The purposes of this study were to investigate the feasibility of improving dosimetry with temporary low-dose-rate (LDR) multicatheter breast implants using directional (iodine) interstitial sources and to provide a comparison of a patient treatment plan to that achieved by conventional high-dose-rate (HDR) interstitial breast brachytherapy. A novel source emitting radiation in a specified direction has been developed. The directional sources contain an internal radiation shield that greatly reduces the intensity of radiation in the shielded direction. The sources have a similar dose distribution to conventional nondirectional sources on the unshielded side. The treatment plan for a patient treated with HDR interstitial brachytherapy with (iridium) was compared with a directional treatment plan using the same data set. Several dosimetric parameters are compared including target volume coverage, volume receiving , , and of the prescription dose (V50, V100, and V150, respectively), dose homogeneity index (DHI), and the skin surface areas receiving , , and of the prescription dose (S30, S50, and S80, respectively). The HDR and LDR prescription doses were 34 Gy in ten fractions delivered over five days and 45 Gy in 108 h, respectively. Similar and excellent target volume coverage was achieved by both directional LDR and HDR plans ( and , respectively). For a target volume, the dosimetric parameters were similar for LDR and HDR: DHI was 0.82 in both cases, V100 was and , and V150 was and , respectively. However, with directional LDR, significant reductions in skin dose were achieved: S30 was reduced from 100.6 to , S50 from 50.6 to , and S80 from to zero. The reduction in V50 for the whole breast was more than ( for LDR versus for HDR). In this case study, compared with HDR, directional interstitial LDR sources allow similar dose coverage to the subcutaneous target volume while lowering the skin dose due to a more conformal dose distribution and quicker falloff beyond the target. The improved dose distribution provided by directional interstitial brachytherapy might enable partial breast treatment to tumors closer to the skin or chest wall or in relatively small breasts.