Feasibility study combining low dose rate (192)Ir brachytherapy and external beam radiotherapy aiming at delivering 80-85 Gy to prostatic adenocarcinoma.

Abstract

Increasing the radiation dose to prostatic adenocarcinoma has provided higher local control rates. A total of 80 Gy seem necessary to achieve this goal but patient set-up and prostate motion remain difficult problems to solve in conformal radiotherapy. Brachytherapy which overcomes these points could be an alternative way to external beam boost fields. We wanted to transpose the irradiation models largely used in cervix cancer treatment combining external beam radiotherapy and low dose rate brachytherapy. In 71 patients with 19.5 and 13 ng/ml mean and median PSA levels, respectively, a dose escalation from 74 to 85 Gy was performed in four groups. Shifting from intraoperative placement of sources vectors (Group I) to positioning under ultrasound controls (groups II-IV), improving the implantation shape and optimizing radiation delivery to urethral bed have reduced the total dose to rectal wall under 65 Gy and to urethra under 100 Gy. Rectal/prostate dose ratio was lowered from 0.7 (Groups I-II) to 0.58 (Groups III-IV) while avoiding problems resulting from pelvic bone arch interference, prostate volume or seminal vesicles location. The mean and median follow-up periods are 28 and 18 months. In Groups III and IV 85% of patients without hormonotherapy treated with 80-85 Gy normalized PSA under 1 ng/ml within 6 months. No severe late effect has been noted for patients implanted under echographic control. The method described allows to deliver 85 Gy. Longer follow-up is however needed but the levels of dose delivered are not expected to induce prohibitive side effects.