CONFORMAL HIGH-DOSE-RATE IR-192 BOOST BRACHYTHERAPY IN LOCALLY ADVANCED PROSTATE-CANCER - SUPERIOR PROSTATE-SPECIFIC ANTIGEN RESPONSE COMPARED WITH EXTERNAL-BEAM TREATMENT

PURPOSE Prostate-specific antigen levels are used to judge disease con trol of prostate cancer. No published data attest to the greater abili ty of conformal brachytherapy to control disease compared with convent ional radiation at a single institution. This report compares the bioc hemical response rates in patients with stages T2b to T3c prostate can cer treated with conformal brachytherapy boost and external beam radia tion with the rates in patients treated with conventional external rad iation alone. MATERIALS AND METHODS From November 1991 through Novembe r 1995, 58 patients received 45.6 Gy pelvic external irradiation and t hree high dose rate iridium-192 conformal boost implants of 5.5 to 6.5 Gy each. They were compared with 278 similarly staged patients treate d from January 1987 through December 1991 with external beam radiation to prostate-only fields (median dose 66.6 Gy). No patient received an drogen deprivation. Patient outcome was analyzed for biochemical contr ol. Biochemical failure was defined as a prostate-specific antigen lev el > 1.5 ng/mL and rising on two consecutive values. If serial posttre atment prostate-specific antigen levels were showing a continuous down ward trend, failure was not scored. RESULTS Median follow-up was 43 mo nths for the conventionally treated group and 26 months for the brachy therapy boost group. The median pretreatment prostate-specific antigen level was 14.3 ng/mL, for the external-beam-radiation-alone group and 14.0 ng/mL for the brachytherapy boost group. The median Gleason scor es were 6 and 7, respectively, for the two groups. The biochemical con trol rate was significantly higher in the brachytherapy boost treatmen t group. Three-year actuarial biochemical control rates were 85% versu s 52% for the conformally and conventionally treated patients, respect ively. In a multivariate analysis, the use of conformal brachytherapy boost and pretreatment prostate-specific antigen level were significan t prognostic determinants of biochemical control. The 3-year actuarial rates of biochemical control for conformally versus conventionally tr eated patients, respectively, were 83% versus 72% for a pretreatment p rostate-specific antigen level of 4.1 to 10.0 ng/mL, 85% versus 47% fo r a prostate-specific antigen level of 10.1 to 20.0 ng/mL, and 89% ver sus 29% for prostate-specific antigen > 20 ng/mL. When the analysis wa s limited to patients in both groups with a minimum 12-month followup, the brachytherapy boost group continued to show a higher biochemical control rate compared with the conventional radiation group (3-year ac tuarial rates of 86% vs 53%). DISCUSSION These preliminary results sho w a significant improvement in the biochemical response rate with conf ormal boost brachytherapy and pelvic external radiation compared with conventional radiation alone. These results, coupled with our previous ly reported acceptable toxicity rates, support the use of this techniq ue.