Validation Of Biochemical Definition Of Cure After Low-Dose Rate Prostate Brachytherapy

322 Background: Prospectively collected outcome data for 14,196 patients with localized prostate cancer treated with LDR brachytherapy (BT) from 7 institutions were analyzed. For the 80% of patients with a 4 year PSA < 0.2 ng/ml, 99% were free of clinical failure at 10 years and 96% at 15 years. We sought to validate this result with 2 independent data sets from mature prospective clinical trials. Methods: In the initial analysis, patients were treated with either BT alone (61%), or in combination with external beam radiotherapy (EBRT:8%), androgen deprivation (ADT:22%) or both (9%). 42% were low risk, 50% intermediate (IR) and 8% high risk(HR). KM analysis was carried out using clinical failure (local, distant, regional or biochemical triggering salvage) as endpoints for each of 4 PSA categories: PSA<0.2 ng/ml, PSA >0.2 to < 0.5, PSA > 0.5 to < 1.0, and PSA>1.0 ng/ml. Results were compared to 12 year follow up data on a phase 2 trial of BT for IR prostate cancer (n=223; MDAnderson Cohort 1) and 10-year data from the BT arm of the phase 3 randomized ASCENDE RT trial (n=160, Cohort 2) for upper tier IR and HR prostate cancer. Results: The results of the initial KM analysis showed that for the 80% of patients with PSA < 0.2 ng/ml at 4 years, 99% were free of recurrence at 10 years (95% CI: 98.4-99.1) and 96% at 15 years (95% CI: 95-97). The association of treatment success with PSA range was highly significant (p<0.0005). Independent validation against BT alone in IR patients (Cohort 1) confirmed that 99% of patients with PSA at 4 years < 0.2 ng/ml were NED at 10 years (CI: 95.8-99.9). For the unfavorable IR and HR patients receiving 12 months ADT + pelvic EBRT and BT in ASCENDE-RT (Cohort 2), PSA < 0.2 ng/ml at 4 years was associated with 96.7% (CI: 89.9-98.9) being failure free at 10 years. Conclusions: As over 80% of patients achieve a PSA < 0.2 ng/ml at 4 years post-LDR BT, and this is associated with 97%-99% being disease free beyond 10 years, we suggest that this biochemical definition of cure be adopted for LDR brachytherapy patients with ≥ 4 years’ follow-up.