Abstract # 2887 Fractional Integral Target Dose Comparison for I-125, Pd-103 and Cs-131 in Prostate Seed Implants

Presenter: Li, Texin

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A total of 15 boost therapy implant cases (10 for each isotope) are utilized for this comparison. The TRUS images are obtained intra‐operatively using B&K Ultrasound unit in 5 mm steps. The planning parameters are defined according to RTOG 0232 guidelines. The prescription doses are: 85 Gy for Cs‐131, 108 Gy for I‐125 and 100 Gy for Pd‐103. The seed strengths employed are: 1.8 U (Cs‐131), 1.42 U (Pd‐103) and 0.279‐0.356 U (I‐125). The prostate volume ranges from 25 to 50 cc. Planning goals are: V100~ 95%, D90~100%, and prostatic urethra D10~ 150%. Dose calculations are performed using Variseed (ver7.1, VMS, Palo Alto, CA) planning system using AAPM TG‐43 formalism. All plans are evaluated to insure the planning goals are met. Uniformity (V200), D90 for urethra, and D30 for rectum are also evaluated. Integral target dose are obtained by multiplying the prostate volume V with mean dose Dmean. Total integral dose is obtained by adopting the point source model from AAPM TG43. The integral dose E delivered by a single source with unit activity is the integration of 4Pi r2 D(r)/r2 dr times 1.44T1/2. Let A be the total activity implanted, the total integral dose will be AE. The fractional integral target dose the prostate receives is defined as DmeanV/AE. We also define a parameter (Dmean‐mPD)V/AE that indicates the fraction of excess energy absorbed in the prostate. This parameter shows the amount of energy that is more than needed to achieve the minimum peripheral dose (mPD).