Lack of radiation dose response for patients with low-risk clinically localized prostate cancer: A retrospective analysis

Purpose: The need for dose escalation for patients with low-risk clinically localized prostate cancer remains controversial. In this study, we report our pooled institutional experience of low-risk patients treated with a range of "standard" radiation doses to assess outcome in regard to biochemical failure and to determine whether a dose-response relationship exists within this conventional dose range. Methods and Materials: Patients with low-risk clinically localized prostate cancer (T1 or T2a. Gleason grade less than or equal to6, and prostate-specific antigen less than or equal to10 ng/mL) treated at Joint Center for Radiation Therapy-affiliated institutions between October 1989 and September 1997 were retrospectively analyzed for freedom from biochemical failure. The dose was prescribed volumetrically with 95% normalization to between 5760 and 6900 cGy (6100 and 7300 cGy ICRU reference point dose). Patients were stratified into 3 groups with relatively equal numbers of patients (<6660, 6660, and >6660 cGy). To ensure that any differences in biochemical failure between patients at the lower and higher ends of the dose range used were not masked by analysis of the entire cohort, patients receiving less than or equal to6500 cGy or greater than or equal to6800 cGy were subsequently analyzed separately. Biochemical failure was defined per the ASTRO consensus definition. The log-rank test was used to assess for differences in follow-up and time to biochemical failure. A Kaplan-Meier plot of time to biochemical failure for the initial 3 subgroups was generated. Results: A total of 264 patients were identified. Sixteen patients whose dose was not recorded in the database were excluded from analysis. The median follow-up was 35 months. No significant differences were found in baseline prostate-specific antigen, Gleason grade, or clinical stage among the groups. The overall actuarial rate of 5-year freedom from biochemical failure was 80.2%. By dose level, the 5-year biochemical failure-free rate was 79.2%, 78.4%, and 84.5% for <6660 cGy, 6660 cGy, and >6660 cGy, respectively. These differences were not significant. Subsequently, 45 patients receiving less than or equal to6500 cGy were compared with 23 patients receiving greater than or equal to6800 cGy. The difference between these groups was not significant. The 5-year freedom from biochemical failure rate for the patients receiving less than or equal to6500 cGy was 89.9%. Conclusion: Within a range of doses considered standard for treatment of low-risk clinically localized prostate cancer during an 8-year period, no improvement in biochemical freedom from failure was noted with the higher doses. The overall 5-year rate of freedom from biochemical failure is consistent with that reported by others with standard and escalated external beam doses used in this low-risk population. A prospective randomized study is necessary to define the optimal dose in this patient population. (C) 2002 Elsevier Science Inc.