Calculation of Air Kerma to Average Glandular Tissue Dose Conversion Factors for Mammography

Abstract

Among the quantities used for dose specification in mammography the average absorbed dose in glandular tissue is most appropriate for risk assessments. Generally, average absorbed dose in glandular tissue is derived from measurement of air kerma free-in-air combined with conversion factors obtained from radiation transport calculations in mathematical breast models. Air kerma to glandular tissue dose conversion factors are published by various authors as a function of half-value layer (HVL) for simple breast phantoms. Calculations by different authors differ in radiation transport codes, photon interaction data, photon spectra, composition and thickness of superficial layer (representing skin and subcutaneous adipose tissue), presence of compression plate, and are not performed with the tissue compositions recommended by the ICRU. Protocols for dosimetry in mammography in different European countries (UK, Netherlands, Sweden) use results from different authors. To study the influence of the parameters indicated, air kerma to glandular tissue dose conversion factors, g, are calculated using the MCNP radiation transport code. Use of different spectra can cause differences in 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 cross sections results in 10% higher g values compared to the use of XCOM data. The influence of the backscatter material (composition and thickness) on the g value is marginal (<1%). The different superficial layers used result in differences in g values of up to 19%. When the breast thickness is changed from 2 cm to 8 cm, g values decrease by a factor of about 4. Employing Hammerstein's tissue compositions compared to those from the ICRU results in differences in g values up to about 14%.