Abstract
PurposeTo describe an implementation of dual-energy computed tomography (DECT) for calculation of proton stopping-power ratios (SPRs) in a commercial treatment-planning system. The process for validation and the workflow for safe deployment of DECT is described, using single-energy computed tomography (SECT) as a safety check for DECT dose calculation.Materials and MethodsThe DECT images were acquired at 80 kVp and 140 kVp and were processed with computed tomography scanner software to derive the electron density and effective atomic number images. Reference SPRs of tissue-equivalent plugs from Gammex (Middleton, Wisconsin) and CIRS (Computerized Imaging Reference Systems, Norfolk, Virginia) electron density phantoms were used for validation and comparison of SECT versus DECT calculated through the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, California) application programming interface scripting tool. An in-house software was also used to create DECT SPR computed tomography images for comparison with the script output. In the workflow, using the Eclipse system application programming interface script, clinical plans were optimized with the SECT image set and then forward-calculated with the DECT SPR for the final dose distribution. In a second workflow, the plans were optimized using DECT SPR with reduced range-uncertainty margins.ResultsFor the Gammex phantom, the root mean square error in SPR was 1.08% for DECT versus 2.29% for SECT for 10 tissue-surrogates, excluding the lung. For the CIRS Phantom, the corresponding results were 0.74% and 2.27%. When evaluating the head and neck plan, DECT optimization with 2% range-uncertainty margins achieved a small reduction in organ-at-risk doses compared with that of SECT plans with 3.5% range-uncertainty margins. For the liver case, DECT was used to identify and correct the lipiodol SPR in the SECT plan.ConclusionIt is feasible to use DECT for proton-dose calculation in a commercial treatment planning system in a safe manner. The range margins can be reduced to 2% in some sites, including the head and neck.
Highlights
Proton therapy provides significant advantages in dose distribution because of the sharp dose falloff near the end of the proton range
We demonstrate a method for dual-energy computed tomography (DECT) implementation that uses the same single-energy computed tomography (SECT) Hounsfield units (HUs)-to-stopping-power ratios (SPRs) curve but with moreaccurate HU values calculated with a script in the planning system
Calculation of the SPR on tissue surrogates using the ESAPI script showed that DECT is more accurate than SECT is
Summary
Proton therapy provides significant advantages in dose distribution because of the sharp dose falloff near the end of the proton range. Dose painting with pencil beam-scanning techniques, combined with the inherent Bragg peak from heavy-charged particles, allows for high dose conformity and sparing of organs at risk (OARs). Precise control of the location of individual Bragg peaks requires accurate material composition information within the beam path. The human body is composed of an array of tissues that have varying atomic number and mass density. Current proton-treatment planning systems use singleenergy computed tomography (SECT) Hounsfield units (HUs) to estimate proton stopping-power ratios (SPRs) by applying a calibration curve. HUs from x-rays are a measure of a material’s linear attenuation and do not have a one-to-one correspondence with SPRs, resulting in errors in the SPR calculation
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