Iodine-125 trans perineal prostate brachytherapy with pre-planning technique: pre and post-implant dosimetry results analysis

Post-implant CT-based dosimetry is the only method of assessing the quality of permanent prostate brachytherapy. As a consequence of our permanent feedback with the pre-planned technique, geometric and dosimetric criteria for optimal seed implantation are proposed and pre and post-implantation dosimetric results are presented. In 2000 and 2001, one hundred and twenty patients with early stage prostate cancer were treated with trans-perineal I-125 pre-planned brachytherapy (RAPID StrandTM, Amersham Health). The prescription dose was 145 Gy to the planning target volume. For the pre-planning and post-implant dosimetry the Variseed 6.7 version software was used (Varian Medical Systems). The D90, V 100 and V 150 values, the position of the dose peak [Dose] peak) and the full width at half maximum (FWHM) on differential dose volume histogram from both planned and post-implant dosimetry were compared for all patients. For pre-planned dosimetry, the mean values for D90, V100, V150, [Dose] peak, FWMH were respectively of 199 Gy, 100%, 70%, 220 Gy, 113 Gy. For post-implantation, these values became respectively of 157 Gy, 90%, 62%, 220 Gy, 194 Gy. In our practice, differences are noted between pre-planned and post-implant dosimetry parameters that should be anticipated to assure optimal definitive result. A working methodology both for performing the pre-planned dosimetry and for evaluating the post-implantation dosimetric results is proposed. (authors)