A new power law for determination of total 125I seed activity for ultrasound-guided prostate implants: clinical evaluations

Abstract

Purpose: The intraoperative planning with peripheral loading approach is an important technique for ultrasound-guided transperineal prostate implant. In this paper a sphero-cylindrical dose model is described to generate a new power law or a look-up table for determination of the total 125I activity required to deliver a prescription dose to a given prostate volume. Methods and Materials: Dose calculations were based on the new standards for 125I seeds (model 6711) implemented by the National Institute of Standards and Technology (NIST) in 1999. Using the sphero-cylindrical dose model with peripheral loading approach, a new power law for calculating total activity of radioactive iodine required to deliver a prescribed dose for the target volume was developed. Accounting for random variation of the seed positioning in the prostate and the current air-kerma strength standard of 125I seeds, this new power law is formulated as follows: A (mCi) = 2.15 d (cm) 2.00 where A is apparent activity in mCi, or A (U) = 1.69 d (cm) 2.00 where A is air-kerma strength in U, required to deliver a cumulative dose of 145 Gy to a prostate gland with an average dimension, d, in centimeters. Results: The efficacy of using the new power law in prostate implants was demonstrated. For clinical evaluations of this new power law, 40 patients were chosen in 1998. The average D 90 of these 40 patients was 172.0 Gy (SD ± 29 Gy). This means that on the average, 90% of the target volume received was 172.0 Gy. The average coverage index (CI) in this study was 94.7 (SD ± 4.7). As a result, 94.7% of the target volume received the prescription dose. The dose homogeneity index (HI) which measured the degree of the dose inhomogeneity was 0.38 (SD ± 0.21). Conclusion: This new and simple power law or a new mCi-volume look-up table for 125I seed prostate implantation has been developed and formulated for clinical use. Clinical evaluations expressed in quantitative parameters such as D 90, CI, and HI in prostate implants have been thoroughly analyzed and clearly demonstrated the efficacy of this approach.