Implantation of Row One Needles 1-2 mm Anterior to the Posterior Margin of the Prostate Gland Results in Excellent PSA Control and Minimal Rectal Toxicity After I-125 Brachytherapy

Abstract

Positioning of the posterior-most row of seeds (row 1) during prostate brachytherapy (PB) determines dose coverage to both the cancer-prone posterior peripheral zone and the anterior rectal wall and therefore impacts cure rates as well as the incidence of late rectal toxicity. Studies correlating row 1 position of needles and seeds and clinical outcomes after PB are lacking. Thus we undertook this analysis to demonstrate the safety and efficacy of our practice of positioning row 1 in patients during PB. We randomly selected 50 patients for this study who had a minimum 4 year follow-up from the 350 patients who have undergone I-125 PB at our institution. PB was performed with loose I-125 seeds to deliver a minimum peripheral dose of 144 Gy using a modified uniform loading technique. In all patients, row 1 on the template was positioned at a distance of 1-2 mm anterior to the posterior margin of the prostate gland from base to apex by angulating the rectal probe posteriorly to match the curvature of the prostate gland. Median patient age was 67 years, eight (16%) had diabetes, 12 (24%) were on blood thinners, one patient had ulcerative colitis. Patient records were evaluated for biochemical relapse free survival (RFS) and late rectal toxicity according to the RTOG criteria. Each patient's day 1 post-implant CT scan was reviewed for prostate quadrant dosimetry and various measures of rectal dosimetry including V100%, V100 Gy, rectal wall surface area dosimetry and seed number/distance from the rectal wall. After median follow-up of 63.5 months (range, 48-96 months) biochemical RFS was 100%. The median V100% of the anterior and posterior quadrants of the prostate was 91.8% and 90.9%, respectively. Median number of seeds < 5 mm, < 3 mm and < 1 mm from the rectal wall was 8 (0-26), 2 (0-17), and 0 (0-4), respectively. Median rectal V100% was 0.94 cc and rectal V100 Gy was 3.67 cc. Median rectal surface area receiving at least 100 Gy, 200 Gy and 100% of the prescription dose was 10.22 cm2, 0.80 cm2 and 4.88 cm2, respectively. The rate of RTOG grade 1 late rectal toxicity was 9/50 (18%). No patients had grade 2 or higher late toxicity. On multivariate analysis late grade 1 rectal toxicity was significantly associated with the presence of any seeds located < 5 mm from the anterior rectal wall (OR 1.19, p = 0.046) and the rectal surface area receiving at least 100 Gy (OR 1.32, p = 0.049). These results confirm the long-term safety and efficacy of implanting row 1 needles at a distance of 1-2 mm anterior to the posterior margin of the prostate gland during PB. This practice can provide good dosimetric coverage of the posterior peripheral zone with excellent biochemical RFS and minimal rectal toxicity in spite of the localized delivery of high radiation doses to the anterior rectal wall.