Prostate brachytherapy: Improvements in prostate volume measurements and dose distribution using interactive ultrasound guided implantation and three‐dimensional dosimetry

Improvements in prostate volume measurements and in dose distribution have led to an increase in the use of brachytherapy for prostate cancer. The addition of interactive ultrasound and three‐dimensional dosimetry has also enhanced the treatment plan. We investigated the effect of these changes on implant outcome in patients with localized carcinoma of the prostate who were treated with these new techniques. Two hundred twenty‐one men with an elevated prostate specific antigen (PSA) or abnormal digital rectal examination (DRE) had transrectal ultrasound evaluation and needle biopsy of the prostate. Prostate volume was determined by ellipsoid and planimetry techniques. Differences of at least 5%, 10%, 15%, 25%, and 50% between the two techniques were found in 170 (77%), 130 (59%), 102 (46%), 53 (24%), and 11 (5%), respectively. In addition, the ellipsoid measurements, which approximate the caliper measurements of the open retropubic implants, were found to underestimate gland volume by 10% in 47 patients (21%), 15% in 39 (18%), 25% in 28 (13%), and 50% in 8 (4%). In 75 patients who had 125I implant the dose to 90% (D90) of the gland increased from 7,619 to 15,806 cGy (P < 0.02) by distributing the majority (70%) of the activity in the periphery and by increasing the total implanted activity by 35%. Despite this peripheral dose placement and increased activity, there was no increase in dose to the anterior rectal wall. We conclude that accurate prostate gland determinations (by planimetry), a peripheral activity distribution, and a 35% increase in implanted activity are necessary in order to ensure adequate dose coverage to the prostate gland when implanting 125I. These changes also may be accomplished without an increase in rectal dose and subsequent radiation proctitis.