Is there reliable experimental evidence for different dicentric yields in human lymphocytes produced by mammography X-rays free-in-air and within a phantom?
We examined the production of dicentrics in human lymphocytes irradiated with 29 kV X-rays to a depth of 13.5 mm in a PMMA phantom. For these irradiation conditions, which are appropriate for the diagnostic application of mammography X-rays, a coefficient of (5.88±0.66)×10–2 Gy–1 of the linear quadratic dose–response relationship was determined. This value does not differ significantly from the coefficient of (6.55±0.97)×10–2 Gy–1 obtained earlier for a free-in-air set-up using blood of the same donor. The results are interpreted in terms of both the energy distributions of the photon fluence of mammography X-rays free-in-air and those in the PMMA phantom. Based on earlier results of experiments with monochromatic X-rays in the energy range 1.83–40 keV (completed here by an additional measurement at 25 keV), a fit function (E) to the measured coefficients as a function of the energy E of monochromatic X-rays was used to calculate weighted mean values for both the mammography X-ray spectra free-in-air and in the phantom. As a result, weighted mean values of (4.9±1.0)×10–2 Gy–1 and (4.5±1.0)×10–2 Gy–1 were obtained, respectively. Although the measured coefficients for mammography X-rays appear to be systematically higher than those calculated as weighted mean values , it can be concluded that the modification of the mammography X-ray spectrum to a depth of 13.5 mm in a PMMA phantom compared with the free-in-air spectrum has no significant influence on the dicentric yields in human lymphocytes.