Clinical implementation of AAPM Task Group 32 recommendations on brachytherapy source strength specification

Area of Science:

• Medical Physics
• Radiotherapy
• Nuclear Medicine

Background:

• Historically, brachytherapy source strength was described using various physical quantities like true activity, apparent activity, and radium equivalence.
• These traditional methods lacked standardization, leading to potential inconsistencies in clinical practice.

Purpose of the Study:

• To establish a single, standardized quantity for describing brachytherapy source strength: air-kerma strength.
• To provide methods and conversion factors for transitioning from historical quantities to air-kerma strength.
• To ensure accurate dose calculations and clinical application of brachytherapy sources.

Main Methods:

• Developed a set of equations for unambiguous conversion of source strength estimates.
• Renormalized published dose-rate tables to be consistent with air-kerma strength.
• Calculated multiplicative conversion factors for commonly used brachytherapy sources (198Au, 103Pd, 125I).

Main Results:

• A universal conversion factor of 7.23 μGy m²h⁻¹/mgRaEq was determined for converting equivalent mass of radium to air-kerma strength.
• Specific conversion factors were provided for apparent activity of 198Au (2.06), 103Pd (1.29), and 125I (1.27) seeds.
• Conversion factors for true activity and radium mass depend on source geometry and radionuclide, necessitating careful review of vendor practices.

Conclusions:

• Air-kerma strength offers a standardized and unambiguous measure for brachytherapy sources.
• The provided conversion factors facilitate the clinical adoption of air-kerma strength.
• Users are cautioned to verify vendor-specific constants when applying conversion factors.