Refinement of the Hounsfield look‐up table by retrospective application of patient‐specific direct proton stopping‐power prediction from dual‐energy CT

Background and Purpose:

Proton treatment planning relies on an accurate determination of stopping-power ratio (SPR) from x-ray computed tomography (CT). A clinically applicable refinement of the heuristic CT-based SPR prediction using a state-of-the-art Hounsfield look-up table (HLUT) is proposed, which incorporates patient-specific SPR information obtained from dual-energy CT (DECT) in a retrospective patient-cohort analysis.

Material and Methods:

SPR datasets of 25 brain-tumor, 25 prostate-cancer and three non-small cell lung-cancer (NSCLC) patients were directly derived from clinical DECT scans with the DirectSPR approach. Based on the median frequency distribution of voxelwise correlations between CT number and SPR within the irradiated volume, a piecewise linear function was specified (DirectSPR-based adapted HLUT). Differences in dose distribution and proton range were assessed for the non-adapted and adapted HLUT compared with the DirectSPR method.

Results:

The application of the DirectSPR-based adapted HLUT instead of the non-adapted one reduced systematic range deviations from 1.2% (1.1 mm) to -0.1% (0.0 mm) for brain-tumor, 1.7% (4.1 mm) to 0.2% (0.5 mm) for prostate-cancer and 2.0% (2.9 mm) to -0.1% (0.0 mm) for NSCLC patients. Due to the intra- and inter-patient tissue variability, range deviations larger than 1% are still present for the adapted HLUT.

Conclusions:

The incorporation of patient-specific correlations between CT number and SPR, derived from a retrospective application of DirectSPR to a broad patient cohort, improves the accuracy of the current state-of-the-art HLUT approach. The DirectSPR-based adapted HLUT has been clinically implemented at our institution, which represents a further step toward full integration of the DECT-based DirectSPR method for treatment planning in proton therapy.