Effect of prostatic edema on CT-based postimplant dosimetry.

Purpose: To investigate the magnitude of edema after prostate brachytherapy and its effect on the CT-based postimplant dosimetry based on the sequential CT scans using dose-volume histograms, dose conformity, and homogeneity indices in patients with prostate cancer.

Methods And Materials: CT scans were obtained for 25 patients who underwent prostate brachytherapy with 125I or 103Pd before implant and postimplant Day 1, Day 7, and Day 28. The prostate, rectum, and bladder volumes on each scan were contoured by the same physician. Posttreatment dose distributions were generated using FOCUS (CMS Inc., St. Louis, MO) brachytherapy planning software. Dose calculations were based on TG43 formalism. Dose-volume histograms for target, rectum, and bladder were created for all patients, and the quality of the implants was analyzed using the dose conformity indices: CT-based target volumes ratios, TVR(1) and TVR(2); dose homogeneity indices, DHI(1), DHI(2), and DNR; dose coverage index, CI; the percentage of the prostate volume enclosed by 100%, 90%, and 80% of the prescription dose: V(100), V(90), and V(80); the volume of the rectum covered by 100%, 80%, and 70% of the prescription dose; and the dose covering 90% of the prostate volume (D(90)).

Results: The prostate volume increased between the prescan and the implant Day 1 scans and then decreased between Day 1 and Day 28 scans. The average increase in prostate volume was 30% between the prescan and implant Day 1 scans for the 25 cases evaluated. The prostate volume decreased 20% between the Day 1 and Day 28 scans. The preplan dose coverage to the periphery of the prostate was 100% for all cases evaluated. V(100) increased from an average of 77% to 85% between the Day 1 and Day 28 scans, respectively. On average, D(90) increased from 84% for Day 1 to 93% for Day 28. The average TVR(1), TVR(2), and CI were 1.99, 2.28, and 0.87, respectively, based on the Day 28 scans. The average DHI(1), DHI(2), and DNR were 0.52, 0.46, and 0.48, respectively, based on the Day 28 scans.

Conclusions: The decrease in prostate volume from Day 1 to Day 28 after the implant markedly improved the prescription dose covering the prostate from 77% to 85%. Day 28 prostate volumes were still about 10% larger than the preimplant CT volumes for the 25 cases evaluated. Postimplant dosimetry using dose conformity and homogeneity indices is dependent on the timing of CT studies, as a result of changing prostate volumes from edema.