AbstractID: 6649 Title: Characterization of ¹⁰³Pd and ¹²⁵I Prostate
Brachytherapy Seeds by Air-kerma Strength, X-ray Spectrometry, and Well-ionization Chamber Response
Measurements
Nineteen designs of radioactive "seed" sources for prostate cancer brachytherapy containing 103Pd or 125I
from thirteen manufacturers have been characterized at the National Institute of Standards and Technology
(NIST). The national standard for seed air-kerma strength is maintained at NIST using the Wide-angle
Free-air Chamber (WAFAC), a variable volume ionization chamber. The photon spectrum emitted from
each seed type was measured by x-ray spectrometry using a high-purity germanium (HPGe) detector. The
responses of three different well-ionization chambers relative to the WAFAC-measured air-kerma strength
were found to depend on both the emitted photon spectrum and internal seed structure. Variations in the
relative response of the WAFAC, HPGe spectrometer, and well-ionization chambers have been attributed
to a combination of differences in measurement geometry, fluorescence x-ray emission from nonradioactive seed components, and anisotropy of x-ray emission and self-absorption (attenuation) effects due
to internal seed geometry. The magnitudes of such effects vary with seed design, and therefore clinical
verification measurements of seed air-kerma strength are subject to an additional component of uncertainty,
the magnitude of which varies with seed type. Changes in the relative responses of the various instruments
for measurements made on the same seed at different times have been observed, most likely due to a
shifting of internal seed components.