Purpose: Dose escalation for prostate cancer by external beam irradiation is feasible by a 160 MeV perineal proton beam that reduces the volume of rectum irradiated. We correlated the total doses received to portions of the anterior rectum to study the possible relationship of the volume irradiated to the incidence of late rectal toxicity.
Methods: We have randomized 191 patients with stages T3 and T4 prostatic carcinoma to one of two treatment dose arms. These were: 1) 75.6 Cobalt-Gy-equivalent (CGE), 50.4 Gy delivered by 107-25 MV photons followed by 25.2 CGE delivered perineally by protons (Arm 1) or 2) 67.2 CGE delivered by 10-25 MV photons (Arm 2).
Results: With a median follow-up of 3.7 years, post-irradiation rectal bleeding (grades 1 and 2 only, none requiring surgery or hospitalization) from telangiectatic rectal mucosal vessels has occurred in 34% of 99 Arm-1 patients and 16% of 92 Arm-2 patients (p = 0.013). Dose-volume histograms (DVHs) for the anterior rectal wall, the posterior rectal wall and the total rectum in 41 patients treated on Arm 1 were calculated from the three dimensional dose distributions. Rectal bleeding has occurred in 14 or 34% of the 41 DVH-analyzed subset of Arm-1 patients. Both the fractional volume of the anterior rectum and the total dose received by fractional volumes of the anterior rectum significantly correlate with the actuarial probability of bleeding.
Conclusions: Clinicians planning dose escalation to men with localized prostate cancer should approve with caution treatment plans raising more than 40% of the anterior rectum to more than 75 CGE without additional effort to protect the rectal mucosa because this late sequela data indicate that more than half of these men will otherwise have rectal bleeding.