MICRODOSIMETRIC EVALUATION OF RELATIVE BIOLOGICAL EFFECTIVENESS FOR PD-103, I-125, (241)AM, AND IR-192 BRACHYTHERAPY SOURCES

Purpose: To determine the microdosimetric-derived relative biological effectiveness (RBE) of Pd-103, I-125, Am-241, and Ir-192 brachytherapy sources at low doses and/or low dose rates. Methods and Materials: Th e Theory of Dual Radiation Action can be used to predict expected RBE values based on the spatial distribution of energy deposition at micro scopic levels from these sources. Single-event Lineal energy spectra f or these isotopes have been obtained both experimentally and theoretic ally. A grid-defined wall-less proportional counter was used to measur e the lineal energy distributions. Unlike conventional Rossi proportio nal counters, the counter used in these measurements has a conducting nylon fiber as the central collecting anode and has no metal parts. Th us, the Z-dependence of the photoelectric effect is eliminated as a so urce of measurement error. Single-event spectra for these brachytherap y sources have been also calculated by: (a) the Monte Carlo code MCNP to generate the electron slowing down spectrum, (b) transport of monoe nergetic electron tracks, event by event, with our Monte Carlo code DE LTA, (c) using the concept of associated volume to obtain the lineal e nergy distribution f(y) for each monoenergetic electron, and (d) obtai ning the composite lineal energy spectrum for a given brachytherapy so urce based on the electron spectrum calculated at step (a). Results: R elative to Co-60, the RBE values obtained from this study are: 2.3 for Pd-103, 2.1 for I-125, 2.1 for Am-241, and 1.3 for Ir-192. Conclusion s: These values are consistent with available data from in vitro cell survival experiments. We suggest that, at least for these brachytherap y sources, microdosimetry may be used as a credible alternative to tim e-consuming (and often uncertain) radiobiological experiments to obtai n information on radiation quality and make reliable predictions of RB E in low dose rate brachytherapy. Copyright (C) 1996 Elsevier Science Inc.