Refining prostate seed brachytherapy: Comparing high-, intermediate-, and low-activity seeds for I-125 permanent seed prostate brachytherapy.

Abstract

To analyze the difference in prostate coverage and dose to the rectum in men with prostate carcinoma treated with permanent seed brachytherapy with different seed activities. Forty-nine patients treated with iodine-125 permanent seed prostate brachytherapy with low-activity seeds of 0.30-0.37mCi were identified. For each of these patients, 2 patients with similar prostate volume (±2 cc) were paired: one treated with intermediate seed activity (0.44-0.46mCi) and one with high seed activity (0.60-0.66mCi). The doses to prostate and rectum were compared using CT on Day30. A total of 147 patients divided into the three seed activity groups were analyzed. Mean prostate volume was 35.7 cc (standard deviation [SD], 11.70). Compared with low-activity seeds, implants with high-activity seeds consisted of an average of 22 seeds and 4.7 needles less. The dose to the prostate (prostate volume receiving 100% of the prescribed dose [V100], prostate volume receiving 150% of the prescribed dose, and minimal dose covering 90% of the prostate volume expressed in Gy) was not higher on Day 30 (p= 0.58-0.97). The mean volume (in cubic centimeters) of rectal wall receiving 100% of the prescribed dose (V100) increased with activity: low activity, 0.34 cc (SD, 0.49), intermediate activity, 0.47 cc (SD, 0.48), and high activity, 0.72 cc (SD, 0.79) (p= 0.009). There was a trend (p= 0.073) toward a higher frequency of clinically unfavorable rectal dosimetry (V100>1.3 cc) in patients with high-activity seeds (16.7%) compared with low-activity (6.3%) or intermediate-activity (4.2%) seeds. High-activity seeds do not result in a higher dose to the prostate but in a higher dose to the rectum.