Calibration Services: Proficiency Tests for X-Ray Air Kerma within the Quality System

Summary

Proficiency tests have been offered by the Dosimetry Group to support air-kerma traceability since the 1980’s. The Quality Management System incorporates the existing proficiency testing routines that are supported by the 17025 measurement services.

Description

To meet the needs of US calibration facilities which maintain traceability to NIST for air kerma measuring instruments, NIST offers proficiency tests upon request. NIST will conduct a proficiency test as part of an accreditation program or on an individual basis with a calibration facility. NIST provides the required proficiency tests for secondary calibration facilities for the DOE, Armed Forces, US chamber manufacturers, NVLAP and the American Association of Physicists in Medicine (AAPM) Accredited Dosimetry Calibration Laboratory (ADCL) programs. NIST provides proficiency testing to support the FDA MQSA requirement that any US mammography facility providing NIST-traceable calibrations must participate in a biennial proficiency testing program directly with NIST. NIST provides the testing facility with a calibrated instrument, provides the test protocol, conducts the test and provides a NIST Proficiency Test report.

General Guidelines for NIST Proficiency Tests Offered for X-Ray Air-Kerma Traceability

1. The calibration facility shall initiate contact with the NIST Dosimetry Group technical contact and request a proficiency test for the reference radiation qualities within their scope of accreditation.
2. NIST will calibrate a NIST-provided reference-class, transfer ionization chamber using mutually decided upon reference radiation qualities which are in the scope of the accreditation or which have been identified to meet a measurement goal of the requesting calibration facility. NIST will then send the transfer standard to the participating calibration facility, along with a proficiency-test protocol, describing the details of the NIST calibration conditions and the required dates and addresses for the proper shipment of the chamber. The NIST calibration results for the chamber are not revealed to the test facility until the completion of the test.
3. Calibrations of the NIST transfer chamber will be made at the participating facility using the beam qualities identified in the test protocol. After the participating facility calibrates the NIST ionization chamber, using their appropriate equivalent beam quality(s), the chamber is returned to NIST, or to the next participating facility if the test involves multiple participants. The participant provides NIST with the calibration coefficient(s) for the NIST chamber measured at their facility in units of Gy/C in terms of the HVL of the specified beam quality(s). The participant’s calibration coefficient(s) should be provided to NIST, normalized to one standard atmosphere (101.325 kPa) and 22 °C for a direct comparison with the NIST calibration coefficients. The participant must provide the uncertainty of the calibration coefficient.
4. Upon receipt of the chamber at NIST, it will be re-calibrated to assure stability and proper working conditions.
5. Each result from a participating facility will be compared to the average of the NIST calibration coefficients for that beam quality. At the completion of the test a NIST Proficiency Test report will be issued to the participating calibration facility. The report will state the level of agreement with NIST under the conditions of the proficiency test. The proficiency test can be used to demonstrate that the participating facility can calibrate a NIST transfer standard, using characterized x-rays or gamma-rays and NIST traceable instruments of sufficient quality permitting acceptable agreement with NIST. The results will reveal the degree to which the participating calibration facility can demonstrate proficiency in transferring a NIST air-kerma calibration under the conditions of the said facility at the time of the proficiency test.
6. The selection of the reference radiation qualities used for the proficiency test depends on the scope of the accreditation. In most cases it is not feasible that an annual or biennial proficiency test cover the entire range of air-kerma rates and beam qualities. NIST advises that the proficiency test cover a representative part of the scope of accreditation, eventually testing the complete scope of the reference radiation qualities, typically using two to four qualities per test. The accreditation guidelines generally state the frequency and extent of the required test.
7. The proficiency test protocol will contain information concerning the air-kerma rate, corrections, calibration distance and beam size used for the NIST calibrations. The protocol will be issued to the participating calibration facility at the time of receipt of the NIST transfer standard. However, the calibration facility should use their routine calibration conditions for proficiency tests. The goal of the proficiency test is to test that the calibration facility is transferring quality calibrations under routine conditions.
8. NIST will conduct the proficiency test using a NIST-owned transfer-quality ionization chamber. The participating facility is responsible for obtaining the necessary electrometer. NIST will not provide a chamber/electrometer unit.
9. NIST will conduct a proficiency test on an individual basis, as well as part of an accreditation program. For a proficiency test involving multiple facilities, questions or concerns should be directed to the NIST technical contact, and no communication should be made amongst the facilities concerning the test results until the completion of the test.
10. If a proficiency-test customer does not meet the level of agreement required by their accrediting guidelines, a retest can be conducted at the customer’s expense. Before a retest is conducted, NIST encourages open dialogue with the customer to assist with the identification of the problem(s) that contribute to the disagreement. Typically, NIST will suggest that the customer review the data for calculation or typographical errors, verify the transfer of traceability of air-kerma, and investigate the calibration history of the transfer chamber.
11. It is the responsibility of the participant to inform the accrediting body of the results of the NIST proficiency test. The results are considered confidential and proprietary, and NIST will not provide copies of the reports to the accrediting body unless written authorization is provided by the customer.

Proficiency Tests for Mammography

1. The calibration facility shall initiate contact with NIST and request a proficiency test for mammography calibrations.
2. NIST will conduct a proficiency test on an individual basis, as well as part of an accreditation program.
3. The proficiency test will consist of three stages. First NIST will calibrate a NIST transfer standard using two appropriate molybdenum and/or rhodium generated x-ray beam qualities. NIST will then send the transfer standard to the participating calibration facility, along with details of the NIST calibration conditions, excluding the calibration results. Calibrations of the NIST transfer chamber will be made at the facility using the appropriate beam qualities. The NIST transfer chamber shall be returned to NIST accompanied with the calibration results, in units of Gy/C, for each beam quality. Upon receipt of the chamber at NIST, it will be re-calibrated to assure stability and proper working conditions.
4. The results, provided by the participating facility, will be compared to the NIST calibrations. A NIST Proficiency Test report will be issued to each participating calibration facility. The report will state the level of agreement with NIST under the conditions of the proficiency test. The proficiency test can be used to demonstrate that the participating facility can calibrate a NIST transfer standard, using techniques, characterized x-rays and NIST traceable instruments of sufficient quality permitting acceptable agreement with NIST. It will be the responsibility of the calibration facility to inform the FDA-MQSA of the proficiency test results.
5. Since medical physicists and inspectors are required to use NIST traceable instruments to conduct inspections of clinical molybdenum and/or rhodium anode x-ray units, the calibrations of the mammography calibration instruments will be conducted using two of the seventeen NIST Mo and/or Rh beam qualities. The selection of the two beam qualities to be used in the proficiency test will be made jointly by NIST and the participating calibration facility. The beam qualities chosen will be within the HVL and energy range of the beams available for calibrations at the participating facility. The facility may choose to compare with NIST for more than the two required beam qualities. Information concerning the air kerma rate, calibration distance and beam size used for the NIST calibrations will be issued to the participating calibration facility at the time of receipt of the NIST transfer standard. However, the calibration facility should use their routine calibration conditions for proficiency tests. The goal of the proficiency test is to test that the calibration facility is transferring quality calibrations under routine conditions.
6. NIST will conduct the proficiency test using a NIST-owned transfer-quality ionization chamber. The participating facility is responsible for obtaining the necessary electrometer. NIST will not provide a chamber/electrometer unit.