Guidance to users of Nycomed Amersham and North American Scientific, Inc., I-125 interstitial sources: dosimetry and calibration changes: recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee Ad Hoc Subcommittee on Low-Energy Seed Dosimetry.

Dose calculations to patients undergoing implantation of 125I interstitial brachytherapy sources are affected by two recent changes in low-energy seed dosimetry: (a) implantation of a new primary air-kerma strength standard at the National Institute of Standards and Technology (NIST) on 1 January 1999 and (b) publication of revised dose-rate distributions in AAPM's Task Group 43 Report. The guidance herein represents AAPM's recommendations for users of 125I interstitial seed products marketed prior to 1 January 1999 (Nycomed Amersham models 6711 and 6702 and North American Scientific, Inc. models 3631 A/S and 3631 A/M. Implementation of Task Group 43 (TG43) 125I dose calculations involves revising data stored in files of radiation treatment planning software and lowering the prescribed dose to be delivered to patients by as much as 15% to avoid modifying the dose actually delivered to patients. The magnitude of the dose prescription change depends on the dosimetry data used prior to TG43 and the implant geometry. Adapting to the revised NIST calibration standard requires the user to increase the dose-rate constant (or its equivalent by 11.5%) but does not require modification of the prescribed dose. Failure to correctly implement these modifications can result in 20% or even 30% errors.