Influence of prostatic edema on 131Cs permanent prostate seed implants: a dosimetric and radiobiological study

To study the influence of prostatic edema on postimplant physical and radiobiological parameters using (131)Cs permanent prostate seed implants.

Thirty-one patients with early prostate cancer who underwent (131)Cs permanent seed implantation were evaluated. Dose-volume histograms were generated for each set of prostate volumes obtained at preimplantation and postimplantion days 0, 14, and 28 to compute quality indices (QIs) and fractional doses at level x (FD(x)). A set of equations for QI, FD(x), and biologically effective doses at dose level D(x) (BED(x)) were defined to account for edema changes with time after implant.

There were statistically significant differences found between QIs of pre- and postimplant plans at day 0, except for the overdose index (ODI). QIs correlated with postimplant time, and FD(x) was found to increase with increasing postimplant time. With the effect of edema, BED at different dose levels showed less improvement due to the short half-life of (131)Cs, which delivers about 85% of the prescribed dose before the prostate reaches its original volume due to dissipation of edema.

Results of the study show that QIs, FD(x), and BEDs at the level of D(x) changed from preneedle plans to postimplant plans and have statistically significant differences (p < 0.05), except for the ODI (p = 0.106), which suggests that at the time of (131)C seed implantation, the effect of edema must be accounted for when defining the seed positions, to avoid the possibility of poor dosimetric and radiobiologic results for (131)Cs seed implants.