Evaluation of dosimetric parameters and disease response after 125 iodine transperineal brachytherapy for low- and intermediate-risk prostate cancer.

<bold>Purpose: </bold>To analyze dosimetric outcomes after permanent brachytherapy for men with low-risk and "low-tier" intermediate-risk prostate cancer and explore the relationship between the traditional dosimetric values, V100 (volume of prostate receiving 100% of the prescribed dose) and D90 (minimum dose to 90% of the prostate), and risk of biochemical failure. <bold>Methods and Materials: </bold>A total of 1,006 consecutive patients underwent implantation between July 20, 1998, and Oct 23, 2003. Most (58%) had low-risk disease; the remaining 42% comprised a selected low-tier subgroup of intermediate-risk patients. The prescribed minimum peripheral dose (MPD) was 144 Gy. All implants used 0.33 mCi 125I sources using a preplan technique featuring right-left symmetry and a strong posterior-peripheral dose bias. Sixty-five percent of patients had 6 months of androgen deprivation therapy. Postimplantation dosimetry was calculated using day-28 CT scans. <bold>Results: </bold>With a median follow-up of 54 months, the actuarial 5-year rate of freedom from biochemical recurrence (bNED) was 95.6% +/- 1.6%. Median D90 was 105% of MPD, median V100 was 92%, median V150 was 58%, and median V200 was 9%. Dosimetric values were not predictive of biochemical recurrence on univariate or multivariate analysis. Analysis of dosimetric values by implantation number showed statistically significant increases in all values with time (D90, V100, V150, and V200; p < 0.001), but this did not translate into improved bNED. <bold>Conclusions: </bold>In contrast to some previous studies, dosimetric outcomes did not correlate with biochemical recurrence in the first 1,006 patients treated with 125I prostate brachytherapy at the British Columbia Cancer Agency. Despite a median D90 of only 105% of MPD, our bNED rates are indistinguishable from series that reported higher D90 values. [ABSTRACT FROM AUTHOR]