AbstractID: 7025 Title: Centennial Recognition of the National Bureau of Standards:The Evolution of Physical
Measurement Standards for Radiology
The Bureau of Standards, later called NBS and more recently NIST, was founded by an act of
Congress in 1901. The Bureau initiated programs to develop physical measurement standards
during the same decade in which x-ray technology became widely used in diagnostic imaging, and
radium-226 and radon-222 found applications in therapy. The first involvement of the Bureau
began on receipt of a standard of radium-226 from Marie Curie and the Radium Standards
Commission in December of 1913. Initially, beam qualities for both x rays, and the gamma rays
from radium and its daughters, were expressed in terms of half value layers of different metals.
The relationship between x-ray spectra and beam qualities were understood, but crystal
spectrometers were not available in the clinics. So, x rays of increasingly higher energies were
characterized by half value layers. In 1927, the Bureau hired L.S. Taylor to develop a program for
x-ray exposure measurements. The early brachytherapy sources were replaced after the advent of
nuclear fission by reactor-produced radionuclides, and then later by iridium-192 and short-lived
low energy photon emitters. Orthovoltage x-rays gave way to mega-electron volt treatment beams
from betatrons, linacs, microtrons and cyclotrons. The quantities and units used to characterize
beams for these different modalities have evolved in discrete steps over the past century. The
evolution of the physical measurement standards in medical physics will be presented in four
parts: Brachytherapy, External Beam Exposure/Air Kerma, Absorbed Dose to Water in
Megavoltage Therapy, and Beam Quality Specification in Diagnostic and Therapeutic Radiology.