Simultaneous characterization of electron density and effective atomic number for radiotherapy planning using...

Relative electron densities of body tissues (ρ_e) for radiotherapy treatment planning are normally obtained by CT scanning of tissue substitute materials (TSMs) and producing a Hounsfield Unit-ρ_e calibration curve. Aiming for more accurate, simultaneous characterization of ρ_e and effective atomic number (Z_eff) of real tissues, an in-house phantom (including 10 water solutions plus composite cork as TSMs) was constructed and scanned at 4 kVps. Dual-energy algorithms were applied to 80-140 and 100-140 kVp combination scans, for better differentiation of tissues with same attenuation coefficient at 120 kVp but different ρ_e and Z_eff. Stoichiometric calibration and closeness of the ρ_e of the 11 TSMs to real tissues (≤ 0.5%) resulted in smaller ρ_e calculation discrepancies, compared to studies with commercial phantoms (p < 0.024). Applying an energy subtraction algorithm further mitigated errors by spectral separation and reduction of beam hardening artifacts and noise, reducing the mean and standard deviation of the absolute difference of ρ_e at 80-140 kVp (p < 0.003) and 100-140 kVp (p < 0.0001) scans, compared to 120 kVp scan, respectively. Moreover, a parametrization algorithm decreased the Z_eff discrepancy from real tissues at 80-140 kVp scans; for thyroid, the residual error was ≤ 0.18 units of Z_eff (vs. 0.2 with the Gammex 467 phantom from a previous study). These results further suggest that a dual-energy algorithm in combination with stoichiometry can decrease errors in calculation of the ρ_e of real tissues to ameliorate inhomogeneity for dose calculation in radiotherapy treatment planning, especially when the energy spectrum of the X-ray tube of the CT machine is not available.