Objectives. To determine the impact of radiation dose on the response of Stage T1/T2 prostate cancer to external beam radiation therapy and to contrast the modern-day clinical and biochemical control rates to those seen after radical prostatectomy or permanent iodine-125 seed implants. Methods. The study cohort consisted of 643 patients with palpable Stage T1/T2, NX/N0, M0 prostate cancer treated with external beam radiotherapy between 1987 and 1995. No patient received neoadjuvant or adjuvant androgen ablation. The radiotherapy isocenter dose ranged from 60 to 78 Gy, with a mean of 67 and a median of 66. Median follow-up was 43 months. The primary end point was freedom from relapse or rising prostate-specific antigen (PSA) level. Results. The patients were divided into two dose groups with the cutpoint based on the mean and median values, and prior analyses. There were 354 patients treated to radiation doses of 67 Gy or less and 289 treated to doses greater than 67 Gy. Those receiving the higher doses had a significantly greater 4-year freedom from failure rate of 87% versus 67% (P <0.0001). Multivariate Cox proportional hazards analyses revealed that isocenter dose was independent of Gleason score and pretreatment PSA level, which were the other significant covariates. Conclusions. Very high freedom from failure rates were achieved when the radiation dose to the prostate was above 67 Gy. These rates are promising when compared to published radical prostatectomy series and most permanent iodine-125 seed implant series in which patients were stratified by pretreatment PSA. Further follow-up is needed to confirm that these promising results are sustained.
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