Vol.5 No.4 2013

Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−232−Synthesiology - English edition Vol.5 No.4 (2013) the half-value layer for AIST’s soft X-ray standard of W/A1 radiation qualities).In the X-ray energy range used in mammography, the sensitivity (calibration coefficient) of ionization chambers with a relatively small energy dependence changes with X-ray energies more than their uncertainties (95 % confidence interval) as shown by the vertical bar for each data point in Figure 3. The change in sensitivity differs according to the type of dosimeter, i.e, the calibration coefficients of the dosimeter A decrease with the HVL whereas those of dosimeter B increase. This change is due to differences in the material and thickness of the X-ray entrance window of the dosimeter as well as the internal structure of the dosimeter. The semiconductor dosimeters, which have a larger energy dependence than the ionizing chambers, are used more often in medical practice; thus, Japanese industries and academic societies were concerned about the effect of the radiation quality and energy dependence of the dosimeters on the calibration coefficient. To manage this problem, AIST developed and disseminated a dose standard based on the radiation qualities of mammography X-rays.3 Scenario for improving the reliability of mammography X-ray dose evaluationTo improve the reliability of mammography dose evaluation in medical practice, it is necessary to (1) establish a dose standard based on the radiation qualities of mammography X-rays used in practice and (2) create a calibration service system to disseminate the dose standard to society. Figure 4 shows a scenario for improving the reliability of mammography dose evaluation.To quickly respond to social demands, time required for standard development was shortened by utilizing the existing primary standard without developing a new primary standard optimized for mammography X-rays. To reduce the uncertainty of dose evaluation in medical practice, to which the standard was mainly disseminated, an X-ray field close to that of an actual mammography machine was developed. Additionally, to confirm the international equivalence of the standard, an X-ray reference field was developed at the same time in compliance with the International Electrotechnical Commission (IEC) standard. In addition to participating in the international key comparison, the calibration capability of the developed dose standard was verified by comparing the dose measurements of X-rays from an actual mammography machine with several dosimeters calibrated using the standard, and the results were evaluated.Because mammography quality control (including dose evaluation) was already being performed in many medical practices, rapid and widespread dissemination of the standard was achieved by evaluating the glass dosimeter that was already being used in this quality control. Therefore, AIST developed a method for calibrating the glass dosimeters.In the conventional traceability scheme of the dose standards, the standards were disseminated through networks of secondary calibration laboratories by means of calibrations. In this case, it would not be useful if the conventional W/A1 radiation quality owned by the calibration laboratories was used as the reference (X-ray) field. However, it costs several tens of millions of yen to develop a reference field for the radiation qualities of mammography X-rays. Therefore, AIST developed a system in which the AIST’s irradiation facility could be used for the calibration/testing service to ensure Fig. 4 Scenario for improving the reliability of mammography X-ray dose evaluation・Focus on rapidity of standard dissemination rather than development of a new standard with high precision→Evaluation of the correction factors of the existing primary standard using mammography X-ray radiation qualityDevelopment of primary standard (free air ionization chamber)Development of X-ray field for mammographyPromotion of the use of the AIST irradiation facilityCollaboration with existing quality control systemsDevelopment of the dose standard for mammography X-rayValidation of calibrationRapid dissemination of the dose standardIncreasing the reliability of mammography dose evaluation in medical practice・Ensuring international equivalence through international comparison・Validation using a mammography machine・Introduction of calibration/tasting service・Evaluation of the glass dosimeter used in medical practice・Development of a reference field approximating the X-ray spectrum of the actual mammography machine (calibration distance, placement of compression paddle, etc.)・Development of an X-ray reference field that is internationally acceptable(complies with IEC standard)


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