Room: Karl Dean Ballroom B1
Purpose: In proton radiotherapy, range uncertainties induced by the conversion from x-ray CT (xCT) Hounsfield units (HU) to relative stopping power (RSP) compromise the precision of dose calculation. This study investigates optimizing the RSPs of individual voxels in the planning xCT iteratively based on multi-projection proton radiography (pRG).
Methods: For a phantom consisting of inserts made of tissue substitute materials, time-resolved dose rate functions (DRF) delivered by a modulated broad beam were measured by an amorphous silicon flat panel imager placed downstream of the phantom. Water equivalent path lengths (WEPL) in the pRG were derived from the root-mean-square (RMS) of DRFs. By rotating the phantom, multiple pRG projections were acquired at angles from 0 to 358 degrees with an increment of 2 degrees. xCT of the phantom was acquired and co-registered with the pRG acquisition coordinates. RSPs of individual xCT voxels were optimized iteratively by minimizing the difference between the measured WEPLs in pRGs and the calculated WEPLs by ray tracing with HU-converted RSPs. Pixels in the measured WEPL images exhibiting severe proton range mixings were rejected for the optimization. Tikhonov regularization was applied under the assumption that HU-converted RSPs are within a few percent (~5%) from the ground truth.
Results: While ~50% of WEPL pixels were rejected due to severe range mixings, RSPs of >90% CT voxels could still be optimized by utilizing 12 pRG projections around the phantom. For the test phantom, percentage errors of the HU-converted RSPs w.r.t. independently measured reference RSPs were reduced from a range of -8% ~ +4% to less than Â±2%. Percentage errors were further reduced to less than Â±0.5%, by tuning the regularization coefficient for different materials.
Conclusion: This study demonstrates the feasibility and promise of optimizing RSPs of individual CT voxels with multi-projection pRGs acquired by a single flat panel imager.