Objective: To determine the rectal tolerance for developing Grade 2 radiation proctitis after 125I prostate implantation based on the rectal dose-volume histogram.
Methods and materials: Two hundred twelve patients with T1-T2 prostate cancer underwent 125I implantation without external beam irradiation. One month after the procedure, all patients underwent CT-based postimplant dosimetry (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 the 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 calculated 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 thep value and finding a < or =5% risk of proctitis in the low volume group. Patients were followed from 12 to 61 months (median: 28 months) after implantation.
Results: Twenty-two patients developed Grade 2 proctitis: 14% within the first year, 72% between the first and second year, and 14% during the third year 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 < or =1.3 cc of rectal tissue resulted in a 5% rate of proctitis 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 prescription dose (160 Gy) increased, so did the proctitis rate: 0% for < or =0.8 cc, 7% for >0.8-1.3 cc, 8% for >1.3-1.8 cc, 24% for >1.8-2.3 cc, and 25.5% for >2.3cc (p = 0.002).
Conclusions: Rectal dose-volume histogram analysis is a practical and predictive method of assessing the risk of developing Grade 2 proctitis after 125I prostate implantation. Delivered dose should be kept below defined rectal volume thresholds to minimize this risk. This information can allow one to decrease rectal morbidity by modifying prostate implant technique.