SU-E-T-501: Perturbation of TG-43 Parameters of the Brachytherapy Sources under Insufficient Scattering Materials

According to the TG-43 recommendations of American Association of Physicist in Medicine (AAPM), the dosimetry parameters of brachytherapy sources are obtained in a water phantom with full scattering conditions. However, in many actual clinical treatments the source are not surrounded with sufficient tissue in all directions to provide the full scattering condition. HDR brachytherapy of the breast or 125I eye-plaque treatment of the ocular melanoma are among the treatment sites that fits this category. In this project, the impact of insufficient phantom material surrounding the 137Cs, 192Ir, and 103Pd brachytherapy source on their TG43 dosimetric characteristics has been investigated.

In this study, the effect of the insufficient tissue (referred here as missing tissues) around the brachytherapy sources on their TG-43 dosimetric parameters have been investigated using MCNP5 Monte Carlo code. The brachytherapy sources were simulated in different locations inside a cubical water phantom with the dimensions of 30*30*30 cm³ . The variation of the dosimetric parameters of the three sources (i.e. Î>, g(r), and F(r,θ)) were compared to the values from full scattering conditions (i.e. source at the center of the phantom).

The results of this study indicate the variation of g(r)and 2D anisotropy function of the brachytherapy sources as a function of the missing tissue thickness. These changes increase by decreasing the energy of the photons emitted by the brachytherapy sources. These differences are mainly due to the lack of full scattering condition for the points near the phantom boundary.

Unlike the published data for symmetric, but insufficient phantom material around the source, the impact of asymmetric phantom materials have been evaluated on dosimetric characteristics of the brachytherapy sources.