Rectal dose constraints for salvage iodine-125 prostate brachytherapy

Purpose Organ-confined prostate cancer recurrences after primary radiotherapy can be treated with salvage iodine-125 brachytherapy. Options include total salvage (TS) or focal salvage (FS). TS often leads to severe late gastrointestinal (GI) toxicity. Differences in rectal dosimetry between TS and FS are presented and dose constraints proposed to reduce late severe GI toxicity (>90 days). Methods and Materials Intraoperative dosimetry and 30-day CT-dosimetry of 20 FS and 28 TS patients were evaluated. GI toxicity was evaluated using the common terminology criteria for adverse events-4. With receiver operating characteristic analysis, dosimetry cutoff values to prevent severe late GI toxicity were assessed. Results FS reduces rectal dose significantly. Median D0.1cc, D1cc, D2cc, and V100 reductions were 38 Gy (p = 0.002), 46 Gy (p < 0.0001), 46 Gy (p < 0.0001), and 0.41 cc (p = 0.0001), respectively, compared with TS. FS patients had no late severe GI toxicity. TS patients with severe GI toxicity (41%, n = 11) showed significantly higher rectal doses than TS patients without GI toxicity (59%, n = 16). Median D0.1cc, D1cc, D2cc, and V100 differences were 29 Gy (p < 0.001), 17 Gy (p = 0.001), 28 Gy (p < 0.001), and 0.45 cc (p = 0.001). With receiver operating characteristic analysis, restrictions for the D0.1cc, D1cc, D2cc, and V100 are <160 Gy (area under the curve [AUC], 0.88; 95% confidence interval [CI] 0.76–1.00), <119 Gy (AUC, 0.87; 95% CI, 0.74–1.00), <102 Gy (AUC, 0.89; 95% CI, 0.77–1.00), and <0.38 cc (AUC, 0.88; 95% CI, 0.75–1.00), respectively. Thirty-day CT dosimetry showed minor overestimation of intraoperative D2cc (median, 10 Gy [p = 0.02]). Conclusions FS reduces rectal dose compared with TS. D0.1cc, D1cc, D2cc, and V100 restrictions were 160 Gy, 120 Gy, 100 Gy, and 0.35 cc. Taking correlation into account, the D2cc <100 Gy might be sufficient for clinical practice. Larger series and multivariable models are necessary to further assess the found restrictions.