Relationship between isotope half-life and prostatic edema for optimal prostate dose coverage in permanent seed implants.

The robustness of treatment planning to prostatic edema for three different isotopes (125I, 103Pd, and 131Cs) is explored using dynamical dose calculations on 25 different clinical prostate cases. The treatment plans were made using the inverse planning by simulated annealing (IPSA) algorithm. The prescription was 144, 127, and 125 Gy for 125I, 131Cs, and 103Pd, respectively. For each isotope, three dose distribution schemes were used to impose different protection levels to the urethra: V120 = 0%, V150 = 0%, and V150 = 30%. Eleven initial edema values were considered ranging from 1.0 (no edema) to 2.0 (100%). The edema was assumed to resolve exponentially with time. The prostate volume, seed positions, and seed activity were dynamically tracked to produce the final dose distribution. Edema decay half-lives of 10, 30, and 50 days were used. A total of 675 dynamical calculations were performed for each initial edema value. For the 125I isotope, limiting the urethra V120 to 0% leads to a prostate D90 under 140 Gy for initial edema values above 1.5. Planning with urethra V150 at 0% provides a good response to the edema; the prostate D90 remains higher than 140 Gy for edema values up to 1.8 and a half-life of 30 days or less. For 103Pd, the prostate D90 is under 97% of the prescription dose for approximately 66%, 40%, and 30% of edema values for urethra V120 = 0%, V150 = 0%, and V150 = 30%, respectively. Similar behavior is seen for 131Cs and the center of the prostate becomes "cold" for almost all edema scenarios. The magnitude of the edema following prostate brachytherapy, as well as the half-life of the isotope used and that of the edema resorption, all have important impacts on the dose distribution. The 125I isotope with its longer half-life is more robust to prostatic edema. Setting up good planning objectives can provide an adequate compromise between organ doses and robustness. This is even more important since seed misplacements will contribute to further degrade dose coverage.