A comparison of radial dose functions for 103Pd, 125I, 145Sm, 241Am, 169Yb, 192Ir, and 137Cs brachytherapy sources.

Recently, encapsulated sources of 103Pd (21 keV average), 145Sm (41 keV average), 241Am (60 keV), and 169Yb (93 keV average) have been introduced as alternatives to conventional brachytherapy sources of 125I Model 6711 (27 keV average), 125I Model 6702 (28 keV average), 192Ir (369 keV average), and 137Cs (662 keV). To illustrate the dependence of the penetrating ability of photons from brachytherapy sources as a function of photon energy, a comparison of their radial dose functions is presented. Using the ITS Monte Carlo simulation code for photon-electron transport, the radial dose functions were calculated for monoenergetic photon sources with energies in the range of 30 keV to 1 MeV. Also, similar calculations were performed using the photon spectra emitted by the encapsulated brachytherapy sources. To verify the accuracy of Monte Carlo calculations, comparisons are made with our new measured data for 241Am and existing experimental and theoretical data from other investigators. A comparison of radial dose functions indicates that for 241Am, 169Yb, 192Ir and 137Cs sources radial dose functions are close to unity for distances up to 10 cm, for 145Sm the radial dose function drops to about 0.4 at 10 cm, and for 125I and 103Pd it drops precipitously to less than 0.20 at 7 cm. At 5 cm, the measured radial dose functions for 103Pd, 125I Model 6711, 125I Model 6702, 145Sm, 241Am, and 192Ir have values of 0.09, 0.34, 0.38, 0.86, 1.12, and 0.97, respectively. While all of these radioisotopes provide adequate penetrating power for interstitial brachytherapy, only the radioisotopes emitting photons with energies greater than about 40 keV can provide adequate depth dose (that is, small or no tissue attenuation) for intracavitary irradiation. Our criterion for choice of minimum photon energy suitable for intracavitary irradiation is that the radial dose function at 5 cm should not be less than 0.90. Also, note that photons with energies around 80 keV exhibit maximum penetrating ability in solid water for distances up to 5 cm.