Investigation of 90Y-avidin for prostate cancer brachytherapy: A dosimetric model for a phase I-II clinical study

Purpose: A novel method for prostate irradiation is investigated. Similarly to 125I or 103Pd seed brachytherapy, 90Y- avidin could be injected via the perineum under ultrasound image guidance. This study inspects the theoretical feasibility with a dosimetric model based on Monte Carlo simulation. Methods: A geometrical model of the prostate, urethra and rectum was designed. The linear-quadratic model was applied to convert 125I absorbed dose prescription/constraints into 90Y dose through biological effective dose (BED) calculation. The optimal 90Y-avidin injection strategy for the present model was obtained. Dose distribution was calculated by Monte Carlo simulation (PENELOPE,GEANT4). Dose volume histograms (DVH) for the prostate, urethra and rectum were compared to typical DVHs of 125I seed brachytherapy, used routinely in our institute. Results: With 90Y-avidin, at least 95 % of the prostate must receive more than 70 Gy. The absorbed dose to 10 % of the urethra (D 10%-urethra) and the maximum absorbed dose to the rectum (D max-rectum) must be lower than 122 Gy. For the present model, the optimum strategy consists in multiple injections of 90Y-avidin 50 μl drops, for a total volume of 3.1 ml. The minimum activity to deliver the prescribed absorbed dose is 0.7 GBq, which also fully respects urethral and rectal constraints. The resulting dose map has a maximum in the central region with a sharp decrease towards the urethra and the prostate edge. Notably, D 10%-urethra is 95 Gy and Dmax-rectum is below 2 Gy. Prostate absorbed dose is higher with 90Y-avidin than 125I seeds, although the total volume receiving the prescribed absorbed dose is 1-2 % lower. Urethral DVH strictly depends on the 90Y distribution, to be optimized according to prostate shape; in our model, BED30%-urethra is 90 Gy with 90Y-avidin, whereas for patients receiving 125I seeds it ranges between 150 and 230 Gy. The rectal DVH is always more favourable with 90Y. Conclusion: The methodology is theoretically feasible and can deliver an effective treatment in T1-T2 prostate cancer. Pharmacokinetic and biodistribution studies in prostate cancer patients are needed for validation. © 2013 Springer-Verlag Berlin Heidelberg.