Dose-volume impact in high-dose-rate Iridium-192 brachytherapy as a boost to external beam radiotherapy for localized prostate cancer--a phase II study.

BACKGROUND AND PURPOSE: Evaluation of dose-volume-time-related factors in 64 patients treated with high-dose-rate brachytherapy (HDR-BT) as a boost to external beam radiotherapy (EBRT) for localized prostate cancer. PATIENTS AND METHODS: Clinical parameters were correlated with morbidity scores of the EPIC (Expanded Prostate Cancer Index) questionnaire. Median time after radiotherapy (HDR-BT up to 18 Gy in two fractions and EBRT up to a median dose of 50.4 Gy) was 1.5 and 3 years (first and second questionnaire). RESULTS: A significant impact of a urethra D1 exceeding 15 Gy in at least one HDR fraction concerning urinary morbidity and a rectum D1 exceeding 6 Gy to the rectal mucosa in the first and second HDR fraction concerning the rectal bleeding rate was found. A higher number of needles was associated with lower urinary and bowel scores after 1.5 years. A prostate length >4.8 cm and a longer duration of EBRT (independently of the dose) predisposed for lower urinary and bowel scores. In contrast to a urethra D1 > 15 Gy as an independent factor, a rectum D1 > 6 Gy per HDR fraction correlated with a higher number of needles and an increased prostate length. CONCLUSIONS: To minimize morbidity in HDR-BT for prostate cancer, a maximum dose to the urethra of 15 Gy and a maximum dose to the rectal mucosa of 6 Gy is advisable. Treatment- and patient-related factors have a major impact on toxicity.