Purpose: To evaluate changes of dose distribution for both the prostate and the surrounding tissues after permanent brachytherapy as monotherapy for prostate cancer.
Patients and methods: In 35 patients, CT scans were performed before, 1 day after (day 1) and 1 month after the implantation (day 30). Changes of prostate volume, dosimetric parameters, and distances between posterior prostate contour and rectal wall as well as prostate contour and prescription isodose were analyzed.
Results: Prostate volume increased from 37+/-11 cm3 (mean+/-standard deviation) to 49+/-12 cm3 on day 1 and dropped to 40+/-9 cm3 on day 30. Prostate V100 increased from 87+/-7% to 90+/-7%, prostate D90 from 138+/-21 Gy to 151+/-30 Gy. Mean rectal volume covered by the prescription isodose rose from 0.4 cm3 to 1.0 cm3; a changing distance between the prostate and rectal wall was excluded as a reason. Prostate D90 (day 1) and rectum V100 (day 30) proved to be significantly higher for larger prostate sizes. The distance between the prescription isodose and the prostate contour increased particularly at the posterior and inferior borders: 1.9 mm and 2.5 mm on average (0.1 mm and -0.7 mm at opposite borders, respectively).
Conclusion: With a decreasing edema of the prostate, an increasing dose both to the prostate and the anterior rectal wall resulted--the postimplant interval is essential for the dosimetry report. Due to a larger edema, a higher prescription dose might be needed for optimal cancer control in smaller prostates. Compared to day 1, the dose to the surrounding tissues increased on day 30, particularly at the posterior and inferior prostate borders.