Probability of late rectal morbidity in 125I prostate brachytherapy

Purpose: Rectal toxicity is a concern in prostate brachytherapy because it is difficult to avoid delivering a dose equal to, or greater than, the prescription dose to the anterior surface of the rectum. The purpose of this study was to define the probability that a patient will experience Grade 2 (bleeding/ulceration) late rectal morbidity after I-125 prostate brachytherapy according to the rectal dosimetry. Methods and Materials: Ninety-eight consecutive patients who received monotherapy I-125 prostate implants for treatment of Stage T1-T2, favorable-risk adenocarcinoma of the prostate were evaluated for Radiation Therapy Oncology Group Grade 2 late rectal morbidity. All reported incidences of late morbidity were retrospectively confirmed by colonoscopy. All patients had at least 15 months follow-up after implantation. The median follow-up was 32 months (range 15-54). The rectal dosimetry was based on a CT scan obtained at 3-9 weeks after implantation. The rectum was contoured on each CT image between the base and apex of the prostate. A dose-surface histogram was compiled for each implant, and the relative surface area that received a dose greater than or equal to100, greater than or equal to150, greater than or equal to200, greater than or equal to300, greater than or equal to400, and greater than or equal to500 Gy was recorded. The probability of developing late rectal toxicity was calculated by logistic regression analysis as a function of dose and the percentage of the rectal surface that received that dose. Results: Of the 98 patients, 10 developed Grade 2 late rectal morbidity. The percentage of the rectal surface that received 100, 150, 200, and 300 Gy was significantly greater (p less than or equal to0.02) for patients who experienced late rectal morbidity. The probability of late rectal morbidity increased with both the dose and the percentage of the rectal surface that received that dose. The probability was less than or equal to1% when 20%, 7%, and 0% of the rectal surface received 100, 150, and 200 Gy, respectively. The probability increased to less than or equal to5% when 31%, 19%, and 9% of the rectal surface received these doses. The probability of late rectal morbidity can also be expressed in terms of the maximal rectal dose. The probability of late morbidity was 0.4%, 1.2%, and 4.7% when the maximal rectal dose was 150, 200, and 300 Gy, respectively. Conclusion: The percentage of the rectal surface that receives a dose greater than or equal to100 Gy is predictive of Grade 2 (bleeding/ulceration) late rectal morbidity after I-125 prostate brachytherapy. The probability of late morbidity depends on both the dose and the percentage of the rectal surface that received that dose. Our results indicate that the rectum can tolerate doses of 100, 150, and 200 Gy to approximately 30%, 20%, and 10% of the rectal surface with a less than or equal to5% risk of late morbidity. Our results also indicate that the practical guideline for limiting the incidence of late morbidity to 1%, 3%, or 5% is to keep the maximal rectal dose to <200, 250, and 300 Gy, respectively. (C) 2003 Elsevier Science Inc.