Extracapsular radiation dose distribution after permanent prostate brachytherapy.

The physical extent of the radiation therapy dose delivered to the periprostatic region was evaluated in 26 consecutive patients with low-risk prostate cancer (prostate-specific antigen <or=10, Gleason score <or=6, and clinical T1c/T2a) who underwent prostate brachytherapy using either 103Pd or 125I without supplemental external beam radiation therapy between October and December 1999. All patients underwent a transrectal ultrasound volumetric study of the prostate gland followed by the generation of a preplan based on a consistent, modified uniform seed-loading philosophy. The planning treatment volume (PTV) consisted of the prostate gland with approximately 5-mm anterior and lateral margins. The radiation dose was prescribed to the PTV with margin. Day 0 computed tomography-based dosimetric evaluation revealed an overall mean 100% isodose margin of 6.5 mm +/- 1.8 mm. For 125I and 103Pd, the 100% isodose margins were 6.8 mm +/- 1.6 mm and 6.3 mm +/- 1.9 mm, respectively. At the 90% and 75% isodose lines, the isodose margins differed by no more than 1 mm for the 2 isotopes. With the exception of the area near the bladder neck and the posterior border of the prostate, the 100% isodose margin was >or=5.0 mm for all slices evaluated. The utilization of preplanning periprostatic margins is strongly correlated with the administration of prescription radiation doses to the periprostatic region. The extent of the postimplant periprostatic margin (6.5 +/- 1.8 mm) satisfies the preplanning margin criterion of >5 mm.