Report of a Large Cohort of Intermediate-Risk Prostate Cancer Patients Treated with Cs-131 Brachytherapy

Abstract

The intermediate-risk (IR) group of prostate cancer patients is extremely heterogenous. Patients are sub-classified into favorable-IR (FIR) and unfavorable-IR (UIR) groups due to variable prognoses, leading to significant treatment variations. We sought to evaluate our institutional outcomes utilizing Cs-131 prostate brachytherapy (PB) for IR patients and elucidate which factors are important for driving more aggressive treatment. We reviewed a prospectively collected database of men treated with Cs-131 PB between 2006-2019. Patients with less than 24-months follow-up were excluded. Patients were classified as IR if they had one of the following factors: Gleason Score 7, prostate specific antigen (PSA) >10 but < 20 ng/mL, or T2b-c on clinical exam. We defined UIR patients as having either Grade Group (GG) 3 or >1 IR factors. Our general treatment schema was to treat FIR with PB alone (115 Gy) and UIR with combined external beam radiation therapy (EBRT) to 45 Gy + PB boost (85 Gy), though this was somewhat variable. The Kaplan-Meier method was used to estimate actuarial event-time probabilities for biochemical failure (BF). BF was defined as a serum PSA at least 2 ng/mL greater than posttreatment nadir PSA. Cox regression analysis was performed to identify predictors for BF. These factors included: GG, UIR subgroup, treatment (PB alone vs combined modality), ADT use, age (≤67 vs >67), clinical T-stage, PSA (<10 vs ≥10-20), and >1 IR factors. A total of 327 patients with a median follow-up of 70.6 months (IQR 45.9-102.7 months) were identified. Median age was 67 (IQR 62-72). ADT was used in 12.5% of patients. Most patients (57.5%) were classified as FIR. Of the 139 patients in the UIR subgroup, 69.1% met UIR criteria by virtue of GG 3 only and 30.9% by having >1 IR factor. Twenty-eight patients (20.1%) had a GG 3 and >1 IR factor. FIR patients had a 5-year BF rate (BFR) of 2.5% (SE 0.012). The 5-year BFRs for FIR patients treated with PB alone (81.4%) and combined modality (18.6%) were 1.7% (SE 0.012) and 5.0% (SE 0.035), respectively (p = 0.18). UIR patients had a 5-year BFR of 10.9% (SE 0.030) and were most commonly treated with EBRT+PB (61.9%). Five-year BFRs were 14.0% (SE 0.078) and 10.1% (SE 0.032) for UIR patients treated with PB alone and combined modality, respectively (p = 0.91). UIR patients by virtue of GG 3 only (n = 68) had a 5-year BFR of 13.0% (SE 0.047). The 5-year BFR for UIR due to >1 IR factors (n = 71) was 8.9% (SE 0.038). Six patients had 3 IR factors with a 5-year BFR of 50% (SE 0.25). On multivariate analysis, UIR subgroup was the only significant predictor for BF (p = 0.02; HR 2.49, 95% CI 1.15-5.41). The FIR subgroup is a distinct entity from the UIR subgroup with excellent outcomes when treated with Cs-131 PB alone. UIR patients have excellent outcomes with combined EBRT + PB. The majority of UIR patients do not require ADT, though the group that would benefit still needs to be defined.