Magnetic Resonance Imaging–Defined Treatment Margins in Iodine-125 Prostate Brachytherapy

Purpose Low-dose-rate prostate brachytherapy achieves a very high and effective intraprostatic dose. Implant quality parameters concentrate on the dose received by the prostate (D90, V100) but not that received by the periprostatic tissue. We calculated implant quality parameters, D90 and V100, for the magnetic resonance imaging (MRI)–defined prostate plus 2, 3, and 5 mm. Methods and Materials A total of 131 men with early-stage prostate cancer treated with iodine-125 brachytherapy represent all those treated with brachytherapy monotherapy in our institution in 2005. Postplan assessment was performed at 1 month using magnetic resonance (MR)–computed tomography (CT) fusion. The prostate V100 and D90 were calculated with 2-, 3-, and 5-mm margins. Results were compared with those in 8 patients with biopsy-proven local failure occurring in an experience of more than 1,100 implants. Results Mean prostate V100 (SD) and D90 (SD) were 95.6% (4.1) and 117.2% (12.7). For prostate plus a 2-mm margin the D90 was 107.9% (14.3) and for a 3-mm margin 96.0 % (14.0). For prostate plus a 5-mm margin, the D90 was only 78.4% (11.0). The 8 patients experiencing local failure, despite adequate implants, had a lower mean V100 of 91.2% (SD, 2.8; p = 0.0008) and D90 of 103.7% (SD, 8.3; p = 0.002) and significantly inferior margin coverage. Conclusions Satisfactory coverage of a 2-mm and 3-mm periprostatic margin is obtained with the described planning approach. Coverage falls off significantly by 5 mm. The 8 patients who experienced local failure had significantly lower doses than the margin cohort. Although the V100 and D90 would be considered acceptable, the fall-off in margin coverage was observed by 3 mm.