Influence of additional PMMA on the calibration of X-ray multimeters for mammography radiation qualities

X-ray multimeters (XMMs) are widely used for quality control measurements in mammography, providing air kerma, half-value layer (HVL), and tube voltage from a single exposure. The energy dependence of their response was comprehensively investigated for various anode/filter combinations and software selections. However, the influence of a change in the X-ray spectrum due to additional material in the beam has not yet been systematically investigated. The aim of this study is to quantify the sensitivity of XMM calibration coefficients to changes in X-ray spectra introduced by additional polymethyl methacrylate (PMMA) of different thicknesses. Four commercially available XMMs were calibrated at the IAEA dosimetry laboratory for air kerma, HVL, and tube voltage using two anode/filter combinations (Mo/Mo and W/Al) at tube voltages ranging from 25kV to 35kV. Calibration coefficients were determined without additional filtration and with PMMA thicknesses of 2.0mm, 2.8mm, and 4.8mm placed close to the X-ray tube to modify the primary spectrum while minimizing scattered radiation reaching the detectors. In relation to the calibration coefficient determined without PMMA deviations of up to 11% were observed for air kerma rate. HVL and tube voltage calibration coefficients exhibited substantially larger deviations, reaching up to 37% and 42%, respectively. We conclude that changes in the X-ray spectra as from additional PMMA filtration, can substantially alter XMM calibration coefficients, particularly for HVL and tube voltage measurements. XMM responses are highly sensitive to spectral modifications beyond standard anode/filter combinations and tube voltage settings.