MO‐FF‐A3‐06: Does KV‐MV Dual‐Energy Computed Tomography Have an Advantage in Measuring Proton Stopping Power Ratio in Patients?

Purpose: To evaluate different combinations of x‐ray energies when using Dual‐Energy CT (DECT) method for calculating proton stopping power ratio (SPR) in the presence of random noise and beam hardening effects. Method and Materials: We selected three representing x‐ray energy pairs: 100kVp and 140kVp for KV‐KV, 100kVp and 1MV for KV‐MV, and two 1MV x‐rays with and without external filtration for MV‐MV DECT imaging. Small perturbations in CT numbers and x‐ray spectrum used for the DECT calculation were introduced to simulate effects of random noise and beam hardening. The CT dose to generate a fixed amount of random noise was determined by Monte Carlo method in simulated CT images of a water phantom. The SPRs of 34 human tissues listed in ICRP 23 and ICRU 44 were calculated using the DECT method with all three x‐ray energy combinations. We also performed error propagation to study the sensitivity to CT number uncertainties for each of the DECT x‐ray pairs. Results: The relative sensitivity of the derived SPR to random noise in CT number was 6.59, 1.27 and 3.34 for KV‐KV, KV‐MV, MV‐MV DECT techniques, respectively. Under the same condition, the root‐mean‐square errors in SPRs derived using the KV‐MV DECT technique were at least 50% smaller than those using the KV‐KV or MV‐MV combinations. Additionally, we found CT number uncertainties for the high energy beam within the dual‐energy pair had a bigger impact to SPR accuracy than the low energy beam, suggesting it is more important to emphasize the accuracy of the high energy CT imaging than the low‐energy one in DECT applications. Conclusion: The KV‐MV combination makes the DECT method more robust in calculating proton SPRs or resolving effective atomic number for tissue composition in the presence of CT number uncertainties. Conflict of Interest: Research partially sponsored by Varian Medical Systems.