Impact of image guidance on outcomes after external beam radiotherapy for localized prostate cancer.

PURPOSE To verify whether rectal distention at the time of planning impacts outcomes in patients with localized prostate cancer treated with daily image guidance. METHODS AND MATERIALS Between 1998 and 2002, a total of 488 prostate cancer patients were treated with intensity-modulated radiotherapy. The radiation dose was 70 Gy delivered at 2.5 Gy per fraction in all cases. All cases were treated with a 4-mm margin posteriorly. In all cases the total rectal volume documented on the CT scan was used for treatment planning. No special bowel preparation instructions were given, either for the simulation or the daily treatments. Before each daily treatment, alignment of the prostate was performed with the B-mode acquisition and targeting (BAT) transabdominal ultrasound system. The median follow-up for all 488 patients was 60 months (range, 24-96 months). RESULTS For all patients the biochemical relapse-free survival (bRFS) rate at 5 years was 86%. The 5-year bRFS rate for the rectal distention<50 cm3, 50 to <100 cm3, and >or=100 cm3 groups was 90%, 83%, and 85%, respectively (p=0.18). To adjust for other potential variables affecting bRFS rates, a multivariate time-to-failure analysis using the Cox proportional hazards model was performed. Rectal distention was not an independent predictor of biochemical failure on multivariate analysis (p=0.80). Rectal distention was not a predictor of rectal or urinary toxicity. CONCLUSION The use of daily image guidance eliminates errors such as rectal distention at the initial planning stage that can affect outcomes after radiotherapy for localized prostate cancer.