CALCULATION OF AIR KERMA TO AVERAGE GLANDULAR TISSUE DOSE CONVERSION FACTORS FOR MAMMOGRAPHY

Among the quantities used for dose specification in mammography the av erage absorbed dose in glandular tissue is most appropriate for risk a ssessments. Generally, average absorbed dose in glandular tissue is de rived from measurement of air kerma free-in-air combined with conversi on factors obtained from radiation transport calculations in mathemati cal breast models. Air kerma to glandular tissue dose conversion facto rs are published by various authors as a function of half-value layer (HVL) for simple breast phantoms. Calculations by different authors di ffer in radiation transport codes, photon interaction data, photon spe ctra, composition and thickness of superficial layer (representing ski n and subcutaneous adipose tissue), presence of compression plate, and are not performed with the tissue compositions recommended by the ICR U. Protocols for dosimetry in mammography in different European countr ies (UK, Netherlands, Sweden) use results from different authors. To s tudy the influence of the parameters indicated, air kerma to glandular tissue dose conversion factors, g, are calculated using the MCNP radi ation transport code. Use of different spectra can cause differences i n g values up to about 7%. The use of a compression plate results in a 4.5 +/- 1.5% smaller g value for the same HVL. The use of MCPLIB cros s sections results in 10% higher g values compared to the use of XCOM data. The influence of the backscatter material (composition and thick ness) on the g value is marginal (<1%). The different superficial laye rs used result in differences in g values of up to 19%. When the breas t thickness is changed from 2 cm to 8 cm, g values decrease by a facto r of about 4. Employing Hammerstein's tissue compositions compared to those from the ICRU results in differences in g values up to about 14% .