Purpose: In this paper, the effective atomic numbers (Zeff), electron densities (Neff), exposure buildup factor (EBF) and energy absorption buildup factor (EABF) of healthy (H), carcinoma (C) and equivalent (E) breast tissues were investigated to quantify differences in radiological parameters between the tissues in the continuous energy region. For these purposes, Zeffs and Neffs, of healthy, carcinoma and equivalent (H-C-E) breast tissues were calculated for interaction of gamma rays and charged particles with selected tissues. EBFs and EABFs of the tissues were also calculated for gamma rays as well and were compared to each other for different photon energies (0.015-15 MeV) up to 40 mfp.
Method: A well-known interpolation procedure has been used for the calculation of Zeff using the mass stopping powers and mass attenuation coefficients for charged and uncharged radiations in the continuous energy region. And the buildup factors of the tissues were calculated by the well-known G-P fitting method based on the interpolation from the equivalent atomic number (Zeq).
Results: The variation of Zeffs, Neffs with energy and differences (%) in the Zeffs between the tissues were determined for photon, electron, proton and C ion, which is commonly used in hadron therapy in the relevant energy region 1 keV-400 MeV. Differences (%) between healthy and carcinoma tissues in Zeff were around 9%, >4%, >5%, >2.5% for C ion, photon, electron and proton interaction, respectively. The relative differences (RD %) in Zeff between healthy and equivalent tissues in the energy range 0.02-1 MeV were found to be less than 1% for photons and C ions. In addition differences (%) between healthy and carcinoma tissues in EABF and EBF were around 25%, but differences (%) between healthy and equivalent tissues were <6% and <4% (at 40 mfp) in the continuous energy region.
Conclusions: From the results, BR12 was found the be available for an equivalent to healthy breast tissue in 0.02-1 MeV for photons and C ions due to RD (%) in Zeff between healthy and equivalent tissues (less than 1%). However, for electron interaction, the RD (%) in Zeff between healthy and equivalent tissues was always greater than 4% in the entire energy range, thus BR12 was found the worse an equivalent to healthy breast tissue in in the entire energy range for electron. Also the reported data in the study should be useful to choose best equivalence for photon, electron, proton and Carbon ion interactions.
Keywords: Effective atomic number; buildup factor; electron and hadron interactions; electron density; photon interaction.