The AAPM dosimetry protocol for LDR brachytherapy seeds that emit x-rays of energies ≤ 50 keV is described in the TG-43U1 report (Rivard et al. 2004). In this protocol, it is specified that the strength of such sources be expressed in terms of air-kerma strength, traceable to the National Institute of Standards and Technology (NIST). NIST maintains and disseminates the primary air-kerma strength standard for all 125I, 103Pd, and 131Cs seed models.
and Soares, C.
Primary Standards for Brachytherapy Sources, 2009 AAPM Summer School, AAPM, College Park, MD, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=901166
(Accessed March 28, 2023)