Calculations of displacement corrections for in-phantom measurements with ionisation chambers for mammography

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Abstract
Displacement corrections for in-phantom measurements with ionisation cham- bers for the purpose of mammography are large and represent a major correction to consider for dose determinations. Experimental data on displacement corrections depend to a large degree on the model used to extrapolate to zero cavity radius. Calculations of displacement correction factors using a Monte Carlo code are presented for different cavity shapes, i.e. spherical, cylindrical and disc-like, in various phantom materials simulating the average breast and breast composing tissues. In addition, the influence of wall material and depth in-phantom are studied. Exponential extrapolation to zero cavity radius should be performed to obtain the dose in homogeneous phantoms. Displacement corrections for photons as used in mam- mography seem compatible with geometrical considerations made previously. A dis- crepancy is found between depth-dose data derived from calculations and those found in experiments.