Phantom size in brachytherapy source dosimetric studies.

An important point to consider in a brachytherapy dosimetry study is the phantom size involved in calculations or experimental measurements. As pointed out by Williamson [Med. Phys. 18, 776-786 (1991)] this topic has a relevant influence on final dosimetric results. Presently, one-dimensional (1-D) algorithms and newly-developed 3-D correction algorithms are based on physics data that are obtained under full scatter conditions, i.e., assumed infinite phantom size. One can then assume that reference dose distributions in source dosimetry for photon brachytherapy should use an unbounded phantom size rather than phantom-like dimensions. Our aim in this paper is to study the effect of phantom size on brachytherapy for radionuclide 137Cs, 192Ir, 125I and 103Pd, mainly used for clinical purposes. Using the GEANT4 Monte Carlo code, we can ascertain effects on derived dosimetry parameters and functions to establish a distance dependent difference due to the absence of full scatter conditions. We have found that for 137Cs and 192Ir, a spherical phantom with a 40 cm radius is the equivalent of an unbounded phantom up to a distance of 20 cm from the source, as this size ensures full scatter conditions at this distance. For 125I and 103Pd, the required radius for the spherical phantom in order to ensure full scatter conditions at 10 cm from the source is R = 15 cm. A simple expression based on fits of the dose distributions for various phantom sizes has been developed for 137Cs and 192Ir in order to compare the dose rate distributions published for different phantom sizes. Using these relations it is possible to obtain radial dose functions for unbounded medium from bounded phantom ones.

[1]  F. Ballester,et al.  A monte carlo study of dose rate distribution around the specially asymmetric CSM3-a 137Cs source. , 2001, Physics in medicine and biology.

[2]  J. Williamson,et al.  Monte Carlo modeling of the transverse-axis dose distribution of the model 200 103Pd interstitial brachytherapy source. , 2000, Medical physics.

[3]  Robert E. Wallace,et al.  Dosimetric characterization of a new design 103 palladium brachytherapy source. , 1999, Medical physics.

[4]  L. Anderson,et al.  Dosimetry of interstitial brachytherapy sources: Recommendations of the AAPM Radiation Therapy Committee Task Group No. 43 , 1995 .

[5]  D Baltas,et al.  Monte Carlo dosimetry of a new 192Ir pulsed dose rate brachytherapy source. , 2003, Medical physics.

[6]  F Ballester,et al.  Monte Carlo dosimetric characterization of the Cs-137 selectron/LDR source: evaluation of applicator attenuation and superposition approximation effects. , 2004, Medical physics.

[7]  L. Sakelliou,et al.  Dose rate distributions around 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm. , 1992, Physics in medicine and biology.

[8]  F. Ballester,et al.  Monte Carlo calculations of dose rate distributions around the Amersham CDCS-M-type 137Cs source. , 2000, Medical physics.

[9]  J. Williamson,et al.  Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations. , 2004 .

[10]  Experimental determination of the TG-43 dosimetric characteristics of EchoSeed model 6733 I25I brachytherapy source. , 2002, Medical physics.

[11]  L. Sakelliou,et al.  Monte Carlo dosimetry of 60Co HDR brachytherapy sources. , 2003, Medical physics.

[12]  L. Sakelliou,et al.  Monte Carlo and TLD dosimetry of an 192Ir high dose-rate brachytherapy source. , 1998, Medical physics.

[13]  A. Meigooni,et al.  Experimental and theoretical determination of dosimetric characteristics of IsoAid ADVANTAGE 125I brachytherapy source. , 2002, Medical physics.

[14]  R Nath,et al.  On the depth of penetration of photons and electrons for intravascular brachytherapy. , 1999, Cardiovascular radiation medicine.

[15]  F. Ballester,et al.  Monte Carlo calculation of dose rate distributions around the Walstam CDC.K-type 137Cs sources. , 2001, Physics in medicine and biology.

[16]  J F Williamson,et al.  Monte Carlo aided dosimetry of the microselectron pulsed and high dose-rate 192Ir sources. , 1995, Medical physics.

[17]  R Nath,et al.  Dosimetric prerequisites for routine clinical use of new low energy photon interstitial brachytherapy sources. Recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee. Ad Hoc Subcommittee of the Radiation Therapy Committee. , 1998, Medical physics.

[18]  J F Williamson,et al.  Monte Carlo evaluation of kerma at a point for photon transport problems. , 1987, Medical physics.

[19]  F. Ballester,et al.  A Monte Carlo investigation of the dosimetric characteristics of the CSM11 137Cs source from CIS. , 2000, Medical physics.

[20]  G. Hartmann,et al.  Monte Carlo calculation of energy build-up factors in the range from 15 keV to 100 keV, with special reference to the dosimetry of 125I seeds , 1988 .

[21]  G Zarris,et al.  Accurate Monte Carlo calculations of the combined attenuation and build-up factors, for energies (20-1500 keV) and distances (0-10 cm) relevant in brachytherapy. , 1991, Physics in medicine and biology.

[22]  F. Ballester,et al.  Dosimetric characteristics of the CDC-type miniature cylindrical 137Cs brachytherapy sources. , 2002, Medical physics.

[23]  J F Williamson,et al.  Comparison of measured and calculated dose rates in water near I-125 and Ir-192 seeds. , 1991, Medical physics.

[24]  J. Williamson,et al.  Monte Carlo-based dose-rate tables for the Amersham CDCS.J and 3M model 6500 137Cs tubes. , 1998, International journal of radiation oncology, biology, physics.

[25]  D. Baltas,et al.  An analytical dosimetry model as a step towards accounting for inhomogeneities and bounded geometries in 192Ir brachytherapy treatment planning. , 2003, Physics in medicine and biology.

[26]  W. Dries,et al.  Measurement and calculation of the dose at large distances from brachytherapy sources: Cs-137, Ir-192, and Co-60. , 1996, Medical physics.

[27]  N. Yue,et al.  Experimental determination of dosimetric characterization of a newly designed encapsulated interstitial brachytherapy source of 103Pd-model Pd-1. , 2002, Medical physics.