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Relationship between isotope half‐life and prostatic edema for optimal prostate dose coverage in permanent seed implants
Author(s) -
Villeneuve Maxime,
Leclerc Ghyslain,
Lessard Etienne,
Pouliot Jean,
Beaulieu Luc
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2900722
Subject(s) - edema , prostate , medicine , urology , prostate brachytherapy , urethra , brachytherapy , prostatic urethra , nuclear medicine , radiation therapy , cancer
The robustness of treatment planning to prostatic edema for three different isotopes ( I125,P103 d , andC131 s ) 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 forI125,C131 s , andP103 d , respectively. For each isotope, three dose distribution schemes were used to impose different protection levels to the urethra:V 120 = 0 % ,V 150 = 0 % , andV 150 = 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 theI125isotope, limiting the urethraV 120to 0 % leads to a prostateD 90under 140 Gy for initial edema values above 1.5. Planning with urethraV 150at 0 % provides a good response to the edema; the prostateD 90remains higher than 140 Gy for edema values up to 1.8 and a half‐life of 30 days or less. ForP103 d , the prostateD 90is under 97 % of the prescription dose for approximately 66 % , 40 % , and 30 % of edema values for urethraV 120 = 0 % ,V 150 = 0 % , andV 150 = 30 % , respectively. Similar behavior is seen forC131 s 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. TheI125isotope 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.