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Patient Specific Optimization of Hounsfield Unit to Relative Stopping Power Calibration Curve Using Carbon Ions

D Shrestha*, L Chen , Y Zhang , J Wang , UT Southwestern Medical Center, Dallas, TX


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: Carbon ion radiotherapy can potentially deliver dose with higher conformality compared to conventional radiotherapy. However, uncertainties in the relative stopping power (RSP) estimation introduced during the conversion of computed tomography (CT) Hounsfield units (HU) of the tissues to their RSP can be a limiting factor. We have developed a method to optimize the HU to RSP calibration curve to reduce the range uncertainties hence improve the accuracy of the dose delivery with carbon ions.

Methods: The phantom used for this work consisted of 28 materials inserts inside lung material with water as the overall background. Carbon ion water equivalent path lengths (WEPL) were simulated using the Geant4 Monte Carlo toolkit using an ion beam of energy 430 MeV/u with cone-beam geometry. An HU-RSP calibration curve with 10 segments created using the material properties from Geant4 was used as the reference curve. A clinical calibration curve was represented by a modified version of the reference curve. An optimization scheme was constructed based on the conjugate gradient method to estimate the intercept and slope (a, b) of each segment of the calibration curve. The objective of the linear optimizer was to iteratively minimize the difference between the WEPL and the forward projection of the RSP phantom constructed from CT using the initial calibration curve.

Results: The relative RMSE associated with estimation of individual RSP values of the inserts were within 0.55% of the ground truth. The optimized calibration curve closely matched with the reference curve. The overall RMSE for the optimized RSP phantom and the ground truth was 0.48%.

Conclusion: Our proposed optimization scheme can lead to reduced range uncertainties and accurate dose distribution hence increasing the efficacy of carbon ion radiation treatment.


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