Updating the Planar Patterson- Parker Table for Ir-192 and Cs-137 Brachytherapy Sources Using the Most Recent TG-43U1 Recommended Dosimetric Parameters

Background: The Patterson-Parker table was created in 1934 to determine mg-hr required to deliver the prescribed dose to the treatment area. These tables were created using the dosimetric data for 226Ra that was determined in air and were utilized for dose calculations around implants with Ra-equivalent radionuclides such as 137Cs and 192Ir. Therefore, the differences of the tissue attenuation and anisotropy of these radionuclides and their impact on dose uniformity of the implants were ignored. Objective: In this study, the Patterson-Parker table has been updated for 137Cs and 192Ir sources using their recent dosimetric data. Furthermore, the dose uniformity for different loading schemes, as a function of the implant area, was tested. Methods: The updated Paterson-Parker tables were generated for 137Cs and 192Ir sources using their published dosimetric parameters that have been determined following the American Association of Physicists in Medicine (AAPM) Task group 43 (TG-43U1) recommendations. The accuracies of the updated tables were examined by two independent methods, Monte Carlo simulation technique and a commercially available treatment planning system. In addition to the dose values along the central axis of the implant, dose profiles along two orthogonal directions were evaluated for the selection of the optimal radioactivity distribution in an implant. Results: We found that the original Paterson-Parker loading scheme of the radioactivity as a function of the implant size has to be adjusted to achieve the optimal dose distribution (1000 cGy ±10%) in an implant. In addition, it was shown that the same implant size for the mg-hr required for 137Cs was not identical to that of 192Ir. Similarly, there were some differences between the updated table and the published Paterson-Parkers tables. Independent Monte Carlo simulations and treatment planning data had excellent agreement with the updated data. Conclusion: The conventional version of Paterson-Parker tables are not useful for the two commonly used brachytherapy sources. The updated version of the tables should be used instead.