The adequacy of an interstitial implant carried out with a new radioactive source, the 169 Yb seed model X1267, has been examined by computing volumetric indices based on dose-volume histograms. The comparison of these indices with the ones computed for 125I seed implantations shows that the use of ytterbium seeds presents an improvement of the dose homogeneity in interstitial implants. This is due to the significant build-up associated with 169 Yb photons that reduces the rapid dose fall-off with the distance from the source. Moreover, relative to 192Ir, the lower photon energy gives 169 Yb the advantage in clinical use of reduced radiation exposure (i) to health care workers, (ii) to relatives of treated patients and (iii) to healthy neighbouring tissues of the patients if appropriate thin shielding is used.