Purpose: In proton therapy, dose deposition is sensitive to both geometry and tissue density along the beam-path. This work investigates the feasibility of rapid daily dose estimation to guide patient setup.
Methods: A reference CBCT (refCBCT) is a synthetic CBCT produced once by deforming first day CBCT to the planning CT, corrected by histogram matching to the planning CT. Same correction is applied to daily CBCT (iCBCT) for the shape of HU-to-RPSP. Software was developed to compare iCBCTs to refCBCT and adjust proton dose based on corrected HU differences along each beam-path. Daily total dose was estimated by summing individual beam doses. 10 patients receiving bilateral neck radiation for oropharyngeal cancer were used for validation. Validation was performed by comparing estimated fractional dose to our off-line standard Monte Carlo calculation using a synthetic CT acquired by deforming the closest in time CT to iCBCT.
Results: After acquiring iCBCT and its registration to the planning CT, dose estimation for 5-field bilateral HN plans took < 1min. Estimated doses for 4 fractions x 10 patients when compared with the standard dose calculations produced Gamma-Index (3mm/3%) with average pass-rates 97.4%, σ= 0.8%. 95th percentile of voxel-by-voxel dose difference was <5% of the daily prescription. There was no statistically significant difference on how the method performed on the second and last treatment fractions.
Conclusion: Using density-based dose deformation along each proton beam, a rapid and accurate fractional dose estimate for oropharyngeal radiation was obtained which could be used for real-time guidance on patient setup.