Background & Purpose: Rectal toxicity is less common after 125I seed implant brachytherapy for prostate cancer,\r\nand intraoperative rectal dose-volume constraints (the constraint) is still undetermined in pioneering studies. As our\r\nconstraint failed to prevent grade 2 or 3 rectal bleeding (bled-pts) in 5.1% of patients, we retrospectively explored\r\nanother constraint for the prevention of rectal bleeding.\r\nMaterials and methods: The study population consisted of 197 patients treated with the brachytherapy as\r\nmonotherapy using real-time intraoperative transrectal ultrasound (US)-guided treatment at a prescribed dose of 145\r\nGy. Post-implant dosimetry was performed on Day 1 and Day 30 after implantation using computed tomography (CT)\r\nimaging. Rectal bleeding toxicity was classified by CTC-AE ver. 3.0 during a mean 29-month (range, 12-48 months)\r\nperiod after implantation. The differences in rV100s were compared among intraoperative, Day 1 and Day 30 dosimetry,\r\nand between that of patients with grade 2 or 3 rectal bleeding (the bled-pts) and of the others (the spared-pts). All\r\npatients were divided into groups based on provisional rV100s that were increased stepwise in 0.1-cc increments from\r\n0 to 1.0 cc. The difference in the ratios of the bled-pts to the spared-pts was tested by chi-square tests, and their odds\r\nratios were calculated (bled-OR). All statistical analyses were performed by t-tests.\r\nResults: The mean values of rV100us, rV100CT_1, and rV100CT_30 were 0.31 �± 0.43, 0.22 �± 0.36, and 0.59 �± 0.68 cc,\r\nrespectively. These values temporarily decreased (p = 0.020) on Day 1 and increased (p = 0.000) on Day 30. There\r\nwas no significant difference in rV100s between the bled-pts and spared-pts at any time of dosimetry. The\r\nmaximum bled-OR was identified among patients with an rV100us value above 0.1 cc (p = 0.025; OR = 7.8; 95% CI,\r\n1.4-145.8); an rV100CT_1 value above 0.3 cc (p = 0.014; OR = 16.2; 95% CI, 3.9-110.7), and an rV100CT_30 value\r\nabove 0.5 cc (p = 0.019; OR = 6.3; 95% CI, 1.5-42.3).\r\nConclusion: By retrospective analysis exploring rV100 as intraoperative rectal dose-volume thresholds in 125I seed\r\nimplant brachytherapy for prostate cancer, it is proved that rV100 should be less than 0.1 cc for preventing rectal\r\nbleeding.
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