Rectal dosimetric analysis following prostate brachytherapy.

PURPOSE: To retrospectively assess the rectal tolerance dose in transperineal ultrasound-guided prostate brachytherapy using easily measured point doses. METHODS AND MATERIALS: Forty-five consecutive patients who underwent prostate seed implantation from January 1996 to October 1996, using either 125I or 103Pd as monotherapy or as a boost following 45 Gy of external beam radiotherapy (XRT), were evaluated. For monotherapy using 125I, the minimal peripheral dose (mPD) was 160 Gy, utilizing dosimetry parameters which are equivalent to 144 Gy under the American Association of Physicists in Medicine Task Group 43 (AAPM TG43) recommendations. Computed tomography (CT)-based dosimetry was performed within 9 days of the implant, with a urinary catheter in place for identification of the urethra, and a rectal obturator positioned prior to the CT scan for identification of the anterior rectal mucosa. Dosimetric parameters relevant to this study were the average, maximal and minimal anterior rectal mucosal dose, and the surface area and length of the anterior rectal mucosa irradiated to 50%, 75%, 90%, 100%, and 120% of the prescribed dose. Rectal complications were determined by patient reporting during office visits and telephone follow-up. Follow-up ranged from 19 to 28 months (median 23). RESULTS: Among the four cohorts, the average anterior rectal mucosal dose as a percent of the prescribed mPD was 82.5% +/- 14.1% (standard deviation, SD), and the average maximum was 120% +/- 35%. The length of the rectum receiving a given dose was found to be correlated with the rectal surface area receiving that dose (r2 = 0.82 - 0.93 over the dose range 0.5-1.2 mPD). Rectal complications occurred in only four patients, and consisted of mild, self-limited proctitis. Three of the four were in the top quartile of average and maximal rectal dose and length of rectum receiving > or = 100% of mPD, while the fourth was in the bottom quartile of these parameters. In terms of surface area, all four patients were in the upper half of the study group, with > or = 90 mm2 at dose levels > or = 100% of mPD. CONCLUSION: Our results indicate that in an implant program which maintains the anterior rectal mucosa point dose averages to about 85% of the prescribed dose (regardless of the choice of isotope or the treatment approach), the average maximum to 120% of mPD, and the length of the anterior rectal mucosa receiving 100% and 120% of the prescribed dose at approximately 10 mm and 5 mm, respectively, the incidence of mild self-limited proctitis will be in the range of approximately 9%, without incidence of rectal ulceration and/or fistula formation.

Influence of timing on the dosimetric analysis of transperineal ultrasound-guided, prostatic conformal brachytherapy.

Postoperative computed tomography (CT)-based dosimetric analysis of transperineal ultrasound-guided conformal prostate brachytherapy provides detailed information regarding the coverage and uniformity of the implant. However, there is no generally accepted standard for the optimal timing of the postoperative dosimetry. This report details dosimetric analysis and the effect of timing based upon CT and orthogonal film evaluation for ten unselected patients implanted with either iodine-125 (125I) or palladium-103 (103Pd). Within 2 hours after implantation, patients underwent a CT scan and the first of four sequential sets of orthogonal films. Subsequent orthogonal films were obtained on days 3, 14, and 28 postimplant. CT-based dosimetry revealed coverage of the prostate to the prescribed minimal peripheral dose (mPD) at 93.1 +/- 3.6% of the volume, the prostate volume receiving 150% of mPD was 38.2 +/- 8.7%, and the urethral and rectal doses were 114 +/- 12% and 78 +/- 19% of mPD, respectively. The implanted seeds seen on orthogonal films acted as markers for temporal changes in prostate dimensions, and the standard deviation of each dimension was used as input in an ellipsoidal volume calculation. Seed coordinates were self normalized to the center of gravity of each two-dimensional view and were measured relative to the linear regression line in the superior-inferior direction. The reproducibility of the anteroposterior (AP) film setup in terms of temporal variation in the angle of the regression line was markedly better than that of the lateral films, 1.8 degrees +/- 1.2 degrees vs. 4.3 degrees +/- 2.6 degrees, respectively. Dimensional contraction from day 0 to day 28 averaged 11.3% in the superior-inferior direction, 8.5% in the AP/PA (posteroanterior) direction, and 2.5% in the right-left lateral direction. This translated into a volume change of 20.9% (ranged 11.6-31.6%), which was determined by using the ellipsoid method. The half-life for edema resolution was 10.6 +/- 1.8 days (range 8.6-14.3 days). However, because of variability in the degree and extent of edema and its rate of resolution, we believe that it may be futile to define a single point in time as the most accurate indicator of the postoperative dose distribution. Rather, it may be preferable to accept universal standardization of timing and methodology for CT-based postoperative dosimetry, which would facilitate comparison of results between centers and maximize the information content of that single measurement. We conclude that day 0 represents the optimal time, because dosimetric evaluation at that time minimizes patient discomfort and inconvenience (a catheter is already in place), provides information about edema when it is near its maximum extent, and provides prompt closure of the learning loop and, as such, hopefully will result in improved implantation techniques and results.

Prostatic conformal brachytherapy: 125I/103Pd postoperative dosimetric analysis.

Widespread replication of the favorable long-term results of prostatic conformal brachytherapy achieved by the Seattle group requires evidence that the technical quality of their implants is achievable elsewhere. Preplanning with a modified uniform loading algorithm using low activity seeds produces virtually no regions within the planning volume at less than the prescribed dose and no interconnected volumes between seeds at double the dose. The operative procedure stabilizes the prostate and locates the prostate targets, needles, and seeds and their relationship to the bladder and rectum using transverse and longitudinal ultrasound as well as contrast enhanced fluoroscopy. A detailed postoperative dosimetric analysis of patients with clinical T1/T2 adenocarcinoma of the prostate gland who underwent transperineal ultrasound conformal prostatic brachytherapy from March through June 1996 was performed. The analysis involved 7 consecutive patients implanted with 125I seeds and 5 consecutive patients implanted with 103Pd seeds. Median coverage to the full minimal peripheral dose (mPD) was 96% (range 80-99%) of the prostate volume. At 80% of the mPD, median isodose coverage was 100% (range 91-100%) of the prostate volume. Regarding hot spots to critical structures, the median maximal urethral dose was 175% of the mPD (range 115-227%) and the median maximal dose to the anterior rectal mucosa was 105% of the mPD (range 83-133%). Analysis of postoperative dose-volume histograms has shown that our maximal dose surface to any volume greater than 5 cm3 is 203% (range 175-247%). These results indicate that good quality transperineal ultrasound prostatic conformal brachytherapy can be accurately reproduced in a community hospital setting and that biochemical no evidence of disease (NED) results and local control rates will be comparable to those of the Seattle group with no unexpected urethral or rectal complications or side effects.