Vol.5 No.4 2013

Research paper : Dose standards for safe and secure breast cancer screening (T. Tanaka et al.)−236−Synthesiology - English edition Vol.5 No.4 (2013) and Canada (the National Research Council, NRC, radiation quality is slightly different) participated in this international key comparison, and equivalence was confirmed among the mammography dose standards at all institutes.[17] It was also found that the uncertainty in the AIST dose standard was comparable to that in the dose standards of other countries.6.2 Validation using a mammography machineThe X-rays produced by mammography machines used in medical practice are generated as a short pulse. However, the dose rate at the reference field is constant over time because stability is required. Additionally, in an actual mammography machine, the structure of the irradiation device is compact and it is difficult to put in the national primary standard, whereas there is spatial allowance in the standard field. The investigation of such differences between an actual mammography machine and the standard field will help improve the reliability of dosimetry in medical practice. Therefore, AIST conducted a comparison of the dose evaluation results between ionization chamber dosimeters (calibrated at AIST) and a glass dosimeter using a mammography machine employed in medical practice.As a result, it was confirmed that the AGDs measured by the ionization chambers and glass dosimeter agreed within the uncertainties (Fig. 10). The dosimeters calibrated at the AIST’s standard field were found to be reliable for dose evaluation of an actual mammography machine.7 ConclusionsAIST has developed a dose standard for mammography radiation qualities and has established a calibration service to improve the reliability of mammography dose evaluation in medical practice. The time required to develop a mammography dose standard was greatly reduced by utilizing the existing primary standard instrument (free air ionization chamber) for soft X-rays. Additionally, AIST participated in an international key comparison to confirm international equivalence. Rapid and widespread dissemination of the standard was possible by fully utilizing the existing calibration service system. The calibration service system will be further developed in the future through collaborations with academic societies and calibration services.Currently, digital mammography, which uses digital images and computers, is becoming common instead of conventional screen-film mammography. Compared to evaluation using a conventional film image, dose evaluation from digital image is thought to be more difficult; therefore, the importance of dosimetry in mammography quality control is expected to increase. Additionally, various radiation qualities (W/Rh or W/Ag) will be used in digital mammography. The sensitivity of the semiconductor dosimeter used in medical practice changes significantly depending on the radiation quality, and the development of a reference field is urgently required. Currently, a reference field for digital mammography is being developed in the US, and focused research is being performed to evaluate the semiconductor dosimeter. AIST will continue developing its reference field to manage the increase of radiation qualities efficiently and to contribute to the quality control and safety of advanced mammography.AcknowledgementsThe excellent quality control system in Japan played a major role in the rapid and widespread dissemination of this standard. We are deeply grateful to the staff members who constructed such an excellent quality control system. We are also grateful to the people of Chiyoda Technol Corporation who assisted in the development and evaluation of the mammography glass dosimeter.TerminologiyTerm 1.Age-adjusted incidence rate: The incidence rate corrected for the age composition of the standard population (1985 population model). This enables comparisons among groups with significantly different age compositions.Term 2.Age-adjusted mortality rate: The mortality rate corrected for the age composition of the standard population (1985 population model). This enables comparisons among groups with significantly different age compositions.Term 3.Linear attenuation coefficient: When a single-Glass dosimeterIonization chamber CIonization chamber BIonization chamber ARadiation qualityRatio of the average of measured AGD using 4 dosimeters0.900.951.001.051.10Mo/Mo 28 kVMo/Mo 26 kVFig. 10 Comparison of dose measurements for the actual mammography machineFor each data point, the vertical bar shows the uncertainty at the 95 % confidence level.


page 12