Dosimetric and radiographic correlates to prostate brachytherapy-related rectal complications

Despite rates of radiation proctitis reported in the 1% to 9% range in most series, there is little information regarding rectal morbidity and dosimetric parameters. Accordingly, we have analyzed computed tomography (CT)-based dosimetric parameters based on a series of patients with endoscopically proven radiation proctitis. Nine patients diagnosed with radiation proctitis on endoscopy were identified in a prior review of 160 consecutively treated patients at the University of Washington in 1997. For each proctitis patient, two patients with no rectal bleeding matched for prostate size, isotope, and dose were selected as controls. Axial CT images obtained 2 to 4 hour postoperatively were used for postimplant dosimetry. Dose volume histograms of the rectum, surface area of the outer rectal wall receiving > or = 100% of the prescribed dose, maximum rectal dose, and length of rectum receiving > or = 100% prescription were obtained. Preimplant CT scans were used to group patients into three categories based on the amount of apparent rectal contact with the prostate. All rectal dosimetric parameters were statistically different between patients with or without rectal bleeding. The mean surface area receiving at least 100% prescription dose was 3.1 cm(2) for the controls vs. 6.9 cm(2) for the rectal bleeders (P = 0.001). The volume of rectum receiving at least 100% of prescription dose was 0.6 cc for the controls vs. 2.5 cc for the bleeders (P = 0.00008). Patients with full prostate-rectal contact had significantly higher rectal dose parameters compared to those with partial or no rectal contact. All nine proctitis patients were in the full-rectal-contact group compared to only seven of 18 (39%) controls. This detailed dosimetric analysis shows higher rectal doses for patients with radiation proctitis, making it a potential method of identifying patients at higher risk for receiving excessive rectal doses based on the anatomic relationship between the rectum and prostate on CT scan. Published 2001 Wiley-Liss, Inc.