Rectal-wall dose dependence on postplan timing after permanent-seed prostate brachytherapy

PURPOSE: Dose to rectal wall after permanent-seed prostate brachytherapy is dependent on distance between posterior prostatic seeds and anterior rectal wall and is influenced by postimplant periprostatic edema. We analyzed the effect of postplan timing on anterior rectal-wall dose. METHODS AND MATERIALS: Twenty patients received permanent seed 125I brachytherapy as monotherapy (145 Gy). Implants were preplanned by use of transrectal ultrasound (TRUS) and carried out by use of preloaded needles. Postimplant dosimetry was calculated by use of magnetic resonance imaging-computed tomography fusion on Days 1, 8, and 30. The anterior rectal-wall dose is reported as the isodose enclosing 1.0 or 2.0 cc of rectal wall and as the RV100 in cc. RESULTS: The dose to rectal wall increased progressively over time. The median increase in dose to 1.0 cc of rectal wall (RD [1 cc]) from Day 1 to 30 was 39.2 Gy (p < 0.001). RV100 increased from a median of 0.07 cc on Day 1 to 0.67 cc on Day 30. The most significant predictor of rectal-wall dose (RD [1 cc], RD [2 cc], or RV100) was the time of evaluation (p < 0.001). CONCLUSION: Although periprostatic edema cannot be quantified by postimplant imaging, the dose to the anterior rectal wall increases significantly over time as prostatic and periprostatic edema resolve. Critical-organ dose reporting and guidelines for minimizing toxicity must take into account the time of the assessment.