Beta versus gamma dosimetry close to Ir-192 brachytherapy sources.

The relative importance of the dose rate component owing to the beta spectrum emitted by 192Ir brachytherapy sources at the short radial distances of interest in intravascular and endobronchial applications is investigated. Separate dosimetric calculations, using Monte Carlo simulations, were performed for the gamma and beta dose rate components of an 192Ir ideal point source as well as real 192Ir source designs used in clinical practice including 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 192Ir, 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 investigated sources at radial distances less than 1 mm, but overall findings suggest that the contribution of beta particles is not significant in 192Ir clinical intravascular applications that are currently carried out. However, since treatment of vessels with smaller diameters, in the future, may lead to the development of 192Ir sources and catheters of reduced diameters, the potential effect of the beta spectrum in terms of dose enhancement to tissues in close proximity to 192Ir sources should not be ignored.