Defining the risk of developing grade 2 proctitis following I-125 prostatebrachytherapy using a rectal dose-volume histogram analysis

Objective: To determine the rectal tolerance for developing Grade 2 radiati on proctitis after I-125 prostate implantation based on the rectal dose-vol ume histogram. Methods and Materials: Two hundred twelve patients with T1-T2 prostate canc er underwent I-125 implantation without external beam irradiation. One mont h after the procedure, all patients underwent CT-based postimplant dosimetr y (3-mm abutting slices). The rectal volumes, defined by an inner and outer wall, were determined from 9 mm above the seminal vesicles to 9 mm below t he prostate apex. All doses were calculated by TG43 formalism. The prostate prescription dose was 160 Gy; A dose response analysis was undertaken for volumes of rectal tissue receiving a given dose. Dose levels examined were 80 Gy, 100 Gy, 120 Gy, 140 Gy, 160 Gy, 180 Gy, 200 Gy, 220 Gy, and 240 Gy. Grade 2 proctitis was defined as rectal bleeding occurring at least once a week for a minimum period of one month. The risk of proctitis was calculate d using actuarial methods. For each dose level, a critical volume cutpoint was chosen to define a low and high volume group of patients. The cutpoint was determined based on two goals: minimizing the p value and finding a les s than or equal to5% risk of proctitis in the low volume group. Patients we re followed from 12 to 61 months (median: 28 months) after implantation. Results: Twenty-two patients developed Grade 2 proctitis: 14% within the fi rst year, 72% between the first and second year, and 14% during the third y ear after the implant date. After the third year postimplantation, no cases of proctitis were reported. Proctitis was found to be significantly volume dependent for a given dose. The prescription dose (160 Gy) delivered to le ss than or equal to1.3 cc of rectal tissue resulted in a 5% rate of proctit is at 5 years vs. 18% for volumes >1.3 cc (p = 0.001). Similar results were found for all doses examined. As the rectal volume receiving the prescript ion dose (160 Gy) increased, so did the proctitis rate: 0% for less than or equal to0.8 cc, 7% for >0.8-1.3 cc, 8% for >1.3-1.8 cc, 24% for >1.8-2.3 c c, and 25.5% for >2.3cc (p = 0.002). Conclusions: Rectal dose-volume histogram analysis is a practical and predi ctive method of assessing the risk of developing Grade 2 proctitis after I- 125 prostate implantation. Delivered dose should be kept below defined rect al volume thresholds to minimize this risk. This information can allow one to decrease rectal morbidity by modifying prostate implant technique. (C) 2 001 Elsevier Science Inc.