iaea human health series publications - SEDIM

8.7.2. Incident air kerma at the entrance surface of PMMA slabs8.7.2.1. Scope— Objective: To estimate the incident kerma in air (without backscatter) at the position corresponding to theentrance surface of PMMA of thicknesses of 20, 45 and 70 mm 23 . The measurement can be done at aconvenient distance from the X ray source and then calculated for the locations of the entrance surfaces usingan inverse square law correction.— References [8, 48, 60, 64, 65].— Minimum frequency: Annually and after change of, or maintenance to, the X ray tube, tube housing and/orcollimation system.8.7.2.2. Instrumentation(1) Dosimeter appropriate for mammography (see Appendix II).(2) Exposure technique data from the AEC test (see Section 8.4.2 for target–filter combination, kV and mAs AEC )for the PMMA slabs with thicknesses of 20, 45 and 70 mm.(3) Metal plate to shield the detector from X rays (e.g. 1 mm steel, 5 mm aluminium or >0.1 mm lead), largeenough to cover the active area of the detector.(4) Tape measure.(5) Thermometer.(6) Barometer.8.7.2.3. MethodologyThe incident air kerma without backscatter is measured using the same exposure factors selected to expose aphantom equivalent to three thicknesses of PMMA representing breasts of different attenuation. The incident kermain air at the entrance surface of the PMMA slab(s) is determined from: (a) the mAs value used to expose the PMMAslab with the target–filter–kV combination used in Section 8.4.2 (mAs AEC ); (b) the measured incident kerma in airfor the given mAs AEC ; and (c) the measured distance from the focus of the X ray tube to the breast support table.(1) Determine the distance, d T , from the focus of the X ray tube to the breast support table and record it in thekerma and dose chart (Chart 11 in Annex II). 24 This can be determined using the manufacturer’s specificationsor by measurement, if necessary.(2) Place the metal shield on the breast support table to protect the digital detector from excessive exposure. Donot scratch the table.(3) Attach the dosimeter to the lower surface of the compression paddle. The dosimeter should be centredlaterally in the beam, approximately 40 mm into the radiation field from the chest wall edge of the beam. Setthe height of the compression paddle so that the centre of the active volume of the dosimeter (themeasurement point) is 45 mm above the breast support table. 2523The measurement is made without the PMMA, because it should be done without scattered radiation. Note that the originalMonte Carlo modelling [63] incorporated a correction to the incident air kerma that would have been present for a thicker breast whoseattenuation corresponds to these PMMA thicknesses. Therefore the entrance kerma to the PMMA is evaluated, rather than the kerma tothe entrance of the breast.24This is difficult to measure accurately with a tape measure, and it may be best to obtain the distance from the manufacturer.This measurement need not be repeated unless there are changes to the mechanical assembly that would affect this distance.25The dosimeter is placed at the same height as the top surface of a 45 mm PMMA slab rather than at the height of theequivalent breast thickness because the inverse square law correction to the surface of the breast is already incorporated in theconversion factors used in Table 18.111