Influence of radiation field size on half-value layer measurements in mammography

Introduction Mammography measurements are commonly conducted in a radiation field of standard size without additional collimation of the primary beam. International standards, protocols and guidelines propose half-value layer (HVL) measurements under narrow beam conditions, which are not applied in clinical measurements. Methods The HVL was measured using an ionization chamber and X-ray multimeters (XMMs) in different field collimations in radiation qualities commonly encountered in mammography. The impact of field size on measured HVL was examined for Mo/Mo, Mo/Rh, W/Rh and W/Ag anode/filter combinations, for X-ray tube voltages of 25 kV, 28 kV, 30 kV and 35 kV. Results Field size has a significant influence on the HVL measured with the ionization chamber. Up to 5 % differences were observed and it was impacted by irradiation geometry (field shape and usage of compression paddle) and radiation quality. The XMMs do not have similar behaviour. Conclusion With ionization chambers, the HVL should be measured following the definition in a narrow beam without scatter. The XMMs are not so sensitive to scattered radiation but they should have a traceable calibration in terms of HVL for specific radiation qualities in use.