Phantom size in brachytherapy source dosimetric studies

Abstract

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.

Kerma and Photons Absorbed-Dose from Ir-192 HDR Seeds

“…Results achieved in this work for g(r) are consistent with the ones calculated in the past by Granero et al [2], Daskalov et al [3] corrected to full scattering conditions [1,29] and Taylor and Rogers [11]. Table III …”

“…The function obtained from our data continues very close t obtained from photons-AD rate profile although stay just above, this difference being less than or equal Results achieved in this work for ones calculated in the past by Granero [3] corrected to full scattering conditions [1,29] and Taylor and Rogers [11]. Table III distance.…”

Brachytherapy treatment planning commissioning: effect of the election of proper bibliography and finite size of TG‐43 input data on standard treatments

“…Figure 2 depicts the effect of the phantom size on the g(r), where agreement better than 2% is achieved only for $\mathrm{r}<4 \text{cm}$ among the simulations with different phantom sizes. Accordingly, it is imperative that this information be used in the dose evaluation for organs over 4 cm from the applicator 13 , 19 , 20 …”

“…Concerning $F(r,\theta )$, the phantom size chosen has less influence in the simulation results than for g(r) because of its definition, 13 , 19 and it appears that the sphere is the best choice of phantom shape to calculate the $F(r,\theta )$ (19) . There was good agreement among different simulations, with discrepancies over 5% distributed in angles very close to 0° and 180° and for large r values (over 10 cm) 10 , 11 , 12 , 13 …”

Comparison of TG‐43 dosimetric parameters of brachytherapy sources obtained by three different versions of MCNP codes

“…A subsequent Working Group from AAPM has recommended the dosimetric evaluation of high‐energy sources (16) using a similar method. Several different Monte Carlo codes have been utilized for the dosimetric evaluation of brachytherapy sources including MCNP4C2, 17 , 18 MCNP5, (19) MCNPX, (20) EGS4, (21) EGSnrc, (22) PTRAN, (5) and GEANT4 (23) . Each of these codes uses a different cross‐sectional library for both photons and electrons for all the chemical elements within the energy range of 1 eV to 1 GeV.…”