Beta versus gamma dosimetry close to Ir-192 brachytherapy sources

The relative importance of the dose rate component owing to the beta spectr um emitted by Ir-192 brachytherapy sources at the short radial distances of interest in intravascular and endobronchial applications is investigated. Separate dosimetric calculations, using Monte Carlo simulations, were perfo rmed for the gamma and beta dose rate components of an Ir-192 ideal point s ource as weld as real Ir-192 source designs used in clinical practice inclu ding wire and seed sources and both Nucletron and Varian, old and new, high dose rate (HDR) source designs. A significant dose rate enhancement due to the beta spectrum emitted by Ir-192, greater than 50% for radial distances r < 2 mm, was observed for an ideal point source. For real source designs, however, the magnitude of this enhancement was found to depend strongly on the sources' geometric as well as compositional details of the active core and encapsulation. A detectable effect was found for the majority of the i nvestigated sources at radial distances less than I nim, but overall findin gs suggest that the contribution of beta particles is not significant in Ir -192 clinical intravascular applications that are currently carried out. Ho wever, since treatment of vessels with smaller diameters, in the future, ma y lead to the development of Ir-192 sources and catheters of reduced diamet ers, the potential effect of the beta spectrum in terms of dose enhancement to tissues in close proximity to Ir-192 sources should not be ignored. (C) 2001 American Association of Physicists in Medicine.