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History Symposium: History of Calibration Protocols and Laboratories

P Almond1*, L DeWerd2*, (1) UT MD Anderson Cancer Center, Houston, TX, (2) University of WI-Madison/ADCL, Madison, WI


(Wednesday, 8/1/2018) 4:30 PM - 6:00 PM

Room: Davidson Ballroom A

Calibration Protocols - A Proud History of the AAPM
Peter R. Almond, Ph.D., DABR, FAAPM, FACMP

By some estimates, each year in the United States some 575,000 cancer patients receive radiation treatments for their disease. Assuming that each course of treatment consists of 20-30 fractions there are therefore 11.5 to 17.25 million individual treatment sessions each year. The correctness and accuracy of each of those treatment sessions are initially guaranteed by the calibration protocol used. In most cases this will be an AAPM protocol. But this was not always the case. In the late 1930s before the formation of the AAPM, physicists and some radiologists had started to believe and suggest that uniform standards in X-ray therapy machine calibrations be used to obtain a known level of accuracy. In general however, the radiology community pushed back against this idea seeing no benefit to their patients from such a move and seeing it as interference in their clinical practice by physicists. However after World War II and the introduction of higher energy X-ray machines and the introduction of electron beam therapy the need for standardization in machine calibrations at a known accuracy became a necessity. Although the AAPM was primarily established as a professional society in1958 it soon became apparent that scientific concerns needed to be addressed and a scientific committee was established in 1962 and in 1963 a subcommittee of the scientific committee was formed to consider radiation dosimetry. This became known as SCRAD. In 1966 SCDAD published its first protocol on the dosimetry of electrons. This was followed by a SCRAD protocol on X-ray calibrations. By the beginnings of the 1980s however the need for a single protocol for treatment machines producing both X-rays and electrons was more desirable. Various task groups had replaced SCRAD and TG-21 produced the first AAPM protocol for both modalities. Finally TG-51 was issued which allowed for different chambers to be used while maintaining the desired accuracy.

Learning Objectives:
1. To appreciate the history of the AAPM’s calibration Protocols, why they were needed and why they changed.
2. To get some understanding of the importance of these protocols in the overall picture of radiation cancer treatment.
3. To appreciate the improvement physician physicist relationship during this period.

The History of Calibration Laboratories: The Transfer from Primary Standards to the Clinic
Larry A. DeWerd, Ph.D., FAAPM

Modern protocols to transfer dose to water have been established to deliver a precise dose to the tumor in a patient. Protocols have gone through a number of iterations. The most recent change was to introduce the quantity absorbed dose to water for the TG 51 protocol. The basis of the precision for these protocols is the determination of the output of linear accelerators, using a calibrated ionization chamber. It was well known that this calibration should be traceable to a primary standard for accuracy. In the beginning, the National Bureau of Standards (NBS), which later became the National Institute of Standards and Technology (NIST), calibrated ionization chambers to exposure and then later air kerma. In 1975, NBS found that they could not keep up with the demand for calibration and petitioned the AAPM to set up secondary laboratories. Thus, was born the Regional Calibration Laboratories which became the Accredited Dosimetry Calibration Laboratories (ADCL). The ADCLs maintain precision traceable to NIST and demonstrate this traceability with proficiency tests. Through this precision, assurance is given that accuracy to a world-wide standard is obtained. The proficiency tests over the years for cobalt calibrations show agreement within 0.5 % among all laboratories. Therefore, the clinic can be assured that their measurement of dose has high precision from the ADCLs and is traceable to an accurate primary standard. The ADCLs have expanded beyond megavoltage, external beam applications to calibrations for brachytherapy and diagnostic x-rays. A review of the history and precision of the secondary laboratories will be discussed.

Learning Objectives:
1. To understand the reason for the establishment of secondary laboratories of the ADCL program
2. To understand the history and precision of the absorbed dose to water quantity
3. To understand that the clinical physicist has accuracy comparable to the rest of the world, when they have a NIST traceable calibration through a secondary laboratory.


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