Detailed urethral dosimetry in the evaluation of prostate brachytherapy-related urinary morbidity

Abstract

To evaluate the relationship between urinary morbidity after prostate brachytherapy and urethral doses calculated at the base, midprostate, apex, and urogenital diaphragm. From February 1998 through July 2002, 186 consecutive patients without a prior history of a transurethral resection underwent monotherapeutic brachytherapy (no supplemental external beam radiation therapy or androgen deprivation therapy) with urethral-sparing techniques (average urethral dose 100%-140% minimum peripheral dose) for clinical T1c-T2b (2002 AJCC) prostate cancer. The median follow-up was 45.5 months. Urinary morbidity was defined by time to International Prostate Symptom Score (IPSS) resolution, maximum increase in IPSS, catheter dependency, and the need for postimplant surgical intervention. An alpha blocker was initiated approximately 2 weeks before implantation and continued at least until the IPSS returned to baseline. Evaluated parameters included overall urethral dose (average and maximum), doses to the base, midprostate, apex, and urogenital diaphragm, patient age, clinical T stage, preimplant IPSS, ultrasound volume, isotope, and D90 and V100/150/200. Of the 186 patients, 176 (94.6%) had the urinary catheter permanently removed on the day of implantation with only 1 patient requiring a urinary catheter >5 days. No patient had a urethral stricture and only 2 patients (1.1%) required a postbrachytherapy transurethral resection of the prostate (TURP). For the entire cohort, IPSS on average peaked 2 weeks after implantation with a mean and median time to IPSS resolution of 14 and 3 weeks, respectively. For the entire cohort, only isotope predicted for IPSS resolution, while neither overall average prostatic urethra nor segmental urethral dose predicted for IPSS resolution. The maximum postimplant IPSS increase was best predicted by preimplant IPSS and the maximum apical urethral dose. With the routine use of prophylactic alpha blockers and strict adherence to urethral-sparing techniques, detailed urethral dosimetry did not substantially improve the ability to predict urinary morbidity. Neither the average dose to the prostatic urethra nor urethral doses stratified into base, midprostate, apex, or urogenital diaphragm segments predicted for IPSS normalization. Radiation doses of 100%-140% minimum peripheral dose are well tolerated by all segments of the prostatic urethra with resultant tumoricidal doses to foci of periurethral cancer.