Further factors for the estimation of mean glandular dose using the United Kingdom, European and IAEA breast dosimetry protocols

The United Kingdom, European and IAEA protocols for breast dosimetry in mammography make use of s-factors which allow for the use of different target/filter combinations. To supplement the existing protocols, a Monte Carlo computer program has been used to calculate s-factors for mammography using a tungsten target with silver filters of thicknesses 50-75 mu m and for the same target filtered with 0.5 mm aluminium. The dosimetry protocols use slabs of polymethyl methacrylate (PMMA) of specified thicknesses to simulate the exposure of typical breasts. The equivalent thickness of PMMA has been calculated using a simplified approach for a wider range of x-ray spectra and for breast thicknesses of 2-11 cm. The results show that for the tungsten/silver target/filter combination, a single s-factor of 1.042 can be used with the protocols, but when the tungsten target is filtered with 0.5 mm of aluminium, it is necessary to select from a tabulation of s-factors against breast thickness. The equivalent thicknesses of PMMA for a given breast thickness show some dependence on beam quality and the values obtained differ from those presently used in the dosimetry protocols by an amount which depends upon breast thickness and half value layer (HVL). For the extreme case of an 11 cm breast and an HVL of 0.62 mm Al, the use of the protocol thickness would give rise to an error of 10%, but for breast thicknesses of 6 cm or less, the error is typically 2-3%.