Rectal dosimetric analysis following prostate brachytherapy

To retrospectively assess the rectal tolerance dose in transperineal ultrasound-guided prostate brachytherapy using easily measured point doses.

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).

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.

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.