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

Displacement corrections for in-phantom measurements with ionisation chambers 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 mammography seem compatible with geometrical considerations made previously. A discrepancy is found between depth-dose data derived from calculations and those found in experiments.