Increasing external beam dose for T1-T2 prostate cancer: Effect on risk groups

Purpose: The aim of this study was to investigate effect of increasing dose on risk groups for clinical failure (CF: local failure or distant failure or hormone ablation or PSA ≥25 ng/ml) in patients with T1-T2 prostate cancer treated with external beam radiotherapy. Methods and Materials: Patients (n = 4,537) were partitioned into nonoverlapping dose ranges, each narrow enough that dose was not a predictor of CF, and risk groups for CF were determined using recursive partitioning analysis (RPA). The same technique was applied to the highest of these dose ranges (70-76 Gy, 1,136 patients) to compare risk groups for CF in this dose range with the conventional risk-group classification. Results: Cutpoints defining low-risk groups in each dose range shifted to higher initial PSA levels and Gleason scores with increasing dose. Risk groups for CF in the dose range 70-76 Gy were not consistent with conventional risk groups. Conclusions: The conventional classification of risk groups was derived in the early PSA era, when total doses <70 Gy were common, and it is inconsistent with risk groups for patients treated to doses >70 Gy. Risk-group classifications must be continuously re-examined whenever the trend is toward increasing total dose.