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Relative biological effectiveness enhancement of a 125 I brachytherapy seed with characteristic x rays from its constitutive materials
Author(s) -
Taschereau Richard,
Roy René,
Pouliot Jean
Publication year - 2002
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1485054
Subject(s) - brachytherapy , relative biological effectiveness , monte carlo method , materials science , dosimetry , nuclear medicine , isotope , radiochemistry , physics , medicine , optics , chemistry , radiation therapy , radiation , radiology , mathematics , nuclear physics , statistics
The isotopes used for permanent prostate implants,125 I and103 Pd , provide about equivalent tumor control. The purpose of this study is to investigate how characteristic x rays may be used to raise the relative biological effectiveness (RBE) of an iodine seed at short distances to increase the differential effect between tumor and healthy tissue. Within the theoretical framework of microdosimetry, the GEANT4 Monte Carlo simulation toolkit has been used to calculate the RBE of experimental seed designs in which shell and core dimensions and composition were varied independently. A new seed model was also simulated based on the best results obtained. The RBE could be enhanced by increasing the shell thickness and for the range considered, optimum results were obtained by using gradually lower atomic number elements. For a practical 50–60 μm shell, molybdenum is the material of choice. The core diameter has little influence on RBE, but maximum effectiveness is obtained with yttrium or zirconium. These results were put together to design a Mo‐shell and Y‐core seed for which the RBE enhancement was at least 5–7 % (close to the source), which is higher than palladium. This enhanced RBE combined with the longer half‐life of iodine could mean comparable tumor control and better protection to organs at risk than with current seeds. The RBE dependence on distance is an interesting feature that could benefit other applications such as ocular melanoma or coronary brachytherapy where a highly localized dose distribution is desired.