Effect of prostate size and isotope selection on dosimetric quality following permanent seed implantation

The aim of this study was to assess retrospectively the influence of prostate size and the effect of isotope selection on implant quality in patients undergoing transperineal ultrasound-guided permanent prostate brachytherapy.

Two hundred forty-eight consecutive patients without prior transurethral resection of the prostate gland underwent permanent seed implantation using either iodine 125 or palladium 103 as monotherapy or a boost following moderate doses of external-beam radiation therapy from January 1998 through November 1999. Postimplant dosimetry was obtained on day 0 using thin slice computed tomography (CT) scans. Dosimetric quality was reported in terms of the following parameters: D90, V100, V150, and V200, where D90 is defined as the minimum dose covering 90% of the prostate volume, and V100, V150, and V200 are defined as the percentage volume of the prostate receiving at least 100%, 150%, and 200% of the prescribed minimal peripheral dose (mPD), respectively. In addition, the urethral dose was evaluated. Preimplant prostate size was divided into the following categories: <20, 20-30, 30-40, 40-50, and >50 cm3. Prostate volume was determined via transrectal ultrasound volumetric study. In addition, within each of the five size categories, the effect of isotope on implant quality was evaluated.

No statistically significant volume dependence in D90, V100, or urethral doses was discerned, whereas V150 and V200 were volume dependent. Between isotopes, the following differences in dosimetric quality were statistically significant: V100, V150, V200, and mean and medial urethral dose. 125I implants had higher values of V100 (95% vs. 94%, p = .004) and urethral dose (118% vs. 110% of mPD, p < .001), and 103Pd implants had higher V150 and V200 (57% vs. 51% and 31% vs. 22%, respectively). Only the isotopic differences in V200 persisted for all the volumetric subgroups. There was no significant overall volume dependence based on neoadjuvant hormone use or nonuse for any of the quality parameters analyzed.

The most important indicators of the quality of dosimetric coverage, V100 and D90, were not dependent on preimplant prostate volume or use of neoadjuvant hormones. The mean, median, and maximum urethral doses also showed no dependence on prostate size. Although there were isotopic differences in day 0 dosimetric parameters following permanent prostate brachytherapy, we do not consider the magnitude of any of these differences to be clinically significant.