Comparing Iridium-192 with Cobalt-60 sources in high-dose-rate brachytherapy boost for localized prostate cancer

Background Dosimetric and clinical comparison of two cohorts of Iridium-192 (Ir-192) and Cobalt-60 (Co-60) high-dose-rate brachytherapy (DR-BT) boost for localized prostate cancer. Material and methods Patients with localized prostate cancer receiving either Ir-192 or Co-60 high-dose-rate brachytherapy (HDR-BT) boost in combination with external beam radiotherapy (EBRT) in the period of 2002-2019 were evaluated for dosimetric differences, side effects, biochemical relapse-free survival (bRFS), metastasis-free survival (MFS), and overall survival (OS). EBRT, delivered in 46 Gy (D-Mean) in conventional fractionation, was followed by two fractions HDR-BT boost with 9 Gy (D-90%) 2 and 4 weeks after EBRT. Genitourinary (GU)/gastrointestinal (GI) toxicity were evaluated utilizing the Common Toxicity Criteria for Adverse Events version 5.0 and biochemical failure was defined according to the Phoenix definition. Results A total of 338 patients with a median follow-up of 101.8 (IQR 65.7-143.0) months were evaluated. At 10 years the estimated bRFS, MFS, and OS in our patient sample were 81.1%/71.2% (p=.073), 87.0%/85.7% (p=.862), and 70.1%/69.7% (p=.998) for Ir-192/Co-60, respectively. Cumulative 5-year late grade >= 2 GU toxicity was 20% for Ir-192 and 18.3% for Co-60 (p=.771). Cumulative 5-year late grade >= 2 GI toxicity was 5.8% for Ir-192 and 4.6% for Co-60 (p=.610). Grade 3 late GU side effects were pronounced in the Ir-192 cohort with 8.1% versus 1.4% in the Co-60 cohort (p=.01), which was associated with significantly lower dose to the organs at risk in the Co-60 cohort. PTV D-90% was 9.3 +/- 0.8 Gy versus 9.0 +/- 1.1 Gy (p=.027) for Ir-192 versus Co-60. PTV V-100% and PTV V-150% were not significantly different between both cohorts. Conclusion Co-60 brachytherapy sources are an effective alternative to Ir-192 in combined prostate HDR-BT boost + EBRT.