The use of directional interstitial sources to improve dosimetry in breast brachytherapy

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Abstract

The purposes of this study were to investigate the feasibility of improving dosimetry with temporary low-dose-rate (LDR) multicatheter breast implants using directional I125 (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 I125 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 I192r (iridium) was compared with a directional I125 treatment plan using the same data set. Several dosimetric parameters are compared including target volume coverage, volume receiving 50%, 100%, and 150% of the prescription dose (V50, V100, and V150, respectively), dose homogeneity index (DHI), and the skin surface areas receiving 30%, 50%, and 80% 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 (99.2% and 97.5%, respectively). For a 170cm3 target volume, the dosimetric parameters were similar for LDR and HDR: DHI was 0.82 in both cases, V100 was 214.4cm3 and 225.7cm3, and V150 was 39.1cm3 and 40.4cm3, respectively. However, with directional LDR, significant reductions in skin dose were achieved: S30 was reduced from 100.6 to 62.5cm2, S50 from 50.6 to 16.1cm2, and S80 from 2cm2 to zero. The reduction in V50 for the whole breast was more than 100cm3 (386.1cm3 for LDR versus 489.2cm3 for HDR). In this case study, compared with HDR, directional interstitial LDR I125 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.

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