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

Andrew Wu, Chen Chiao Lee, Mark Johnson, Douglas Brown, Ronald Benoit, Ralph Miler, Jeffery Cohen, Paul Geis, Alex S.J. Chen, Shalom Kalnicki

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


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 D90 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 D90, CI, and HI in prostate implants have been thoroughly analyzed and clearly demonstrated the efficacy of this approach. Copyright (C) 2000 Elsevier Science Inc.

Original languageEnglish (US)
Pages (from-to)1397-1403
Number of pages7
JournalInternational Journal of Radiation Oncology Biology Physics
Issue number5
StatePublished - Jul 15 2000
Externally publishedYes


  • Brachytherapy
  • Implant
  • Iodine-125
  • Prostate

ASJC Scopus subject areas

  • Radiation
  • Oncology
  • Radiology Nuclear Medicine and imaging
  • Cancer Research


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