A comprehensive review of CT-based dosimetry parameters and biochemical control in patients treated with permanent prostate brachytherapy

Purpose: The American Brachytherapy Society recommends that postprostate im plant dosimetry be performed on all patients undergoing transperineal inter stitial permanent prostate brachytherapy (TIPPB) utilizing CT scan clinical target volume reconstructions. This study was undertaken to assess the rec ommended dosimetry parameters from a large cohort of patients undergoing TI PPB that would predict for PSA relapse-free survival (PSA-RFS), Methods and Materials: Seven hundred nineteen consecutive patients with cli nical stage T1/T2 adenocarcinoma of the prostate underwent TIPPB using eith er I-125 or Pd-103, Postimplant dosimetry was performed at 2 to 3 weeks wit h CT scan 3-dimensional reconstructions obtained on all patients. The D90 a nd D100 doses (defined as the minimum dose covering 90% and 100% of the pro state volume, respectively) and the V100 (defined as the percent of the pro state receiving 100% of the prescribed dose) were obtained for each patient . Regression analysis was performed on the D90 dose, D100 dose, and V100 to test for cutoff points that would predict for PSA-RFS, defined by a modifi cation of the American Society for Therapeutic Radiology and Oncology conse nsus panel statement. A cutoff value was found and was subjected to subset analysis to assess for its robustness, Treatment-related factors were teste d for their ability to achieve dosimetry at or above the cutoff dose. Results: The median follow-up from this cohort is 30 months (7-71 months) w ith a 48-month PSA-RFS of 89.5%, A D90 dose-response cutoff value greater t han or equal to 90% of the prescribed dose was identified. Prostate implant s with a D90 dose < 90% of the prescribed dose had an 80.4% 4-year PSA-RFS, while those with a D90 dose greater than or equal to 90% of the prescribed dose had a 92.4% 4-year PSA-RFS (p = 0.001), No cutoff value was found for the V100 and D100 dose that predicted for PSA-RFS. Using the cutoff value, the D90 dose at 90% of the prescribed dose, a difference in 1-year PSA-RFS survival was identified for patients treated with I-125 (p = 0.04), Pd-103 (p = 0.01), TIPPB as monotherapy (p = 0.001), the addition of hormone ther apy (p = 0.005), and TIPPB without hormone therapy (p = 0.001), The D90 dos e was not significant for the group of patients treated with external beam radiotherapy and TIPPB (p = 0.15), The only significant finding from Cox re gression analysis to predict for a poor D90 dose (<90% of the prescribed do se) was a CT/TRUS volume ratio <greater than>1.5 (p = 0.02), Conclusions: The American Brachytherapy Society recommends that postimplant CT-based dosimetry be performed for all patients treated with TIPPB, This prospective study identified that the D90 dose greater than or equal to 90% of the prescribed dose can be used as a factor for predicting PSA-RFS in p atients treated with brachytherapy, A dose-response using the D90 dose was observed for several typical clinical treatment variations used in the prac tice of TIPPB. Using the D90 dose appears to be a satisfactory parameter fo r predicting outcome in patients treated with TIPPB, (C) 2001 Elsevier Scie nce Inc.