Permanent prostate brachytherapy extracapsular radiation dose distributions: analysis of a multi-institutional database

PURPOSE: Periprostatic brachytherapy doses impact biochemical control. In this study, we evaluate extracapsular volumetric dosimetry following permanent prostate brachytherapy in patients entered in a multi-institutional community database. MATERIAL AND METHODS: In the database, 4547 patients underwent brachytherapy (3094 – (125)I, 1437 – (103)Pd and 16 – (131)Cs). Using the originally determined prostate volume, a 5 mm, 3-dimensional peri-prostatic anulus was constructed around the prostate (except for a 2 mm posterior margin), and evaluated in its entirety and in 90° segments. Prostate dosimetric parameters consisted of a V(100) and D(90) while the annular dosimetry was reported as a V(100). RESULTS: The intraprostatic V(100) and D(90) for (103)Pd, and (125)I were statistically comparable when stratified by isotope and/or monotherapy vs. boost. The overall mean V(100) for the periprostatic annulus was 62.8%. The mean V(100) at the base (51.6%) was substantially less than the apex (73.5%) and midgland (65.9%). In addition, for all patients, the anterior V(100) (45.7%) was less than the lateral (68.8%) and the posterior (75.0%). The geometric V(100) annular differences were consistent when evaluated by isotope. Overall, the V(100) was higher in the (125)I cohort. CONCLUSIONS: The optimal extracapsular brachytherapy dose and radial extent remains unknown, but will prove increasingly important with reductions and/or elimination of supplemental external beam radiation therapy. The large multi-institutional community database demonstrates periprostatic annular doses that are not as robust as those in selected high volume brachytherapy centers, and may be inadequate for optimal biochemical control following monotherapeutic brachytherapy, especially in higher risk patients.