192-Ir or 125-I Prostate Brachytherapy as a Boost to External Beam Radiotherapy in Locally Advanced Prostatic Cancer: A Dosimetric Point of View

PURPOSE: This work aims at comparing the dosimetric possibilities of 125I or 192Ir prostate brachytherapy (Bt) as a boost to external beam radiotherapy in the treatment of locally advanced adenocarcinoma. METHODS AND MATERIALS: From 1/1997 to 12/2002, 260 patients were treated. Until 12/2001 a low dose rate (LDR) treatment with 192Ir wires was used, later replaced by a high dose rate (HDR) delivered with an 192Ir stepping source technology. For the present work, we selected 40 patients including the last 20 treated, respectively, by LDR and HDR. The planning CT Scans of all these 40 patients were transferred into the 3D Prowess system for 125I permanent implants design according to the Seattle method. The reference data for dosimetric comparisons were the V100 and the prescribed dose for 192Ir as well as the dose delivered with 125I techniques to the 192Ir V100. We compared V100-150 data as well as doses to the organs at risks (OR) and cold spots (CS). RESULTS: The V100 is 85.3+/-8% for 192Ir LDR and 96+/-2% for HDR techniques (P < 0.0001). In comparison with 125I, the 192Ir LDR mode induces higher hyperdosage volumes inside the CTV but also more CS, while maximal doses to urethra and rectum are, respectively, 17 and 39% less with 125I (P < 0.0001). In comparison with the 192Ir HDR mode, 125I Bt induces higher hyperdosage volumes and slightly more CS deliberately planned around the bladder neck. If delivered doses to urethra are identical, those to the 20% anterior part of the rectum are 33% less with 125I (P < 0,0001). The 125I Bt technique was only possible in 24 out of the 40 patients studied due to pelvic bone arch interference. CONCLUSIONS: At the present time, there is no evident dosimetric superiority of one Bt method when all the criteria are taken into account. However, improving Bt techniques to implant any prostatic size could found the superiority of the 125I or permanent implants. 125I indeed allows large hyperdosage volumes inside the CTV in comparison with 192Ir HDR techniques while lowering doses to OR and minimizing CS.