National Air-Kerma Standards for Mammography

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Summary for MPD A.1 (Completed)

In 1992, the US Congress passed Public Law 102 530, the Mammography Quality Standards Act of 1992. This Act requires that all screening and diagnostic mammo¬graphic facilities be certified by the Secretary of the Department of Health and Human Services by October 1, 1994. This certification process will involve accreditation by an approved nonprofit private organization or approved State organization. There must be a yearly on site evaluation by a credentialed medical physicist and a yearly inspection by a credentialed government inspector.

Detailed Program Characteristics

Mammographic units used in the United States commonly use molybdenum for both the X ray tube anode material and the additional filter used to remove unwanted low energy bremsstrahlung X-rays that contribute to patient dose but not significantly to image quality. One problem in calibrating instruments used to measure the air-kerma rate from mammographic units is that the National Institute of Standards and Technology (NIST) presently does not yet have a national standard for those mammographic beams. In fact, the only national standards laboratory in the world having appropriate national standards is the Physikalisch-Technische Bundesanstalt (PTB), the German standards laboratory. All the reference X ray beams at NIST are produced by tungsten-anode X ray tubes. The spectra (and therefore any measure of beam quality) are quite different for these two anode materials. For a tungsten target, aluminum filter system operated at voltages appropriate for mammography, most of the dose results from the thick-target tungsten bremsstrahlung (i.e., low energy X-rays), although the L fluorescent tungsten X-rays are present. For a molybdenum target, molybdenum filter system, the K fluorescent X- rays dominate the spectra and there is very little thick-target molyb¬denum bremsstrahlung. For a reasonable choice of operating voltages, one can match either the half value layer or the homogeneity coefficient but not both beam quality parameters for molybdenum anodes.

In the United States, the Food and Drug Administration’s Center for Devices and Radio¬logical Health (CDRH) is responsible for calibrating all the instruments that the govern¬ment inspectors will use during the yearly inspection of each mammography facility. The CDRH X ray Calibration Laboratory is accredited by NIST’s National Voluntary Laboratory Accreditation Program. CDRH is establishing a new facility within the Mammography Calibration Laboratory explicitly to calibrate instruments in appropriate X ray beams. Since there are no suitable national standards in the United States, CDRH has opted to send its reference ionization chamber to PTB to establish traceability to a national standard.

To perform the annual on site evaluation, the medical physicists will presumably have their instruments calibrated at one of the American Association of Physicists in Medicine’s (AAPM) Accredited Dosimetry Calibration Laboratories (ADCL). One of these laboratories, at the University of Wisconsin, is developing a free-air chamber to measure air kerma from their mammography X ray units. In principle, the free-air chamber is an absolute device, but in practice it is necessary to determine a number of correction factors. Preliminary comparisons of this chamber with NIST standards have been made in tungsten-anode beams, and measurements of selected mammography chamber response have been made in the molybdenum and rhodium beams at CDRH.

To be able to provide national standards for all secondary laboratories wishing to calibrate mammography probes, it is desirable for NIST to develop suitable reference X ray beams. An Interagency Agreement has been established with the Food and Drug Administration to develop these national standards. At a minimum, these new reference beams should be identical to the beams recommended by the International Electrotechnical Commission for measuring the characteristics of diagnostic X ray equipment and for verifying the performance requirements of ionization chambers and semiconductors used in medical radiography.

US Facilities, Staffing, and Funding

The appropriate US facilities can be organized into three groups:

1. NIST: As indicated above, NIST needs major new resources in equipment and personnel to carry out this program. With the tight deadlines of MQSA, this program needs high priority. A minimum requirement is 2 person-years and $250,000 for each of two years.

2. CDRH: Most equipment for the new mammography facility has been ordered. Two additional person-years will be required: one to finish developing the automated computer system and the other to do routine calibrations, maintain in house quality control, and maintain inventory. Equipment costs are estimated to be about $130,000 for each of two years.

3. ADCLs: To set up laboratories for calibrating instruments to measure air kerma from mammography units, it is estimated that each ADCL will need at least $100,000 for equipment and a person to operate the calibration facility. Two of the ADCLs have expressed an interest in developing mammography calibration facilities.

Figure A.1 National standard calibration range for mammography testing