1. Relationship between isotope half-life and prostatic edema for optimal prostate dose coverage in permanent seed implants Maxime Villeneuve Ghyslain Leclerc Etienne Lessard Jean Pouliot Luc Beaulieu

2. Edema leads to wrong dosimetry Prostate permanent seed implants: insert radioactive seeds via needles inserted through the perineum into the prostate. Edema induced from: Physical trauma induced by needle puncture Radiation trauma induced by the seeds No predictors (age, prostate volume, etc.) for magnitude Generalized V(t): Vmax ~ [10%,200%] V 0 λ (edema half life) ~ [10,30+] days V residual ~ 10% Time Volume V max t max V residual λ

3. Edema, Robustness, & DVHs Evaluated change in initial clinical pramaters (D 90, V 100, etc.) as a function of: Isotope ( 125 I, 103 Pd, 131 Cs) Maximum edema volume (Vmax) Edema resorption half-live (t (1/2) ) Seed activity Clinical planning objectives t DVH Time Volume Not to scale Average over the DVHs of each time period. t DVH = 1 day. Edema was modeled, 33 scenarios: λ = {10, 30, 50} V max = 0-100% increase in steps of 10%.

4. Results: 0.75 U 125 I Prostate D 90 (a) Require uV 120 =0% Rx. (c) Require uV 150 =30% Rx (b) Require uV 150 =0% Rx. D 90 < 120 Gy, poor clinical outcome.

5. Results: 0.75 U 125 I Urethra V 100 (a) Require uV 120 =0% Rx. (c) Require uV 150 =30% Rx (b) Require uV 150 =0% Rx. As prostate V↑, seeds ↔. Cool center.

6. 0.75 U 125 I 0.50 U 125 I At right: 0.50 U 125 I vs. 0.75 U 125 I Note that dosimetric degradation has no dependence on seed activity Results: Independent of seed activity 0.75 U 131 Cs Below, 131 Cs. Effect of edema more pronounced due to shorter half life. To = 125 I ( 103 Pd), Cs Rx ↑ 10(25)%

7. Discussion Need to account for edema in dose planning. Avg. edema = 1.5 V 0. >1.5V 0 more likely than <1.2 V 0 ↓ half life, ↑ effect of edema. Robustness to edema: 125 I > 103 Pd > 131 Cs May want to extend plan with larger margin (r 3 vs r 2 ) Actual dose to organ < than prescribed. But the Rx dose works (good cure rates) The actual effective Rx is lower than we think. Need to generate plans consider edema a priori.

8. Selected Background Papers R. G. Stock, N. N. Stone, A. Tabert, C. Iannuzzi, and J. K. DeWyngaert, “A dose-response study for I-125 prostate implants,” Int. J. Radiat. Oncol. Biol. Phys. 41, 101–108 1998. S. Nag, W. Bice, K. DeWyngaert, B. Prestidge, R. Stock, and Y. Yu, “The American Brachytherapy Society recommendations for permanent pros- tate brachytherapy postimplant dosimetric analysis,” Int. J. Radiat. Oncol. Biol. Phys. 46, 221–230 2000. Z. Chen, J. Deng, K. Roberts, and R. Nath, “Potential impact of prostate edema on the dosimetry of permanent seed implants using the new 131 Cs (model CS-1) seeds,” Med. Phys. 33, 968–975 2006.