Postimplant dosimetry for I-125 prostate implants: Definitions and factorsaffecting outcome

Objective: An analysis of CT-based dosimetry was performed to assess the ef ficacy of the real time method of prostate implantation, explore the relati onship of various dose descriptions and determine implant factors affecting outcome. Methods and Materials: Between 7/95 and 8/99, 297 patients underwent I-125 implants for T1-T2 prostate cancer and had CT-based dosimetry performed (TG 43 formalism). Dosimetry was performed 1 month postimplant. Using a dose-vo lume histogram, doses delivered to 100%, 95%, 90%, and 80% of the prostate (D100, D95, D90, D80, respectively) as well as percentages of the gland rec eiving 240 Gy, 160 Gy, 140 Gy (V240, V160, V140, respectively) were reporte d. Correlations between the various dose parameters and D90 were generated. The effect of the number of seeds implanted, seeds/volume, prostate volume , experience as assessed by time (8/01/99-date of implant), ultrasound prob e (mechanical sector vs, dual phased electronic), and the ratio of the CT d osimetry prostate volume/ultrasound implant volume (CT/US vol) were analyze d. Results: The median D100, D95, D90, and D80 values were 10,200 cGy, 15,655 cGy, 17,578 cGy, and 19,873 cGy, respectively. The median V240, V160, and V 140 were 56%, 94%, and 98%, respectively. Correlations of dose descriptions found a close relationship of D95, D80, V240, V160, and V140 with D90 with r values of 0.928, 0.973, 0.911, 0.816, and 0.733, respectively. D100 corr elated poorly with D90 (r = 0.099). Using a stepwise regression analysis, C T/US vol ratio, prostate volume, and seed number were the only significant factors affecting D90 with CT/US vol ratio having the greatest effect. The dual-phased electronic probe was associated with fewer D90 values of less t han 140 Gy (2%) compared to the mechanical sector probe (14%) (p = 0.02). Conclusion: CT-based dosimetry results reveal the real-time implant techniq ue to be an effective method of prostate implantation. Factors associated w ith more precise implantation, such as decreased postimplant edema, new tec hnology, and increased number of seeds will lead to higher D90 values. (C) 2000 Elsevier Science Inc.