Rectum dose reduction and individual treatment plan optimization for high-dose-rate prostate brachytherapy

PURPOSE: Different doses and techniques used in high-dose-rate (HDR) prostate brachytherapy make it difficult to define universal quality parameters. The aim of this study was to develop individual, objective parameters for the evaluation of an HDR brachytherapy plan for prostate radiation. METHODS: Fifty-three patients who received an HDR brachytherapy boost after external radiation were analyzed in this study. Brachytherapy was performed with a (192)Ir source after ultrasound-guided, transperineal metal needle application followed by removal of the ultrasound probe to reduce organ dose levels at the anterior rectum wall. The rectum and prostate locations as well as the dose at the anterior rectum wall were estimated under the anatomical conditions of HDR prostate brachytherapy. The doses at the organs at risk (rectum and urethra) were analyzed for several parameters, which were compared to values of former patients before the start of treatment. In cases of major deviations, modifications of the treatment plan were performed before starting the treatment. RESULTS: Deflating of the water balloon led to an increase of the space between the anterior rectal wall and the dorsal margin of the prostate (mean, 6mm; 1-10mm). The dose of the introduced "virtual rectum," represented by the ventral surface of the ultrasound probe, in the treatment plan correlated to dose measurements in the rectum. Pretreatment evaluation and comparison of the established individual quality parameters led in two cases to a treatment plan modification. CONCLUSIONS: This method allows a fast and objective individual brachytherapy treatment plan evaluation and improvement.