp-ton scattering and pair production. The total cross-section ofe2e1 pair production has been parametrized from a least-square fit to the data of Hubbellet al.7 and the BetheHeitlerdifferential cross-section with the Coulomb correction hasbeen assumed.8 For the total cross-section for Compton scat-tering an empirical cross-section formula is used which re-produces the cross-section data fairly accurately well downto 10 keV. Electron binding energy is ignored. The totalcross-section for photoelectric effect was fitted as SANDIAparametrization.9 After photoelectric emission a fluorescentphoton or Auger electron is emitted. The decay scheme for

    192Ir was taken from nuclear data sheets.10 The cut-off energyfor photons was taken at 10 keV. The validation of the codefor Ir-192 energies have been described in previous publica-tions by our group.1113

    For electrons, Gaussian multiple scattering, bremsstrah-lung production~in the source materials! and continuous en-ergy loss were assumed.6 The cut-off energy for electronswas taken at 10 keV. For the Ir-192 energies electronic equi-librium exists for radial distancesr .1 cm from the source,and then dose and water kerma are equal.

    C. Monte Carlo calculations and data analysis

    The coordinate systems used in this study are shown inFig. 1. The origin was taken at the center of the active coreof the source, the long axis of the source has been chosen asthe z-axis ~which coincides with the polar axis! and they-axis along the transverse bisector. The cylindrical symme-try of the sources allows us to present the data as a twodimensional dose rate distributions although Monte Carlocalculations have been done in three dimensions.

    1. Air-kerma strength simulation

    The air-kerma strength of a source,Sk , is specified interms of air-kerma rate at a point along the transverse axis ofthe source in free space. It is defined14 as the product ofair-kerma rate at a calibration distance,d, in free space,kair(d), measured along the transverse bisector of the source(y-axis!, and the square of the distance, that is,Sk5 kaird

    2 inU units ~1 U 5 1 mGy m2 h21!. To evaluateSk , the sourceswere positioned in a 63636 m3 dry air cube and cells toscore air-kerma were defined for transverse axis distancesranging from 2 cm to 150 cm. The air-kerma has been scoredat distances from the source large enough so as to treat thesource as a mathematical point. Cell sizes were taken asrings of cross-sectional area withDy5Dz51 cm, and 23109 photon histories were simulated in four calculations toestimate statistical fluctuations. The approach to point esti-mation of the collision air-kerma was the analog estimatormethod.

    The source strengthSk was estimated by fitting the air-kerma data4 to the linear equationkair(r ,u590)r

    25Sk1br. The slope b describes the deviation inkair(r ,u590)r 2 due to the build-up of scatter in the air and theintercept is an estimate of the product of the air-kerma rate infree space and the square of the distance.

    2. Dose rate simulations in water

    In order to reach full scatter conditions, a cylinder of wa-ter of 40 cm height and 40 cm in diameter was assumed. Twogrid systems were set up to score the absorbed dose rate: onein Cartesian coordinates,D(y,z), and the other in polar co-ordinates, D(r ,u). The purpose of this double scoringmethod was to avoid interpolation problems because the cal-culation of the anisotropy functionF(r ,u) from the Carte-sian dose rate distributionD(y,z) do not provide anF(r ,u)function for uniformly distributedr andu values. For Carte-

    FIG. 1. Mechanical design of the 12i and Plus Gammamed PDR192Irsources. Dimensions are in mm.

    2577 Perez-Calatayud et al. : Plus and 12i Gammamed PDR 192Ir sources 2577

    Medical Physics, Vol. 28, No. 12, December 2001

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