Late rectal toxicity after prostate brachytherapy: influence of supplemental external beam radiation on dose-volume histogram analysis.

PURPOSE

To describe the rate of gastrointestinal (GI) toxicity after prostate brachytherapy and describe how external beam radiation therapy (EBRT) may influence the association of rectal dose-volume histogram (DVH) parameters with rectal toxicity.

METHODS AND MATERIALS

One hundred ten patients with prostate cancer were treated with I-125 brachytherapy alone (n=62, 144 Gy) or as a boost (n=48, 108 Gy) after 45-Gy EBRT. CT-based dosimetry was performed a median of 29 days after implantation. GI toxicity was evaluated by Radiation Therapy Oncology Group criteria. Median followup was 41 months.

RESULTS

Eleven patients developed Grade 2+GI toxicity. Men treated with EBRT had an increased risk of GI toxicity, with freedom from Grade 2+ toxicity of 82% vs. 91% for implant alone, but this difference was not statistically significant (p=0.3044). Of the DVH parameters analyzed, only the rectal volume receiving the prescription dose (rV(100)(%)) was associated with late Grade 2+GI toxicity. Men with rV(100%) >or= 0.05 cc had a 4-year freedom from Grade 2+ toxicity of 77% vs. 100% for those with an rV(100%) <0.05 cc (p=0.0248). However, this relationship was only significant for the subset of patients treated with EBRT, where men with rV(100%) >or= 0.05 cc had a 26% risk of Grade 2+ toxicity compared with 0% for rV(100%) <0.05 cc. Additional DVH parameters, including dose to the hottest 0.1 cc (p=0.0199), 1% (p=0.0086), and 3% (p=0.0043), were also associated with GI toxicity but only in men treated with EBRT.

CONCLUSIONS

Supplemental EBRT may lower the threshold for rectal toxicity after prostate brachytherapy. Morbidity can be minimized by observing rectal constraints.