DOSIMETRY GUIDELINES TO MINIMIZE URETHRAL AND RECTAL MORBIDITY FOLLOWING TRANSPERINEAL I-125 PROSTATE BRACHYTHERAPY

Purpose: To establish dosimetry guidelines to minimize urethral and rectal morbidity following permanent I-125 prostatic brachytherapy. Methods and Materials: Dosimetric parameters were correlated with long-term morbidity for 65 patients following transperineal I-125 implantation for Stage T1/T2 prostatic carcinoma. The prescribed minimum prostatic dose was 150 Gy. The median total activity used was 48 mCi (range: 32-77 mCi). Patients were followed for 1-5 years (median follow-up: 2 years). Postimplant computerized tomography (CT)-based dosimetry was performed for 45 patients. Morbidity was evaluated using a modification of the RTOG grading system, with a scale of grade 0-5. Results: The incidence of grades 2 and 3 urinary morbidity was associated with the maximum central urethral dose (p = 0.03), and with the length of urethra that received greater than 400 Gy (p = 0.07). Patients with larger prostates had more long-term urinary morbidity (p = 0.06). Rectal bleeding (RTOG grade 1) or ulceration (RTOG grade 2) was associated with irradiation of the rectal wall to doses in excess of 100 Gy (p = 0.02). There was no relationship between the matched peripheral dose (MPD), mCi/source or total mCi implanted and long-term morbidity. Conclusions: Postimplant, CT-based dosimetry can predict which patients are at higher risk of radiation-related morbidity. More simplistic parameters including the MPD, total activity implanted, or mCi/source, had no relationship with morbidity. To decrease the risk of long-term morbidity, an effort should be made to keep the central urethral dose below 400 Gy, and the rectal surface dose below 100 Gy.