Rectal dosimetric analysis following prostate brachytherapy

Purpose: To retrospectively assess the rectal tolerance dose in transperine al ultrasound-guided prostate brachytherapy using easily measured point dos es. Methods and Materials: Forty-five consecutive patients who underwent prosta te seed implantation from January 1996 to October 1996, using either I-125 or Pd-103 as monotherapy or as a boost following 45 Gy of external beam rad iotherapy (XRT), were evaluated. For monotherapy using I-125, the minimal p eripheral dose (mPD) was 160 Gy, utilizing dosimetry parameters which are e quivalent to 144 Gy under the American Association of Physicists in Medicin e Task Group 43 (AAPM TG43) recommendations. Computed tomography (CT)-based dosimetry was performed within 9 days of the implant, with a urinary cathe ter in place for identification of the urethra, and a rectal obturator posi tioned prior to the CT scan for identification of the anterior rectal mucos a. Dosimetric parameters relevant to this study were the average, maximal a nd 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 report ing 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 a s 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 rec eiving a given dose was found to be correlated with the rectal surface area receiving that dose (r(2) = 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 aver age and maximal rectal dose and length of rectum receiving greater than or equal to 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 h alf of the study group, with greater than or equal to 90 mm(2) at dose leve ls greater than or equal to 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 prescri bed dose (regardless of the choice of isotope or the treatment approach), t he average maximum to 120% of mPD, and the length of the anterior rectal mu cosa receiving 100% and 120% of the prescribed dose at approximately 10 mm and 5 mm, respectively, the incidence of mild self-limited proctitis will b e in the range of approximately 9%, without incidence of rectal ulceration and/or fistula formation. (C) 1999 Elsevier Science Inc.